Moreton Cheyney RF Repairs

General circuit tracing and RF fault finding

 I took a break from repairs to the set and started to trace the circuit. I immediately realised that some fairly extensive changes have been made by a previous owner and in doing this there were a few bad solder joints that may have resulted in the set being stood down from service. I initially began in the rear corner of the receiver which is occupied by an L63G (equivalent to the 6J5) and two metal 6J5 valves. I had overlooked the fact that there is no trace of an output transformer and no evidence of one having been fitted so I must assume that the output transformer was supplied with the loudspeaker which, from the 1946 Wireless World advert above, indicates it was supplied only with those sets fitted in cabinets. The transformer must have been fairly special because I cannot recall ever seeing a pair of 6J5s being asked to supply 10 watts. The next thing I spotted was that there are no direct connections between either rear socket and the 6J5 anodes. There clearly should be something and indeed I found a small condenser which I measured as about 4.7nF connecting one 6J5 anode to a pin on a rear socket. The wiring to this particular 6J5 is a little odd in that both its heater pins are wired, not to the sets LT line and ground, but to the rear connector to which that condenser is wired. Maybe this was done to reduce hum? Below, I've shown the area occupied by the output valves.

In the picture below the resistors are marked as follows: Individual anode feed 51Kohm, joint anode feed 10Kohm, cathode 3.3Kohm (see notes below for what this means). I'm guessing the functions of the two potentiometers.

I soon realised that to trace the circuit of the whole receiver I'd need to assign reference codes to the valves and components and you can see the results below.

 

 Valves

 Condensers

 Resistors

 

 

 
 Above is what I traced for the two 6J5 valves that were originally used for audio output which I suspect have been modified in some way, as yet undetermined. At first it looks like the left valve provides a single phase output to a external amplifier via a pin on one of the two rear 5-way sockets. The change was either done by the manufacturer or the user. I guess the external amplifier has a phase splitter so that this one is redundant but further work is needed before this is certain. If these two valves are supposed to directly drive a pair of push-pull power output valves their output voltages should be in anti-phase but they appear to be in-phase. It's therefore possible that not only has one anode been grounded it's possible a grid feed has been rewired, maybe as an interim change not followed up? To see the amplifier click the above circuit. Read on for some thoughts...

Initially I assumed that this chassis carried the complete receiver circuitry; although a couple of important things are clearly missing ie. an audio output transformer and a power supply. I assumed however that the statement about the audio output being 10 watts push-pull with a maximum of 2% distortion meant that two of the 6J5 valves were responsible for this, but later, having found that none of the 6J5s were wired for this I realised that either some later modifications had been carried out, or more likely the audio output was carried on a separate chassis and that this chassis also housed the mains PSU.

Firstly, working at some theoretical figures based on published information we can say that if the push pull audio output is given as 10 watts and assuming this is actually 10 watts RMS, then each 6J5 valve would need to deliver 5 watts of audio at max output. Assuming an efficiency of say 70%, each valve (because the max anode dissipation is 2.5 watts) would draw about 7 watts. Given an HT rail of 300 volts each valve would draw 23mA or from an HT rail of 250 volts, 28mA.

Taking an average an HT rail of 275 volts each valve would draw 25mA.The valves have an auto-bias cathode resistor of 3.3Kohm so the grid of each would be negatively biased at 82 volts which seems rather odd. I would expect the negative bias to be circa 4 to 6 volts so the cathode resistor should be 160 to 240 ohms. I'm unsure about the anode resistors. If they are original then they must have been fitted to protect the valves when operated without their transformer. Looking at the 3.3Kohm cathode resistors I can only assume that these were fitted by the last owner so that the valves would be acting as a driver for an external power amplifier. In that role an anode current of between 1 and 2mA would be typical. Assuming an anode current of 1.5mA the common 10Kohm would drop 30 volts (both valves total 3mA) and each 51Kohm would drop 77 volts leaving an anode voltage of about 170 volts which seems sensible.

Now the circuitry around the ganged pots... I'm still not entirely clear about the purpose of these two pots. Given a slight complexity in producing a decent treble control these pots may be used for that function?

 

I've yet to trace these circuits, but I did notice the left hand pot is wired to a large grounded electrolytic.  

 

 Next steps: identify all the components and trace the circuit diagram. This will help to work out the purpose of the four mystery valves. There are a few possible functions.. the TRF receiver, maybe an anode bend detector, and a "loudness" amplifier. V12 and V13 look like they are a pair of audio drivers for the PA fitted on the amp/PSU chassis. A brief check of the wiring tells me only a single audio feed is now wired to the output connectors and a new phase splitter circuit fitted to the modified external amplifier.

See the components listings which relate to the numbers shown on the following four pictures

Note X1 and X2 noted in IFT4 can are copper oxide "Westector" diodes whose "6" marking refers to the number of elements.

 

 

 

 

 

 
   This area is hidden under wiring and parts and carries some of the AVC components connected with white sleeved wires.

 

 

 

 Now that I've identified 99% of the components I can start to trace the circuit. Once I started I soon found modifications made by the previous owner plus a number of poor solder joints. I decided to find out the actions of the bandwidth switch as this should lead me to the TRF section of the receiver. The switch has four wafers spaced widely apart and I discovered it needs switch cleaner treatment as I found continuity tests didn't make sense. Each wafer has a pair of single pole 5-way contacts. Starting at the wafer nearest the front panel, one switch is not used and the other strangely has all 5 contacts wired together.. so why bother with a switch? The wiper goes to the HT line via R32 and the output contacts to IFT1 where it is routed via the primary coil to the anode of the mixer valve, V1.

The second wafer is where the TRF receiver is switched in (this, I imagine is an anode bend detector). 4 output contacts are wired together and route via IFT2 primary coil to R30 to HT and the TRF contact wired to R13 which is the anode load resistor for V4. Therefore, this switch removes HT from V6, the second IF amplifier and connects instead to V4, presumably the TRF receiver valve. Again the other 5-way switch is unused.

The third wafer has both 5-way switches in use. One switch has its first position unused then selects either R39, R40 or R41 which are wired together and connect to the fifth switch position. Here we find a modification. One of a pair of gold coloured wires connects to position 5 and the second to the output contacts (which are all wired together) of the other 5-way switch then are routed through a hole in the front of the chassis. The wiper connections of both switches are as yet untraced, however that combination of all 5 outputs connects to pin 3 of V9. This valve was broken, but I believe it to be a KTZ63, making pin 3 its anode. I've marked this as a "Loudness" valve which is a modern term recently given to enhancing speech.

The fourth wafer has both 5-way switches used.One handles the normal bandwidth settings plus an output for TRF and is associated with the Radio/Gram switch. The wiper of the other 5-way switch connects to a condenser. This is C27 wired to the anode of V9. Two other resistors are wired to V9 anode viz. R51 and R48 (the anode load resistor for V9). The normal bandwidth outputs for the 5-way switch connect sequentially from R68 (to ground) to R33, R34 & R35. The TRF position is not used.

 

 Above is a first pass at what I believe to have been the original receiver block diagram

I've assumed volume, bass, treble and loudness are carried out at V9, V11, V12 and V13, but I'm clueless about the existing circuitry.

AVC or Automatic Volume Control is the feature which uses the detected carrier level to feed back a negative bias voltage to earlier stages in the receiver to maintain the carrier at a preset level. The carrier of any broadcast station will produce an AC voltage at the last IF amplifier which will be rectified by a diode whose anode produces a negative voltage representing the signal strength of the station. A negative voltage is used because this can readily be used to reduce the gain of input amplifiers. In order for this to work certain of the amplifier valves are variable mu types whose bias determines their gain. Strictly speaking the feature should be termed AGC or Automatic Gain Control because, in a receiver aimed at high fidelity reception the audio output will not be fixed but should be linearly passed through the receiver to produce exactly the same audio output as exists in the broadcasted audio. I would expect this receiver to have fast AGC so that gain is increased instantaneously to combat any fading in the signal, hence I've postulated an AVC amplifier (its old term).

 

RF Front End

 Front end circuitry seems to be fairly standard except that the wavechange switch selects a smaller tuning condenser for the two higher frequency wavebands than that used on the other three wavebands.

The coils are standard with the oscillator having a tapped winding and the RF amplifier coils a tuned primary with an untuned coupling coil.

From inductance measurements the IF seems to be 465KHz.

Resistor R7 looks distressed and has lost its banding. It measures far higher in value than I'd expect, probably due to failure of condenser C49.

I suspect it should be 100 ohms?

 

TRF Receiver 

V4 started off as a real puzzle. Some connections were hidden by components but having sketched out the wiring I realised that Pin 6 was a tie-off point and not a weird valve electrode. The valve in this position was either a KTW63 or KTZ63 and is wired as a very interesting cathode-follower triode.

The anode current is set to a very low value giving the valve a large reverse grid bias. For example with an anode current of 1/4mA the grid bias would be minus 25 volts.

The RF input from the tuned mixer input is rectified by anode-bend characteristics and filtered by R12/C7/C10 and passed to the receiver audio amplifier via DC blocking condenser C8. The anode circuit is grounded to RF by C37 and C74.

This receive mode is selected on the front wafer of the bandwidth switch where IF stages 2 & 3 (V6 & V7) are deselected at the same time as HT is applied to V4.

 

Complete RF Front End (less switching)

Putting the two front-end circuits together, here's a schematic. There's a fair bit of switching involved in coil and bandwidth selection which I've omitted for simplicity.

At this point I hadn't spotted a provision for AFC, reported in a Wireless World article to have been used in the "Silver Dragon". If it's there it may be located within the coil switching circuitry?

Otherwise, logically if there is no AFC, my example must be a "Silver Knight" rather than a "Silver Dragon".

 

 Next I'll re-install the repaired variable bandwidth IFTs, rewire them, sort out the various broken solder joints and the bent trimmers mentioned earlier.

IFT1 and IFT3 had two and one 0.05uF wax condensers respectively; in my listing C59/C60 & C64. Testing these with a 200Kohm series resistor put about 30 volts across each condenser with an HT set at 300 volts. I noticed that this voltage, in all three cases, rose continuously at a very slow rate (about 0.1 volt per 5 seconds). This may mean that the inside of the condenser has some dampness which is slowly evaporating from heating due to the leakage current. Whatever is happening the basic leakage through the old condensers is way too high at around 1mA (making the the condenser the equivalent of a 22Kohm resistor).The final test was to measure the capacitance of the three condensers which was 0.15uF to 0.2uF instead of the marked 0.05uF. I fitted three new capacitors marked 0.047uF x 250VAC, rewired the IFTs where the old 18SWG wires connecting to external circuitry had perished insulation, then screwed them back on the chassis. I'll delay resetting the cams which currently are pushed out of the way until I'm ready to align the IF strip.

One problem I met in fitting IFT1 and IFT3 was probably also encountered when the receiver was in production. IFT1 was OK but IFT3 had a jammed plunger once it had been reassembled. The adjusting screws for the upper coil mounting plate are inaccesible so I'll need to adjust this with fine-nosed pliers to set the dust core perfectly parallel to the hole in the coil. Once the coils were mounted perfectly in-line the plunger was able to move without jamming.

Now to tackle the various broken solder joints and those bent trimmers... The damage has resulted from the chassis being rested on something other than a flat surface. Ideally the underside of the chassis should have been fitted with a metal plate. Fortunately nothing was actually broken, just bent and I was able to twist the trimmers back in place. A couple of coils have been detached from their mounting brackets but as they are held in place by wiring this is not important.

 
 

 QAVC Amplifier and Audio detector

I looked at the circuitry around the socket in the corner of the chassis which didn't have a valve in place when I got the receiver. I initially thought that the socket was wired for a double diode triode, so based on the other valves will be a DH63, but later I spotted that Pin 6 was not a tie-off point but a feed for g2 so revised my idea and nominated a 6B8 (a double-diode pentode) because this is the only valve whose connections match the wiring and components. The circuit is not especially recognisable and is probably some sort of AVC amplifier rather than a signal detector and LF amplifier. As voltage gain is not really needed in that application the valve is wired as a cathode follower which essentially provides a lot more current than the usual AVC diode. The cathode feed of the 6B8 includes a preset potentiometer (located in the rear corner of the chassis) which also sets the grid bias of the second IF amplifier valve. I guessed this control is used to preset the maximum overall gain to minimise distortion on very strong signals. In other words arrange for AVC action to be linear and not result in saturation from high level signals thus preventing insufficient negative feedback.

Later I suspected it has something to do with the QAVC feature which I describe later. In that respect the preset control may be for inter-station muting?

There may be a few errors in the circuit shown. but a revised version is to be found further down this page within the IF amplifier circuit diagram.

 

 

Audio Pre-Amplifier 

After another session of peering into the wiring I came up with this circuit. The switch marked QAVC off/Radio/Gram is a four pole 3-way affair with a long extension made of tufnol which is not entirely suitable (like the ones used for tone controls) because it flexes giving a wobbly feeling to switching. Also, because the switch is fairly stiff the knob has twisted over use and its securing screws have made a deep gouge in the tufnol (below).

Of interest is V9 which must operate with a very low anode current (circa 1 or 2mA). Oddly its heater connections (x, y) are not wired to the remainder of the valves, instead being brought out to one of the two 5-way chassis connectors. V9 appears to be an audio pre-amp (maybe fitted to work with a gramophone deck using a very low output) and it's possible its heater is provided from a 6 volt DC supply to minimise hum?

 

 Just a passing thought.. what exactly is "QAVC off" (engraved on the mode selector knob)?

Looking this up it stands for a rarely used term "Quiet Automatic Volume Control". Ordinary AVC must be noisy? Then again, delayed AVC is common so could the Q stand for "Quick"? AVC is used to maintain the same audio level from the speaker for different broadcasts which is what a cultured listener wants.

Fast AVC would indeed be noisy when tuning across the band, and when a strong station fades out and background noise pops up so I guess this receiver uses another form of AVC (ie. QAVC) which can be switched to normal AVC for night time listening or searching for weak stations. One common AVC feature is its amount of "hang". Tuning to a strong station puts a large negative bias on the RF and IF amplifier valves and adding capacitance to the line carrying the voltage will result in a time delay for this to discharge. Tuning away from the strong station will result, not in an instantaneous rise in audio from the signal skirts and background noise, but will make the transition more gentle which could be what is possibly meant by "quiet". Another form of AVC known as "delayed AVC" is when the AVC rectifier is reverse biased so that a feedback voltage is only developed once a pre-determined carrier level is tuned. The receiver will amplify signals at the maximum amount until the carrier gets to say S6, then the feedback comes into effect to reduce the overall gain.

Once I've understood the whole of the receiver circuit and the dry joints and squashed connections have been tidied up, plus maybe changing some of the wiring to plastic where the original insulation is in poor condition, I could fit a set of valves and apply 6.3 volts for the heaters then carefully apply an increasing HT voltage and monitor any leakage. It may even be possible to work out the function of the various controls before starting on component changes?

I replaced a few wire links that had cracked or missing insulation with plastic covered wire (I use the stuff from old computer power supplies as it has a decent voltage rating), then straightened the bent RF trimmers. Without any valves plugged in I applied 105 volts to the HT line. Initially the current was 30 to 40mA but this soon dropped to around 15mA after there was a faint pop from somewhere down in the audio section.. but no smoke. I measured the voltages at the valve anode pins after the current had settled down with the following results. I've noted the likely leaky component where I've already traced a circuit. V6 appears to have a s/c condenser or an open circuit IF coil (I found later that I had missed soldering new HT and AVC control wires when I'd refitted the repaired IFT). V4 has very high value anode resistor which will accentuate the effect of any leaky condenser. V5 has a new capacitor at C24 (hence the correct voltage). The voltage regulator, V3 feeds several screen grids plus the local oscillator and C15, C19, C21 and C75 will all contribute to HT leaks.

 VALVE

 V1a

V2

V3

V4

V5

V6

V7

V8

V9

V10

V11

V12

V13

 Anode volts

 42

 100

 97

 1.5

 105

 0.3

 100

 46

 87

82

68

98

98

Expected

 105

105

105

105

105

105

105

105

105

105

105

105

105

 Leaky

 C21

 C49

various

 C37/C74

 C24

C59

C72

 C14

C27

C79

C42

 C29

C29
 

 
 

 

Does this receiver have AFC (Automatic Frequency Control)? At this point I decided to look for a treatise on AFC and found exactly what I was looking for. In fact I wouldn't be at all surprised if the Moreton Chayney designers hadn't themselves used this very book in their design of the Silver Dragon. It was written in 1937 and explores circuits used in contemporary receivers. Click the dial above to read this very informative book.

Once I'd looked at established AFC circuits a couple of puzzling facts may have become resolved (but maybe a "red herring"). The previous day I was tracing the circuitry of the IF strip and thinking it was straightforward sketched a general circuit from which to identify the components. When I looked at IFT4 I found something odd and also in IFT2 I found something else that was puzzling. IFT2 has grid connections to not one, but two valves, V6 and V7. One connection looks normal, feeding V6 from the IF transformer secondary coil, but the other to V7 has a small condenser feeding from the anode of the IF amplifier V5.

IFT4 includes the two miniature Westectors (X1 and X2) which I'd initially assumed were for AVC and AM detection but I now realise that these might form part of a discriminator. V5 and V6 are standard IF amplifiers but V7 could be a discriminator used for AFC. Time to re-check the circuit of the mixer valve and look for a connection to the oscillator grid. Another puzzle is also resolved. I found at V7, a KTZ63 which I thought strange as it is not a variable mu pentode and would be unsuited in a standard IF strip because it would not be controlled by the AVC line. I've now amended the block diagram but I'm still unsure of its accuracy. Back to circuit tracing... In the corner of the chassis is a valve socket whose valve was missing no doubt because it's in a vulnerable position and had been smashed. I initially thought it might have been a DH63, double diode triode but the socket has pin 6 used. Clearly not an anchor point as is customary with the likes of the 6K7 etc because only a 1K resistor plus a decoupling condenser are present. The only valve type that fits is the 6B8 which has identical connections to the DH63 plus g2. Pins 4 and 5 are wired together and the anode at pin 3 and g2 at pin 6 are fed by 1K resistors. The anode is decoupled to ground by a 4uF condenser meaning that the valve function is that of a cathode follower. This, in practical terms, means that the normal AVC voltage is produced across a low impedance and therefore able to supply much more current than a standard AVC diode rectifier.

Given a combined anode and screen current of say 13mA the cathode voltage will be 4.68 volts. As the diode anodes rectify an AM carrier the current will increase say by 5mA resulting in a new cathode voltage of 6.48 volts.

After much puzzling and a review of traced circuitry, I decided that AFC is not a feature in this example, but instead I'm looking at inter-station muting.. called QAVC. Below is what I've discovered to-date, suitably marked up to indicate what's going on.

 

 Above is the latest revised circuit diagram for the IF amplifiers. The area around IFT4 is tricky to trace because the underside of the transformer is masked by a resistor tagboard. Westectors X1 and X2 are the diodes connected to V7 anode and the transformer secondary. The earthy side of IFT3 secondary carries demodulated audio (rectified by V8 diodes), and filtered to remove the RF (believed to be 465KHz).

If this is the version of the receiver that has AFC then this may be associated with V7 and its two Westectors. This area is still being checked but the tangle of components and partly concealed wiring is making things difficult. There are six connections plus ground originating from within the IFT4 can and I had to buzz them out to determine where they went. I now realise that the designer has been quite (slightly) kind... white sleeving is AVC and the like, red is HT+, dark maroon is 150v, green is usually audio, blue is local oscillator and oddments such as audio valve anode wiring, and black is ground. Yellow screened wire is used for critical anode connections and valve heaters (slightly confusing!).

Whilst I was tracing the AVC connections I spotted a concealed wire carrying the 150 volt stabilised supply, so stabilised HT isn't confined to circuits at the rear of the chassis, but also to the mixer and possibly the screen of the RF amplifier.

Now that the circuit is becoming more complete, you can see that the two Westectors are for AVC plus QAVC and the 6B8 diodes for audio. No AFC seems to be provided, unless I've missed the method by which the local oscillator is adjusted. Looking at the key passive components: R49 and R50 set the amount of AVC delay and VR5 is used to preset the action of QAVC. Audio is extracted from the AM carrier by V8's diodes and filtered via a low pass filter C68-R83-C69 . The job of V7 is to amplify the IF signal purely for providing AVC and QAVC and you'll note that the gain of this valve is not governed by feedback; being a KTZ63 which does not have variable mu characteristics. So how does V8 work? You'll notice that there's a link between V8 and V6 via their shared cathode resistor R23, so V8 bias will be partly governed by V6 and vice versa. V8 pentode is a QAVC amplifier designed for interstation muting and it's linked to volume control because its two integral diodes will be controlled by its cathode voltage. V8 grid is driven by the RF voltage produced by the AVC-controlled RF amplifier and mixer plus the first two IF stages V5 and V6. Setting aside QAVC (or switching it off) will result in Westector X2 controlling the gain of the receiver. X2 is reverse biased by R49/R50 which places about 7.5 volts (assuming an HT of 250 volts and zero diode leakage) on the diode cathode and will only conduct if its anode voltage is greater then about 8 volts so any AVC action will be delayed until a strong signal is tuned in. This means that the receiver gain is pretty high when no signals are tuned so inter-station noise level will be pretty loud so the designers introduced QAVC. Once QAVC is turned on the audio level will now be dependent on the biasing of the V8 diodes. "QAVC off" places a ground at V8 cathode but when that ground is removed, when "QAVC off" is deselected, V8 cathode rises to a voltage governed by its anode and screen currents and somewhat modified by the cathode current of V6 (controlled by normal AVC). With QAVC in operation V8 cathode voltage is always positive with respect to ground and Westector X1 will be turned off unless the RF voltage across IFT4 secondary exceeds a certain level. V8's diodes will also be turned off until the voltage across IFT3 secondary reaches a certain level. With V8's diodes turned off the receiver will be muted and only unmuted for strong signals which turn back on V8's diodes. Because of component variations and variations in HT voltage, the designers fitted VR5 which sets the quiescent current for V8 and hence the quieting level. That level dictates the strength of the weakest station for which the receiver will produce audio output with QAVC active. Note the common earth return resistor mentioned above which couples together V6 and V8. Because we are looking at decoupled voltages the AVC and QAVC lines are interdependent because the currents drawn by V6 and V8 will add together arithmetically if QAVC is active. If QAVC is turned off you'll note that the ground return for V6 is provided by R23 and R24 but V8 plays no part in V6 biasing because its cathode is grounded. In this state V8 diodes are no longer reverse biased and provide audio through R72 controlled only by normal AVC.

 

 Next I'll attempt to trace the remaining circuitry around V9-V11. Easier said than done however because of the way the set had been built by adding parts on top of wiring. For example the ganged volume controls VR3/VR4 (if that's what they are?) have been fitted over the top of several resistors and part of V10 valveholder. Wiring also passes into and under the rear component tag board masking connections and a couple of resistors. With some difficulty I managed to see the connections to V10 which had a KTZ63 fitted when I received the set. I noticed first that although Pin 3 looked to have typical anode components, Pin 6 had a resistor R63 and 4uF decoupling (block) condenser C75. Is this a tie-off point? It looks not because the resistor goes to HT+ and the condenser to ground with nothing else wired to the pin so it appears to be a g2 connection. What about Pin 4? That's wired to Pin 5 and then goes off somewhere. This is much like I'd found at V8 which cannot be anything other than a 6B8. V10 then must also be a 6B8 as this is the only valve whose connections match the wiring and that KTZ63 would not have worked very well plugged into V10's holder. Hidden under C42, that large yellow condenser I discovered an 0.05uF wax condenser (now C79) wired from V10 anode to V11 grid. At least V11 does seem to be an L63 (the valve that was plugged into V11's socket... and substantially similar to the 6J5) because of its connections.. although it's a very strange L63 because its anode is substantially decoupled to ground via the 4uF yellow condenser C42 and the 620 ohm cathode resistor R61 plus the 1Mohm grid leak R80 are wired, not to directly to ground, but to ground via a 47Kohm 2 watt resistor R69. This means that V11 is a cathode follower and must be running at a very low anode current to avoid non-linearity.

I'm assuming V10 and V11 are audio amplifiers and therefore their output is through the brown wire to C45, that chunky 0.5uF condenser, in series with VR3 via that green-sleeved wire. That then dives off towards what I believe must be the treble and bass tone controls. The green sleeving is consistent.. green=audio.

Bringing down a copy of V9 from above and the latest on the right....as I discover more I'll update the circuits below...

Can you spot any mistakes here?

 

 

 

VR3 and VR4 are ganged together.

 Below is the second attempt at the full audio circuitry. There are several puzzling features, some of which may be because the last owner carried out modifications. Of course there's a possibility the receiver never did leave the factory fully inspected and tested, leaving the last owner to try and fix it.

Because some of the connections are hidden behind components I made a few mistakes. The connection at the chassis end of C42 for example actually went to the junction of a pair of 120Kohm resistors, not to ground. Plus... somewhere in the wiring there must be a ground return for V10 grid leak?

The circuit below, around the components R77, R66, R65, R76, R67 etc looks very odd, perhaps because a chassis connection is missing? Once this is added the components will form two separate circuits viz. the input to V10 grid from V9 and self bias for V10. But what action do the two diodes within V10 perform?

 

Below... here's a puzzle. Through a hole in the front of the chassis is a twisted flex with gold coloured insulation.. clearly a modification. I guess the guy that did this is no longer with us, but he seems to have made a mistake. The left switch is now disconnected but the right hand switch has been messed up.Working out what was happening then figuring out what was intended is necessary in order to re-establish the original MC circuit.

The switch wiper connects via a blue wire to V10 anode and the 18Kc + TRF contacts go to V9 anode. Was the intention to carry V9 and V10 anodes out to the front for headphones? That way you could listen to either audio output? But... only if one didn't switch to the 18Kc and TRF settings. In those settings V9 anode would be connected to V10 anode. "NC" means there's no connection but two tags (left and lower) carry solder so were used in the past. Wafer 4/2 carries an arrangement of resistors is possibly designed for various settings of top cut, but if so, are they the wrong way round (more top cut for 5Kc would need the resistors 1M and 120K to be reversed)? These are R33/34/35/68 with C27. Two more puzzles are evident. First, V10 anode current passes through V10 anode resistor and this connection is decoupled to ground.

 
   

 The bandwidth switch has 4 wafers each carrying two switches.Wafers 1 and 2 each have only a single switch used, but wafers 3 and 4 use both switches. Their tags are not positioned entirely logically and are easy to confuse.

I think the last owner carried out a modification and misread the tags.

Maybe the left switch was originally connected to V9 anode and perhaps connected to the grid of V10 via a condenser and a grid leak with the selected resistor arranged as a potentiometer for reducing signal level. The right switch may have had a top cut circuit for V10, perhaps carrying two condensers to ground, one for 8/11/15Kc and the other for 18Kc/TRF?

Looking at the fairly complete audio circuit you'll notice that there is no output from V9 to V10 so the above suggestion does make sense, almost as if the two blue wires from V9 and V10 anodes were confused?

My aim is to put the Moreton Cheyney circuit back to its original state and see how it performs. I'll be fitting modern parts as necessary because these will have much the same characteristics as the original parts when they were brand new, and being generally physically smaller in size will let me see more of the hidden areas. I'm fairly sure some mid 1950s parts already replace those from the 1940s. From the disposition of parts I think the various sub-assemblies were made perhaps by local workers who were supplied with drawings and sets of parts. This would explain why for example, Sprague 0.1uF and 0.05uF condensers are used in some areas but wax covered types and the odd Hunts types are used elsewhere. Dividing testing to RF/IF then just audio may be the best approach because not many changes seem to have been carried out in the former but there are lots of changes in the latter.

Here are some puzzling features that need resolution.

The circuit for the output valves does not make sense. Maybe the two 4700pF condensers should be connected together or, depending on the following amplifier, brought out to separate pins on the 5-way chassis connector?

Bandwidth switch wafer set 3 circuitry is wrong.

V10 circuitry looks very odd & What is the function of V10s diodes and was it a 6B8 as I guessed or something different?

Should V11 be a phase splitter? My guess is no.

And why are some audio valve heaters brought out to a different supply source.. was this to reduce hum by feeding these with DC rather than AC? No, after examining the matching amplifier and power supply it seems a few valve heaters are fed from a second AC heater supply.

 

 Click the picture to see the latest full circuit diagram. A few bits to be added (magic eye and 0D3 circuit) and some checking around the switches and AVC lines.
 

 

 I'm close to fitting the RF and IF valves to see if this section of the receiver is serviceable. Initially I changed the two large condensers inside IFT4. These are marked 0.1uF x 600V. One drew 4mA at 200V and measured 193nF.. the other drew 0.4mA at 200V and measured 160nF. I fitted two new 0.22uF x 275VAC capacitors because these fitted better mechanically than smaller types.

What exactly do these leakage values mean in practice. As the test voltage was changed it was apparent that the leakages were basically resistive. One condenser had a resistance of 50Kohm and the other 500Kohm and its job in the receiver circuit will detemine its effect. The worst one was decoupling the anode of V7 and the other smooths the voltage providing delayed AVC at the cathode of X2. Taking the latter.. this condenser is in parallel with a 3.3Kohm resistor and is fed by 250 volts of HT via 100Kohm. Its effect is to reduce the value of the 3.3Kohm resistor by less than 4%. Bearing in mind the 3.3Kohm resistor will read high because of its age, the leaky condenser will actually counteract the higher than optimum AVC delay voltage.. or you might say it has negligible effect. The anode voltage decoupler will draw HT current in parallel with that drawn by V7 anode and will reduce the anode voltage of V7 from 220 to 195 volts. Again, the 6.2Kohm anode resistor will be higher than the marked value from aging, so in this case the leaky condenser will make matters worse but the difference in the voltage will be insignificant; however in both cases the old condensers would not be too reliable and likely to get worse rather than better, although they are much better than the typical wax covered variety.

  Connecting 300V across the HT line and chassis resulted in a current of about 33mA so the majority of the old condensers are not too bad. Previously I'd applied 105 volts and seen 15mA HT current. I'll repeat the table shown earlier, but before I did this I decided to look at the matching amplifier so I could figure out the audio section of the receiver. I reduced the HT to about 250 volts and monitored the current. This slowly deceased until it read 20mA, and I checked the valve anodes and screen voltages.. below.

Much later after lots of component changes I measured the voltages again and some of the results are definitely iffy. I've used generic valve types but some are KTW/Z series. It looks like there are a few anomalies. Some voltages i didn't check and the HT used was about 280 volts. The HT current during the later test at 280 volts was something like 60mA. The valves account for about 45mA with some guesses marked *. Condenser leakages probably account for 10 to 20mA.
 

 VALVE

 V1a

V2

V3

V4

V5

V6

V7

V8

V9

V10

V11

V12

V13

 Anode volts

 250

 250

 250

 92

 250

 250

 250

 236

 115

217

217

-

-

Expected

 250

250

250

250

250

250

250

250

250

250

250

-

-

 Leaky

 C21

 C49

various

 C37/C74

 C24

C59

C72

 C14

C27

C79

C42

 C29

C29

 LATER

 6K8

 6K7

 0D3

 6J7

 6K7

 6K7

 6K7

 6B8

 6K7

 6B8

 6J5

 6J5

 6J5

 Anode volts

 258

 257

 127

 156

 260

 273

 253

 190

 30

 30

 243

 129

 134

 screen volts

 93

 123

 na

 156

 29

 24

 163

 125

 64

 -

 na

 na

 na

cathode volts

 2

 1.0

 na

 2

 0.235

 2.1

 5.9

 1.4

 3.7

 0

 63.9

 5.2

 5.4

 current mA

 7

 3

 15

5*

 0.5

 2

 5

 1*

 1

1*

  1

 1.5

 1.5

The amplifier is driven from two audio signals suitable for driving the output valves in push-pull rather than a single feed to be later phase split. These are fed from the rear of the receiver at Socket P2, pins 3 and 4 with pin 5 grounded. Socket P1 carries HT (pin 3) and heater voltages (Pins 2 and 4 plus 1 and 5) with ground at pin 1. 

 

 I decided to test some aspects of the receiver before going further. Since the HT leakage wasn't too bad I plugged in a set of RF valves and the regulator. These were V1, V2, V3, V5, V6, and V7. The HT current read around 50mA but dropped to 40mA as tests proceeded (note that some of this current will be due to V3 the voltage regulator. I immediately found that the heater supply of V5 was missing which limited testing somewhat. Injecting 465KHz at the anode pin of V5 through a capacitor, and grounding the input via 100 ohms to protect the signal generator, proved I could tune the IF transformer trimmers nearest the chassis edge to peak the signal. I discovered the inboard trimmer at IFT4 had a broken slot so I had to re-cut this with a small hacksaw. Turning to the front-end, I found that the local oscillator would only run with the tuning condensers at minimum capacity, and on only two wavebands. Typically I measured 2.87MHz at 510mV on one and 30MHz at 94mV on the other. On these bands the signal rapidly dropped in ampitude as the tuning condenser capacity was increased. Applying 465KHz to the top cap of V1 but not at the top cap of V2 brought up a signal at the top cap of V5.

Next I'll trace the reason for V5 not heating up and check generally for bad soldering in the IF strip. As it was awkward monitoring a signal at IFT4 I'll add V8 which carries the audio detector diodes and monitor B/W switch wafer 4/1 for audio. I'll also add V4 which should give me a shortcut to the state of the RF stage, V2, as this is the TRF anode bend detector. V5 wasn't heating up because the valve holder sockets were not gripping the valve pins. I bent the heater sockets inwards but it worked for only a short time so I removed the old socket and fitted a better quality octal holder. After a lot of testing I noticed the HT current had drifted up to around 75mA from the original 40mA. I traced this (helped by a burning smell from R7) to the anode circuit of the RF amplifier. I snipped off the decoupling condenser but found it was OK and discovered it was one of the anode coils (either medum or long wave coil) which intermittently measured around 60 ohms to ground. Snipping the link between medium wave coil and the short-wave coils removed the intermittent short.

Checking the local oscillator, it seems only to be working on the 2nd short-wave band at around 2.7MHz so all is not well with the 6K8.

 

 Checking for an intermittent short in the RF coil area I found an open circuit coil in the set connecting to the anode of the RF amplifier. I managed to remove this although, because its fixing screw was directly underneath one of the tuning condensers, it wasn't easy. The coil turned out to be a standard Wearite part marked PHF2. I then noticed that all the coils carried Wearite labels.

The coupling winding was open circuit and had 52 turns of enamelled copper wire measuring 0.13mm in diameter. This will be either 39 or 40 gauge. It has a max current capacity of about 30mA. The limiting resistor is R7 which is 10Kohm and fed from 250 volts will result in a current flow of 25mA to ground. The point where the wire is open circuit has a green patch of verdigris at the hole where the wire passed through the former and being slightly corroded reduced its current handling.

 

 I had trouble finding a suitable chart of enamelled wire information so I've copied details below for reference. Note that the resistance reading is per metre of wire at a temperature of 20 degrees Centigrade and, as there are several types of enamelled coating, you may find actual measurements for diameter are greater or less than those shown by around 10%. For example the coupling winding for the Wearite coil above had a measured diameter of 0.135mm but was still 40SWG.

 

 SWG

 Cross Sectional Area

 Resistance

 Diameter

 14

 3.49mm²

 0.00532

 2.108mm

 16

 2.086mm²

 0.00831

 1.63mm

 18

 1.169mm²

 0.0148

 1.22mm

 20

 0.6567mm²

 0.0263

 0.914mm

 22

 0.3973mm²

 0.0434

 0.711mm

 24

 0.2453mm²

 0.0703

 0.558mm

 26

 0.1641mm²

 0.105

 0.457mm

 28

 0.1109mm²

 0.155

 0.376mm

 30

 0.0779mm²

 0.221

 0.315mm

 32

 0.0591mm²

 0.292

 0.274mm

 34

 0.0428mm²

 0.402

 0.234mm

 36

 0.0292mm²

 0.589

 0.193mm

 38

 0.0182mm²

 0.945

 0.152mm

 40

 0.0117mm²

 1.48

 0.122mm

I wound 52 turns of 40SWG enamelled wire onto the coil former. I had to do this by "scramble-winding" rather than the original method which is wound at a slight angle, first a group of turns one way then the other. Mine will probably have more self-capacity but being a coupling winding shouldn't be a problem.

 After fitting the repaired coil I noticed a bare wire close by almost touching the metal shaft of the wavechange switch, almost certainly the cause of the intermittent short circuit that most likely damaged the medium wave coil. Before carrying out testing I replaced three wax 0.1uF condensers, C48 and C52 (AVC decoupling) and C49 (coupling coil decoupler).

Turning on the power I experimented with applying signals to the RF front end. I found the local oscillator would only run when the tuning condenser was set within the top third of full mesh. There seemed to be no connection between the aerial input and the RF stage, but by connecting a signal generator to the top cap of the RF stage with the grid lead not fixed in place, I managed to see RF at the first IF stage. By experimenting I tuned the local oscillator to about 1.25MHz and found a signal of 785KHz produced an IF signal of 465KHz. This was present at the anode of the 2nd IF amplifier so the receiver mixer and IF stages are working to some extent. Tuning the dial I found I could peak the IF signal, but transferring the input from the top cap to the aerial input and replacing the top cap clip failed to produce anything at all. Something is clearly wrong in the grid circuit of the RF amplifier or perhaps there's a missing connection to an electrode of the 6K7.

The next step is to find out why there is no connection between the RF input circuits and the RF amplifier, or alternatively what's wrong with the RF amplifier. Once this has been sorted out I'll investigate the lack of oscillation in the mixer. I'm using a triode hexode 6K8 (similar to an ECH35) but the original valve may have been something like a heptode such as 6A8 (similar to an X63).

 I resolved the lack of amplification through the RF stage. I had the top caps for the RF amplifier V2 and the mixer V1 reversed. The various grid leads are quite long and easily confused. Having corrected this the RF amplifier worked after a fashion. Only one waveband is working, namely medium waves and that only over two thirds of the tuning range from the LF end. The lowest frequency tunes about 800KHz resulting in a decent IF response set at 465KHz which is odd because it should be 545KHz. To respond to 800KHz the oscillator needs to be either 800KHz + 465KHz or 800KHz-465KHz equating to either 1265KHz or 335KHz; clearly the former. The local oscillator fails to run on any other waveband and looking at the padder for the MW oscillator it looks very odd because it's made up from a small (unmarked but low value) condenser in series with three others in parallel. Either someone has attempted to alter the coverage or attempted to replace the original. One option is to remove these and measure the value, but I remembered using a GDO for testing tuned circuits, so rather than swap this odd padder, I'll see if it produces the correct frequency range. Wearite oscillator coil data is listed below assuming an IF of 465KHz. Something is clearly wrong, but then again.. what value tuning condenser and what value padders were assumed when Wearite made the coils?

Later I found a datasheet and copied details from this into the tables below, adding extra info. * For completeness I've added PO6/PO7/PA6/PA7 which are not used in this receiver. As the original document wasn't very clear some numbers may be wrong. I understand the tuning condensers for both RF and oscillator are 500pF. Obviously, in the Moreton Cheyney there are two tuning condensers, one of which is used for the three lowest frequency bands and the smaller for the two higher bands. This means that the coils for the two upper bands will have trouble tracking and the oscillator padders will most likely have been changed to match the dial markings. PA and PH coils can be assumed roughly equal.

 

 OSC COIL

 INDUCTANCE

 COVERAGE

METERS

 COVERAGE

KHz/MHz

 OSC FREQ

HIGH

 OSC FREQ

LOW
 TRIMMER  OSC PADDER

 PO1

 390uH

 700-2000m

 429-150KHz

 894KHz-615KHz

 not feasible

 75pF

 150pF

 PO2

 85uH

 200-557m

 1500-538KHz

 1.97MHz-1.00MHz

 not feasible

 76pF

 450pF

 PO5

 4uH

 34-100m

 8.8MHz-3MHz

 9.26MHz-3MHz

 8.33MHz-2.53MHz

 60pF

 2400pF

 PO3

 1.15uH

 16-47m

 18.75MHz-6.4MHz

 19.21MHz-6.86MHz

 18.28-5.93MHz

 50pF

 5000pF

 PO4

 0.5uH

 12-35m

 25MHz-8.57MHz

 25.465MHz-9.03MHz

 24.53-8.10MHz

 60pF

 5000pF

PO6 *

 27.45uH

 91-261m

3.3MHz- 1.15MHz

3.765MHz- 1.515MHz

2.835MHz- 1.05MHz

65pF 

 900pF

 PO7 *

 44.2uH

 250-750m

1200KHz- 400KHz

1665KHz-865KHz 

not feasible

73pF 

 350pF

RECEIVER

WAVEBAND

 DIAL READINGS

  RF COIL

 INDUCTANCE

 COVERAGE

METERS

 COVERAGE

KHz/MHz

 TRIMMER

RANGE 5 

 800m-1900m

 PA1

 2200uH

 700-2000m

 429-150KHz

 72pF

RANGE 4 

 200m-550m

 PA2

 170uH

 200-557m

 1500-538KHz

 65pF

RANGE 3 

 40m-100m

PA5 

 5.5uH

 34-100m

 8.8MHz-3MHz

 60pF

RANGE 2 

 20m-34m

 PA3

 1.2uH

 16-47m

 18.75MHz-6.4MHz

 55pF

RANGE 1 

 10m-17m

 PA4

 0.5uH

 12-35m

 25MHz-8.57MHz

 60pF

NA
 

 PA6 *

 27.45uH

 91-261m

 3.3MHz- 1.15MHz

 60pF

NA 
 

 PA7 *

 44.2uH

 250-750m

 1200KHz- 400KHz

 60pF

 From the dial reading column you can see that some wavebands do not correlate with the coil specifications. The dial is calibrated fairly well on LW, MW and SW3 (Ranges 5, 4 and 3) but on the top ranges it's not very accurately calibrated, showing mainly the SW broadcast bands and major broadcast stations. As far as precise alignment is concerned, not all coils are fitted with dust cores and rely solely on trimmers for setting up so there may be some vagueness in the finished dial accuracy.

See the results of the repair to PA2

During the initial tests I connected headphones (via a capacitor) to various points in the circuit that were easy to access and found that the cathode of V8 provided a good audio signal (the tone from my signal generator). I initially thought this was because the AM detector is the pair of diodes at V8, but this was incorrect. There should not have been any audio at V8 cathode, but there was, because condenser C13 which is supposed to filter out any audio was open circuit. I removed C13, C14, and snipped C15. I fitted new condensers at these positions but of course I'll need to find a new convenient point at which to temporarily monitor audio so, time to fit V9, the audio preamp and listen to the test signal a little better than before. In fact the signal to which I was listening was being provided not by the AM demodulator diodes at V8, but by the action of X1 (the QAVC detector).

Note that the purpose of V8 is to amplify the AVC in order to set the QAVC level. I did notice that IFT4 which provides the RF voltage for AVC tuned nicely to 465KHz so at least that part of the circuit seems to be working, and the lack of C13 action enabled me to conveniently check IFT4 tuning.

In progress... things to sort out (1) V1 local oscillator and (2) some IFT trimmers are not peaking the 465KHz signal. The bandwidth switch appears to work and the TRF receiver setting does cut off IF amplification so I could also plug in V4 and see how well this functions. I tried this and it's not working, or at least the audio level is too weak to hear in my test headphones connected to V9.

I was disappointed in the audio level from V9. Shorting its cathode resistor improved the level, but it was still really too weak to comfortably hear a test signal. Connecting a long wire to the aerial socket brought in a few medium wave broadcasts but again the audio level was much like that from a crystal set so there's more work to be done. I have a suspicion that the volume control (one of the pair coupled by gearwheels) is faulty, but even so, shorting the input to output didn't change the audio level. During tests I found the AVC voltage responded to changes in test signals, shifting by several volts as the input level was raised.

I'd noticed one or two valves weren't seating properly. This includes the mixer so I decided to change the holder and even when this had been removed it was impossible to fully plug in the 6K8 into the holder. I fitted a new standard black composite IO base, but before refitting the wires and components I tested the Sprague condenser C4 which decouples the screen of V1. This is marked 0.1uF x 500 volts and when fed with 350 volts via a limiting resistor of 2.2Kohm I measured only 0.03 volts across the resistor. This equates to a leakage of only 14uA or a condenser resistance (=leak) of 25Mohms, infinitely better than the wax condensers.

Looking at the circuit diagram, in particular the volume control. This is rather unusual because of course it comprises two ganged controls, but also VR4 does not adjust the audio level from the earlier stages, but instead reduces the value of the grid leak from about 500Kohm to zero and at the same time allows more feedback at higher audio frequencies (because of C35 being only 0.001uF) to influence the output from V9. As VR4 is reduced in value so the value of VR3 is increased. I suspect either VR3 or VR4 (or both) may be open circuit, and that being so, any small leak in C52 might drive V9 grid positive to the extent the valve cannot properly amplify a weak signal. An easy enough point to check by measuring the voltage at the top cap of V9. The list of jobs is steadily growing, as is the list of lift repairs in the build up to Christmas!

 I tackled the pretty awkward job of removing the standoffs which brought the mixer below chassis level. This was probably a retro-fit in manufacturing because the magic eye was in the way of the mixer valve. Dropping the 6K8 by a couple of inches sorted the problem, but another then showed up ie. it was very difficult to insert the valve because there was only a tiny clearance between the glass and the original mounting hole for the valveholder. I didn't want to risk breaking a valve and cutting myself so I bought a metal 6K8 which is shorter then refitted the valveholder direct to the chassis. Fortunately the critical RF connections didn't need extending but I had to use longer wires for cathode, anode and screen connections because these tie the valvebase to the adjacent tagboard on which are mounted the various resistors. The job was successful and the new 6K8 managed to oscillate right across the medium waveband (or at least from end to end of the dial) plus the adjacent shortwave band and 50% of the next. Still no oscillation on long waves or the highest frequency band.

There might be a problem or two with the wavechange switch and the weird combination of condensers forming the medium wave padder are definitely providing too low a value. I checked the tuning range and it measured 1.676MHz to 2.01MHz instead of the correct range of circa 1.00 MHz to 1.97MHz. A rough calculation gives the padder to be around 160pF instead of the Wearite value of 450pF. The long wave padder should be about 150pF. I then found the combination of capacitors were missing a connection to the medium wave coil (this had broken off when the top of the coil had been bashed). Once I'd put the padder wire back in place I was able to tune down to around 550KHz but found the oscillator packed up before getting past 1.5MHz (it needs get up to about 2MHz). I tried everything.. changing resistors, capacitors and tweaking trimmers but with no success. Thinking that damp may be affecting the Q of the coil, I heated up the area with a hair drier. The amplitude of the oscillator at 1.3MHz rose from 8 volts to 12 volts pretty quickly then started to go down again. Removing the heat had improved the amplitude of the oscillator but it still refused to get higher than 1.5MHz. At this point long waves had stubbornly remained oscillation free, SW1 had worked OK and SW2 and SW3 refused to oscillate. Raising the HT voltage and shorting the 6K8 cathode resistor and even disconnection the oscillator grid leak all improved the oscillator amplitude but failed to provide oscillation over the whole tuning range.

To give me a rest from struggling with the front end I again looked at the IF strip. Previously I'd found some trimmers hadn't peaked. Injecting 1000mV of RF at 465KHz into the aerial socket provided enough leakage to drive the IF amplifiers and the first two IFTs tuned nicely. Adjusting the third proved there was a problem. By now I could reduce the input to only 10mV the 2nd IF amplifier was now dealing with several volts of RF and as the screening cans were not fitted and I was using unmetallised valves of course led to instability. The IF signal rose slightly in amplitude and then proved to be untunable. The MC valve cans are rather odd, being aluminium cylinders having a flange allowing them to be screwed to the chassis. All well and good except the valveholders are extremely tight which makes valve insertion difficult and valve extraction leading to loose glass envelopes. The simple solution was to wrap baco-foil around V5, V7 and V7, connecting pin1 on each valve to a wire and wrapping this around the base/glass junction. This solved the problem of instability. Most of the trimmers peaked their coils, but another problem surfaced. There are two types of AVC. The QAVC system can be switched off but standard AVC can not and, because its amplified AVC it interferes with alignment. There are two options which I'll consider when I complete IF alignment... first I can ground the AVC line and secondly, reduce the level of the input signal. Before leaving IF setup I'll cover an annoying fault which is some sort of instability in the AVC circuitry causing the IF signal to vary intermittently by a factor of maybe five. I should perhaps investigate the two Westectors and if these are acting up swap each for a germanium or silicon diode.

Now back to the local oscillator problem....

 
 

 Lack of oscillation on the long waveband turned out to be a damaged coil. It's a Wearite PO1.

I found the problem by checking coil continuity.

At some time the receiver had been placed on something that had bent under chassis parts. The Wearite coils cannot be mistreated and in this example tab A had been bent downwards with sufficient force to pull clip B from its securing slot and the thin 40SWG wire C soldered to the opposite end of the tab had broken. Thankfully I was able to solder it back in place.

I then measured the inductances. The primary or tuning coil was 392.8uH and the feedback coil 231.5uH. From the chart above the primary is reckoned to be 390uH so the coil is only 0.7% high in value.

 Before I refitted the long wave coil I checked the medium wave coil. Now this of course works over part of the band so must be basically OK. Wrong! the medium wave coil also had an open circuit feedback coil. This one however is rather odd. Instead of measuring its inductance once I'd resoldered the broken wire (again close to the solder tag so was easy to fix) I found the tuning coil measured 88uH and 4.2 ohms, but the repaired feedback coil measured 66 ohms which was too high for my LCR meter to check its inductance. I have heard of adding resistance to tame the oscillator, but this is the first time I've met it in practice. Did I mention removing the coils? Each of these was secured by a 6BA screw whose head was hidden under the tuning condenser for the LW coil and under the rear panel of the slow motion drive, but luck was on my side as the two mounting plates in the coils were free enough for me to unscrew them. Before refitting I added a star washer and 6BA nut to each screw enabling the coils to be screwed back in place (adding a spacing washer to bring the top contacts into the correct orientation).

After replacing the wiring and the padders (I temporarily used a 470pF padder for medium waves) but before fitting the trimmers I turned on the power. To my surprise the 6K8 started oscillating on long waves as the HT passed about 30 volts and continued to rise in amplitude as the HT settled on 250 volts. I tried medium wave and SW1. Both worked perfectly. I've supplied a table below to show the oscillator output. I used a x10 probe at the grid condenser and the trimmers are 3-65pF. I'm using a modern oscilloscope which displays, not only traces, but also their frequency and amplitude.

 Initial tuning test results

 No trimmers fitted

 Max Frequency

 Tunes

 Wavelength

 Volts RMS

 Min Frequency

 Tunes

 Wavelength

 Volts RMS

 Long Wave

 952KHz

 487KHz

 616m

 15.7

 605KHz

 140KHz

 2142m

 13.3

 Medium Wave

 2332KHz

 1867KHz

 160m

 11.3

 1040KHz

 575KHz

 521m

 16.6

 Trimmers fitted max C

 Max Frequency

 Tunes

 Wavelength

 Volts RMS

 Min Frequency

 Tunes

 Wavelength

 Volts RMS

 Long Wave

 700KHz

 235KHz

 1276m

 13

 506KHz

 41KHz

 7317m

 15.1

 Medium Wave

 1700KHz

 1235KHz

 243m

 11.5

 940KHz

 475KHz

 631m

 15.6

 Short Wave 1

 9.7MHz

 9.23MHz

 32m

 4.9

 3.1MHz

 2.63MHz

 114m

 4.3

Look back at the coil information

I tuned the dial so that the display on my oscilloscope read 198KHz + 465KHz = 663KHz and connected an aerial and there was Radio 4 on 198KHz. Certainly not loud and clear, because the audio stages are not yet fixed, but as clear as a bell.. tuning higher I heard the usual broadcasts and then BIA, or local airport beacon. Medium waves were full of stations all too faint to hear properly so now the next stage... sort out those weird audio stages. I'll postpone alignment until I can hear what I'm doing and drive my audio wattmeter from demodulated AM. Later I'll use my spectrum analyser to help align all the various bandwidth settings.

Before closing and proceeding to check the audio stages I decided to solve the problem of a jittery display when looking at the IF signal waveforms. It's quite possible, because the receiver uses amplified AVC as well as their novel QAVC that the jitteryness is caused by a component within those circuits. The drivers for them are a pair of Westectors X1 and X2 located within IFT4. These are marked with a red end and look original so there's every chance they are at least the correct way round.

Turning to QAVC (with no signal tuned) and looking at X1 we see that its cathode voltage is positive and defined by V8 cathode current (if QAVC is operative at S1/1). Let's say V8 anode current is 10mA and screen current 2mA (from the 6B8 characteristics with grid voltage at minus 3 volts). The voltage across R23 is then made up from 12mA from V8 plus whatever current V6 is drawing (because V6 shares V8's cathode resistor). Let's say V6 is drawing 11mA (assuming its grid is also minus 3 volts). R23 voltage is therefore about 8 volts. But I set V8s grid at minus 3 and now its minus 8 so V8 anode current will be less because its more cut off and this will better define R23 voltage (which will be less). What actually happens is that V8 and to some extent V6 will quickly attain a steady state and define the action of QAVC. In fact the purpose of QAVC is to reduce the overall receiver gain when no broadcast is tuned. VR5 allows the QAVC action to be preset.

Once a signal is tuned X1 will suddenly find that its rectifying properties have overcome the reverse bias at the junction of R55 and R56 (decoupled to DC by C39). The cathode of X1 begins to rise in a positive direction as the signal is tuned in and reduces the reverse bias to V8 whose anode current increases. This increases the voltage across R23 and a new steady state is rapidly attained. This by itself is not the end of the story however, because also acting to control overall gain is standard AVC action via Westector X2. The job of normal AVC is to increase overall receiver gain when there isn't anything tuned in (in fact quite the opposite of QAVC). In this receiver the two feedback systems are interlinked through a network of resistors and these together with the action of X2 will actually define the steady state for no-signal, tuning into a broadcast and once tuned, the steady state for a tuned broadcast. That jitteryness I'm trying to pin down is therefore quite likely to be oscillation within the QAVC/AVC system caused by a bad smoothing condenser, bad resistor or even a dry solder joint. As an example, AC at V8 grid from a bad C39 may result in instability caused by modulation from a test tone.

Looking at X2 which is the standard AVC detector you can see the cathode (marked red) is connected to a fixed potentiometer comprising R49 and R50 which together define a potential of 8 volts given an HT rail of 250 volts (note that the voltage is not supplied by the voltage stabiliser). This positive voltage will cut off the Westector until 8 volts is produced from the RF voltage appearing at IFT4 secondary. This is generally known as "Delayed AGC". So superimposed in the overall steady state I mentioned previously is what happens at X2 which will only conduct once its anode sees an RF signal capable of being rectified into a voltage in excess of that at the junction of R49/R50. Yet again, this isn't the whole story because X2 anode is connected, not only to the anode of V7 via a DC blocking condenser, but also into the QAVC circuit driven by V8. That means the whole network of resistors and condensers looking after gain is governed by the setting of RV5.

What about audio recovery from the broadcast signal? You'll notice that even the detector for this (V8's diodes) is linked into the QAVC/AVC system. Firstly because the diodes within V8 are governed by V8's cathode voltage and secondly via R82. In this circuit we also have a smoothing condenser C60. I need to check this area because the RF output from 500pF at V7 anode appears to be shunted to ground by C60. This is a fixed potentiometer of 0.5nF and 50nF resulting in only 1% of RF reaching X2 so surely not right?

At the moment it's all slightly academic because I removed X1 and X2 and found both seem to be open-circuit!

I've split off audio section commissioning to another page (see below) but as that exercise proceeds I'm finding the odd problem with the RF area, for example the trimmers across some of the coils have too low a maximum value to peak signals. Easily remedied by adding small fixed capacitors. During RF testing I had trouble peaking the coils in the RF stages. The receiver has very potent AVC and, as I discovered later, very potent audio compression. As you align the RF coils the combined action of these two features is to defeat any attempt to peak the RF coils. You could see these auto-gain effects when rapidly twiddling trimmers. As the tuning of the RF coil passed through resonance the audio would suddenly get louder then within a second or two drop back. As I explain on the audio page a spectrum analyser might reveal all?

There are two groups of curves below from Setting 1-6.. First the QAVC responses and secondly the response curves for the receiver proper.

See the Receive curves

 BANDWIDTH SWITCH SETTING

 1

 2

 3

 4

 5

 6

 BANDWIDTH IN Kc/s

 5

 8

 11

 15

 18

 TRF

And here are the IF curves using a tracking generator set to 465KHz at a level of -20dB fed into the receiver aerial socket, so considerable losses before reaching the first IF amplifier. Note that the signal is monitored at the anode of the QAVC detector diode X1. The peak of the first curve (marker 1) was adjusted to be exactly 465KHz at a bandwidth setting at the minimum of 5KHz and then successive IFTs were adjusted to keep the peak at 465KHz. Marker 2 was set at the peak before making adjustments. No effort was made to adjust the curves to the widths marked on the bandwidth knob as I guess the QAVC response should be sharper than that for recovered audio. Curve 2, 3, 4 and 5 are the curves at wider settings with the final picture showing the response at the TRF position of the bandwidth knob. The distance between marker 1 & 2 doesn't appreciably get any wider but the IF output drops as the bandwidth is increased. Horizontal grid setting in each picture is 20KHz per division so the 3dB points for curve 1 are about 16KHz apart, curve 2 about 20KHz, but the others are less wide with TRF having sharpest response.

 
   

 

 

 

 

 Below are a couple of curves showing the 465KHz IF response to Radio 4 on 198KHz. I used 3 markers with 2 and 3 at 5KHz either side of 465KHz making the width of the first response to be 10KHz at 50dB down. Each horizontal division is 10KHz. The second curve shows Radio 4 at the TRF setting with horizontal divisions at 50KHz making the response about 10KHz at 20dB down. Note that all these curves show the response of the receiver for the QAVC control signal and not the response for audio detection. Read the AVC Page which explains the AVC system

 

 

 

 

 And now the curves for received signals. The tracking generator was set to 0dBm and connected to the aerial input with the receiver set to 200KHz long waves. The TRF curve is not shown,but instead I repeated the widest bandwith setting with QAVC off. In this group of pictures I set Marker 1 to 465KHz and Marker 2 and 3 to roughly 3dB down from Marker 1 for pictures 4-6 and for pictures 1- 3 at +/- 10KHz from 465KHz. The span is 50KHz so each division represents 5KHz.

To recap: The bandwidths marked on the knob of the bandwidth switch is my only source of information and it's fortunate that this knob is original as, to the best of my knowledge, no other record exists.

 

 BANDWIDTH SWITCH SETTING

 1

 2

 3

 4

 5

 6

 BANDWIDTH IN Kc/s

 5

 8

 11

 15

 18

 TRF
 
   
   
   

 Whilst matching exactly to the markings on the knob the results shown above are a lot different to the QAVC curves and indicate that the adjustable IF tuning is working tolerably well. After testing I noticed the audio from Radio 4 Long Waves on the 5KHz setting sounded muffled, I suspect one of the wafers of the bandwidth switch selecting audio filter components was wired to a faulty part. Having tuned the IFTs to peak at V8 diodes I was able to hear all the normal long wave stations. A bonus was that VR5 can now be adjusted to set the quieting level as with it set to minimum quieting I can now hear background noise when not tuned to a broadcast.

 The long waveband is now fully operational and all the usual broadcasts are audible at good strength but the RF coils don't seem to peak up. This might be due to too much overall gain. Unlike usual receivers, whose coils and capacitors can be adjusted for best tracking, the coils on the lowest three bands have no dust cores. I also checked medium waves and again the whole band tunes nicely. The padder is a substitute and a little high in value so the tuned band is larger than the dial markings so that needs some sorting out before final alignment. The lowest frequency shortwave band fully tunes but the highest two need sorting out as their local oscillator cuts out at as frequency rises with the tuning condenser at half mesh. These coils use slugs so it may be that these have degraded?

After another session checking over the receiver, but now armed with a new can of switch cleaner, I found that the two tuning condensers are not wired in parallel for the longer wavelengths but used individually. I noticed that the local oscillator on the two higher frequency bands stopped at exactly the same point on the dial as I tuned to about half scale, and of course the reason was obvious... a bent vane on the smaller tuning condenser. After carefully bending it straight the receiver worked on all bands across the whole dial. Next, I looked at the mechanical parts of the tuning condensers. These, like all of this type of tuning condenser design, rely on perfect earthing between the moving vanes and the metal frame. Because of general corrosion these moving parts looked a bit dodgy and tuning a station and rocking the tuning condenser back and forth showed changes in volume which lessened as I applied switch cleaner. I then noticed the screws holding in place the tuning condenser to the chassis were moving as the tuning knob was pushed. In addition, the tuning control shaft passes through a bush secured to a bracket screwed to the chassis, and both the screws holding the bracket were loose allowing movement of the tuning condenser when tuning. Most of the eight screws securing the two tuning condensers to the chassis were loose and all looked rusty. Again, the volume of a received station changed as the tuning condenser was rocked but lessened as the mounting screws were turned. Ideally the tuning condenser frame should be soldered to the chassis using copper braid. The high resistance between the moving parts clearly accounts for weaker than expected reception and explains why the various RF coils didn't cleanly peak up as their trimmers were adjusted. Once the switch cleaner has finished its job I'll repeat alignment. As far as tightening screws is concerned... not an easy job because of the position of their nuts buried in the coil pack and wiring, and the nuts, like all used in this receiver are 4BA locknuts.

After cleaning up the poor chassis connections and the poor tuning condenser parts I found I could now hear stations with the bandwidth switch selecting the TRF receiver.

Because the set is designed to feed an external amplifier and produces only a low level audio output, which is damped somewhat by low impedance headphones, it will be a good idea during further testing, to use a small audio amplifier and loudspeaker.

 
 

 For the next phase of fault finding and refurbishment see Audio Repairs

Review of Moreton Cheyney AVC System

Return to the Moreton Cheyney home page

 Now.. for the matching amplifier/power supply

Return to Reception