Moreton Cheyney Circuits 

 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 are 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.

 

 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?

 

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?

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.
 

 

  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.
 

 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

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 this will need re-cutting. 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. 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.

 
 

 Return to the Moreton Cheyney home page

 Now.. for the matching amplifier/power supply

 It will be a(nother) real challenge... continues

 Return to Reception