Murphy 62B Repair

 Not a refurbishment as such, but a continuation of a job I started in 2014 (click to read) to make my Murphy 62B into a reliable working receiver.



 Anyone familiar with this type of receiver can see that it's been got-at. The smaller knobs have been changed and a multi-turn pot fitted below a new meter. I don't like the knobs but the S-Meter is a useful addition, as is the new control which I believe is for fine tuning SSB. I need to check on the exact version of the 62B as there are two, the AP67757 and the AP67757A. I initially chose "A" but later I discovered my example has some features of the earlier version without the "A". Below are the appropriate circuit diagrams in three sections. Click the picture above to see the full manual which includes all versions of the receiver.

Preceeding the serial number and engraved at a later date are the letters "EMD/R", which Andy, G8JAC suggests is "Refurbished at the Devonport Dockyard" (Electronic Maintenance Dockyard)


Note that these sets of drawings are not quite right for my 62B variant which is coded AP677571. Mine is a mixture of AP67757 and AP67757A of which the latter carries a cross-mod control and a radar suppression circuit AND uses all glass valves not the B8B types used in the non A variant. For completeness I've copied the A versions first (Fig 66) then the non-A versions (Fig 65) below each example. Slightly confusing....

Note that this receiver would probably have been used on a ship with this type of low pass filter helping to reduce interference.

 AP67757 RF UNIT






 I'd been quite happy with the work I'd carried out over five years ago on this old set and it was now residing on display at the end of my work bench which was probably one of the reasons I decided to renew my acquaintance with it because, if it had been elsewhere, the sheer weight would have put me off tackling it. I needed to temporarily unscrew my desk lamp and some other fittings before dragging it out and removing its outer case. I then noticed the odd Belling Lee mains connector. The previous owner had obviously modified the set and in doing so had replaced the Plessey mains plug (a dangerous 2-pin affair using its shell as earth). I recall having repaired a similar radio for a customer many years ago and had fitted a much safer IEC plug (like the ones used in a PC). I must have had a proper mains lead with a miniature Belling Lee plug back in 2011 but I couldn't find it so made a new mains lead with a new connector which I recalled I had in a box labelled "power connectors".

As enthusiasts like to know what type of valves are used in this type of equipment, I've included the table below, where the columns cover the sub-assembly viz. 1 is RF Unit, 2 is the IF Unit and 3 is the Output/PSU. The radio is the 62B/AP67757A which I assume is very similar to my example the AP677571. Note that the 6BA6W is similar to the EF93, and the 6AL5WA is like the EB91.

































































 I plugged in the mains lead, poked a long wire into the aerial socket, and switched on. The dial lamps came on and after a short while the S-Meter registered half scale but there was no sound at all... just silence no matter which switches I pressed or turned. Switching to the long wave setting and tuning to Radio 4 on 198KHz showed a nice peak at the S-Meter which on tapping gently proved to be a bit sticky. Looking at the rear top panel I found a 2-pin socket marked "speaker" but poking wires from my HRO speaker proved this output was defunct. After a search I found a pair of low impedance headphones and plugged them in and waggled the volume control only to be deafened by a terrible ear-splitting crackling noise. I removed the phones within a few milliseconds and waggled the pot. It gradually got a little quieter but refused to get much better.. but at least I could adjust the volume setting then gingerly put the phones back on. It was obvious that all was not as I'd left the receiver years ago. Tuning across Radio 4 showed the IF response was lumpy and after switching to 80 metres (drat-not quite covered!) indicated the set was as deaf as a post. All the valve shields seemed warm enough so my guess is a resistor or two, and probably a few condensers need replacing. Pictures of the 62B follow. The exterior views are not especially interesting.



 Not a lot to see in the underside view, below. A bit boring.. which may sum up the 62B, although the mechnical designers must have had a field day with this series of receiver?



A nice feature

At each rear corner is fitted a ball bearing race (now devoid of lubrication) which enable the receiver to be moved on a workbench without gouging the surface.

The outer case does not extend to cover the underside of the receiver.




One side is completely screened, and doesn't warrant a picture here, but from this side you can conveniently see the IF strip connections and make measurements with the receiver switched on.

Compare this with the R109 which is incredibly difficult to work on.


 This example carries light rust and a grey deposit that smells like "essence of government surplus shops" and difficult to remove from ones fingers.. Nicely laid out and you can see the odd plug and socket which are fitted so that complete assemblies can be detached for remedial work. Not obvious in these views is the large turret (carrying the RF coils) and the heavy gears etc for turning it.

When I last worked on the receiver I'd overlooked some sloppiness in tuning. In a receiver of this quality I imagine there's provision for sorting this out.


 Below the IF area you can access the power supply valves.






Below is the power supply component layout.

 All the RF coils in the 62B receiver are mounted in screened boxes fitted into its turret.

 I decided to test the receiver using a signal generator rather than a random wire, especially because the wire aerial I have is suspiciously poor when connected to my SDR. I then discovered that I couldn't find a suitable RF plug for mating with the socket fitted to the 62B. Clearly, one desirable change, other than fitting an IEC mains connector is to fit either an SO259 or a BNC socket for the aerial. However, by poking a wander plug into the aerial socket I was able to fit a BNC lead between the 62B and my HP signal generator. The latter fired up and produced 600KHz AM which was detectable with the 62B set to the same frequency. The S-Meter read +20dB with the HP set to +20dBm and dropped to a just detectable audio output at -20dBm. The meter showed the same reluctance to move freely without tapping so that's another job I need to tackle.

Looking at the various controls on the front panel I noticed the unusual one labelled "Anti-Cross-Mod". The 62B RF amplifier valve, specified as a CV4014 is an M8083, a special version of the CV138 or EF91, a common variable-mu RF pentode (V101 above) which is operated in accordance with the values of resistors used to set its electrode voltages. It's important to operate an amplifying valve so that it works in a perfectly linear fashion otherwise its output will carry not only a stronger version of its input signal, but harmonics of this and, more importantly, combinations of several input signals if more than one is visible to its control grid. It's important to realise that although the overall receiver bandwidth may be quite narrow, the RF amplifier bandwidth relies on the quality of its tuning coil and other factors, so that its control grid will see many strong unwanted signals, perhaps in the whole shortwave broadcast band to which the receiver is tuned. If the valve characteristic is not perfectly linear the anode current will carry a whole host of small currents associated with products of many input signals, not just the current associated with the desired signal. One critical result might be to activate the receivers AGC line when we are trying to maximise weak signal amplification.

The specification requirement for the 62B clearly must have asked for some provision for weak signal reception in the presence of interference. Whether this was strong adjacent channel broadcasts or something found in a ships overall electronics package I can't say, but this may have been radar signals or powerful local RF radio communications transmissions. The 62B was earmarked for receiving domestic-type broadcasts and playing these to a ships company so interruptions from the ships radio tansmitters or noise from radar transmissions would be really annoying. So, the special anti-cross-mod control was fitted in order to fine tune the linearity of the RF amplifier to minimise interference. In fact, studying the circuit of V101 shows it was not fed with the set's AGC voltage, instead relying on the bias voltage set by RV101 and its cathode resistors. You'll also note the connection to V101 suppressor grid brought out to SK101 (described in the manual as being used by R.I.S. outfits which I understand is "Radar Interference Suppression", in that a pulse is provided from an external equipment for muting the RF amplifier for very small periods to eliminate pulse noise).


 Now to diagnosing the dead loudspeaker and what a clever idea to make the various modules extractable from the chassis so easily. I initially forgot to remove the knobs but once this was done, and four leads unplugged, the IF amplifier just slipped out of the chassis. I followed a yellow wire from the speaker to a Jones plug and found Pins 3 & 5 measured 3 ohms/0.3mH so the loudspeaker appears to be fine. Next, I removed the RF gain control knob, slackened the two securing scews and slid out the power supply module from the chassis. Dead easy... and checked the loudspeaker on/off switch. Mechanically perfect but it measured open circuit in both ON and OFF. Clearly a build up of oxide or dirt was preventing it from working.



 Upper left, the loudspeaker switch in the OFF position and right, in the ON position but both positions gave open-circuit across the switch terminals.

I thought the action might be lazy, but the switch appears to work normally, that is mechanically.. electrically not.

The lower switch is a ganged pair of identical type and this also read open circuit before I plugged the receiver into the mains supply. In that case however, whetting current soon fixed switch continuity.

Now that the power module has been extracted from the chassis I'll clean both switches then fit an IEC connector in place of the Belling Lee type fitted previously.

Below is the detached IF module.




 Above, a top view of the power supply module and below, the underside.. so easy to work on.


I fitted an IEC mains socket (see below), a new 2Mohm volume control (RV224) and swapped several resistors that read high. These were R206,100Kohm (open circuit V202- g2 ground connection); R228, 330Kohm (412Kohm BFO-g2); R226, 68Kohm (81Kohm V205 detector); R225, 680Kohm (800Kohm V205 detector); R312, 3.3Kohm (oddly 612 ohm and lost its markings from excessive heating, HT feed). The LS switch turned out to have an open circuit at each of its terminals (tinned solder tags screwed to copper terminals). Reassembly was straightforward and after jiggling the local oscillator valve the receiver worked well enough to pull in Radio 4. The dial indicated a peak at about 210KHz (with a very wide tuning coverage) and loss of treble when tuned in precisely so alignment is definitely needed.





  The original 2-pin Plessey plug had been replaced years ago with this miniature Belling Lee type (same mounting requirement, but rather flimsy) which is easily changed for an IEC type. I used a clip-in type because the flange holes on the other type are in an awkward position.

The power supply circuit is shown above, where you can see a pair of filter condensers C309/C310, which decouple the mains input to the chassis. With the old 2-pin plug it's not an uncommon practice to fit a live mains lead whilst holding the receiver case. If the shell connection did not make perfect contact with the connector screw before one of the 2-pins touched one of the mating pins you'd get a nasty shock. On board a ship the receiver would almost certainly be grounded so a shock would be a rare occurrence. The leading earth pin on the IEC connector grounds the chassis before live pins make contact.

Disconcertingly the new volume control was nearly as crackly as the original and I noted 4 volts DC at the input to the pot which probably means C229 is leaking. This is rated at 0.005uF x 1000 volts (note that these high voltage condensers were often used in critical parts of circuits known to be prone to failure), As C229 is readily accessible I'll swap it tomorrow.

 The next morning, before I cut C229, I measured the voltage at its end again and it was sitting at 10 volts which I imagined was due to C229 being colder, but as I looked the meter suddenly shot up to 220 volts which didn't make sense so I peered at the live end of C229 and noticed a red insulated wire (from R204, 4.7Kohm) touching the anchoring tag. I moved it away with a screwdriver and the voltage changed from 220 to 0.5 volts. Maybe C229 is ever so slightly leaky but certainly not responsible for the elevated voltages. Over the last 60 years or so pressure of the insulation against the sharp solder tag had cut through the plastic. As the new volume control is over 30 years old, having arrived in a job lot of about 100 Radiospares pots in little boxes, and having a gap in the top, I was able to squirt in some switch cleaner and, after a few waggles, the crackling went away and audio control was perfect. In fact any slightly odd sounds and the loss of treble when Radio 4 was tuned in had disappeared leaving Radio 4 perfectly clear and undistorted. I waggled the cable harness to the larger chassis plug and I could hear sharp crackling sounds so no doubt there's another potential short lurking in the wiring.


 Now that most of the problems with this old receiver are fixed I decided to check the IF tuning to see if this would help explain its deafness. Sure enough the best IF response was about 497KHz and, after setting all the IF transformers to peak at 500KHz in the 1KHz bandwidth position, reception was transformed. The High and Low BFO settings worked fine on SSB but the tuning was far too imprecise for comfortable listening. This is probably due to wear and poor lubrication in the mechanism which is a very complicated affair. I then remembered the added multi-turn pot at the top right of the front panel and found it was perfect for tuning SSB. This extra control transforms the receiver and, with this, 40m LSB signals can be precisely tuned and received loud and clear. Without this extra multi-turn pot the tuning backlash is so horrendous it makes it impossible to resolve SSB.

Switching back to long waves, and using a length of wire as an aerial, Radio 4 measured a stonking S9+50dB on the S-Meter whereas previously it registered only a measily S5. Switching between the three bandwidth settings showed all was not ideal however; with best reception in each setting requiring a retune to see a peak on the S-Meter. In the past I've discovered that it's virtually impossible to properly align an IF strip using an AM signal generator and audio wattmeter. You definitely need a spectrum analyser to do this properly otherwise, nine times out of ten, the receiver characteristics result in each IF transformer ending up off-frequency. This also happens because of the way the designers produced the different bandwidths (here they just added 470pF across each transformer secondary). In this set as is the case with many that use a crystal filter, you need to align around the crystal response. The 1KHz setting in the 62B. If you're lucky the crystal filter will be close to 500KHz but if not it may be better to align at whatever frequency the crystal filter dictates, then check the wider bandwidth settings match this and finally tweak the local oscillator to match the dial calibrations. Not exactly the final step though as you also need to tweak the BFO to match the response of the crystal filter because the three settings (Tune, High and Low) are preset and not adjustable from the front panel. All this can take an inordinate amount of time but with a satisfying end result.

One thing of note I discovered was the excellent rejection of the receiver to out of band signals. Usually I'm lazy and use the aerial terminal for injection of the IF test signal, but even with 1000mV of 500KHz at the aerial, the IF couldn't be adjusted comfortably and I had to connect the signal generator via a small capacitor to the front of the IF amplifier. Surprisingly, I couldn't see an IF trap in the RF amplifier so the rejection must be due to the excellent screening from the chassis and the turret design.

Sadly 80m isn't covered in this model, with the nearest frequency being 3.9MHz, and now that I've got this far I'm starting to lose interest because the 62B is a bit boring. I'm sorely tempted to just stick the covers back on and shove it back into its slot at the end of my work bench and find something more interesting to fiddle with... but at least I can listen to Radio 4 on the 62B whilst I'm working. After all.. that was the main reason for introducing this particular version of the B40...



 Now that I've removed a side panel, my interest has returned somewhat. This drum is very similar to that used in my favourite receiver, the R206. This one could well be a later development of the same drum?


 These drum contacts are pretty good and, compared with those R206 rhodium plated contacts, would be a lot cheaper to produce and repair.







Below.. the RF amplifier chassis (and the drawing from the manual) which is very accessible especially with the receiver on its side. You can see how the drum simplifies wiring and layout.



 A quick check of the various resistors above revealed most were high in value as per normal. A quick check of the condensers indicated C120 was short-circuit. This brings me to something obvious (and sort-of helpful?). Each of the three main assemblies, the RF Unit, IF Unit and AF & Power Unit have components etc numbered in accordance with their assembly, starting with 1, 2 or 3 respectively. C120 is therefore within the RF amplifier chassis seen in the drawing above (top left). Below the row of those five 0.1uF condensers you can see three resistors numbered R124 (430), R117 (4.7K) and R115 (220) which I measured as 56K, 58K and 72 respectively. The picture shows these as 3.9K, 5.6K and 62. I looked at the circuit for the AP67757/A.. where's C120? After puzzling over this I eventually realised I'd assumed my B40 was the /A but it's not, it's the version without the /A and sure enough, after swapping to the correct set of three circuits (now in place above) there was C120, and following the connection (confusingly) from SK102/8 to PL202/8 (and PL205/8) I see it's shorted to ground by SW206c (AGC ON/OFF). Hence C2120 is grounded when AGC is OFF. The mystifying resistor values are no longer mystifying because the non- /A has different values viz. R124 (47K), R117(4.7K) and R115(220)... at least, partially explained except for R115 which you can clearly see (in the photo) is 61 ohms. Perhaps this is/was the subject of a later mod?

What's really confusing though is the valve complement. The /A version of the 62B has "modern" B7G and B9A-based valves and the non /A uses the rather unusual B8B type of valve. Until I resolve the puzzle I'll assume my 62B is the non /A with /A valves.




This trimmer replaces the rotary switch, SW103, used for crystal control of the local oscillator. Was this, and the S-Meter circuit above the trimmer perhaps taken from a published article on making the receiver more SWL friendly, I wonder?

Having exposed the RF coilsets I guess it will be worth tweaking the trimmers and cores to align RF input with dial readings. Before doing this I'd like to see if I can sort out sloppiness in tuning and also check IF response which varies between bandwidth settings. I tried a quick check to see roughly what the IF response was like. I'd noticed several tuning points after peaking everything and sure enough, when I looked at the response it had three equal peaks in all three bandwith settings. Most significant was the narrow range, supposed to be 1KHz which had steep sides but with three equal peaks with troughs either side of the centre peak. The response to 500KHz was nowhere near the centre of the trace but after twiddling the crystal trimmer to see its effect I found at one critical point the width of the response reduced to a quarter. The picture below shows the response with the tracking generator set to -20dBM and connected via a 220pF capacitor to V201 control grid, with V204b (the detector) being the output monitoring point. This is certainly not ideal but good enough to investigate what's possible. Below, the scan is 200KHz so one horizontal division is 20KHz, making the response about 1KHz, which is correct for the narrow setting. I measured bandwidths of around 4.5KHz and 12KHz at the other settings.


 With the IF correctly on 500KHz I then checked the low frequency band to see if the dial readings corresponded with input frequencies. Rather than using a signal generator at this stage I used three long wave broadcasts viz 252KHz, 198KHz and 162KHz. The results weren't too bad and with some work I can carry out decent alignment later.

At this point I decided to read the manual to see what was suggested by way of alignment back in days the receiver was being used. Of course the alignment needed to be carried out with test gear available back in the 1950s in Naval dockyard repair shops and, as this wasn't a patch on modern test gear, much better results can (theoretically) be gained nowadays. I say theoretically because overall degradation in components will show up when modern test equipment is used. Coils, condensers and resistors will have changed over the last 60 odd years and. as it's not a viable task to fit new parts, the optimum final results may be not quite as good as the designers intended.

The next step is to make a special tool with which to adjust the IF transformers for what the manual refers to as "final touch-up". The IFTs use what appear to be Tufnol screws for adjustment of their cores and these are locked with something like 7/16 inch nuts. These screws are sticky in operation and not all are accessible without angling the adjusting tool. Below are the scans from the three bandwidth settings. I'd read the manual and noticed a second adjusting screw for the crystal filter and, following the guide given in the manual regarding the changing shape of this filter as the adjustments are made, experimented with the effect of twiddling both screws rather than, as previously, just to one on the side of the can.




 A word about the response curves above: You'll note in the initial (blurry) scan the vertical divisions are 4dB with a baseline of -88dBm, giving only 14dB to the output signal peak, but the three newer scans have vertical divisions of 10dB and a baseline of -110dBm giving better than 35-36dB. Readings could have been slightly better if I'd used a VBW/RBW of 300Hz or less. Note the "centre" frequency of 501.666KHz. This seemed to be the natural resonant frequency of the crystal in the filter and because of this I aligned the IF amplifier to that frequency rather than 500KHz. The manual does obliquely refer to this technique which will give a flat response rather than a top carrying a few peaks and troughs which I'd noticed when aligning on 500KHz.

I then tackled alignment of the 6 wavebands. A tip here is to carry out all core adjusting at the low frequency end of all wavebands before using the trimmers for all the high frequency ends, otherwise you'll spend an inordinate amount of time spinning the dial from one end to the other and back.

After carrying out this job I tuned to 40m and tweaked the band edge to 7.000MHz on the dial. Further tests showed again the sticky tuning mechanism noted before when trying to resolve SSB. I now have a suspicion that the cause might be a seizing tuning condenser. The thing is not readily accessible so I'll leave this for now, but another possibility is oscillator pulling, something that plagued my R206 when using it as a second IF on 24-26MHz for my nuvistor converter. Something else I noted.. oddly on the long waveband Radio 4 peaks on different frequencies at each of the three bandwidth settings. This does not seem to happen on the medium waveband. A clue might be the very high signal strength of Radio4 with my aerial.

A coupe of other minor faults to sort out.. the limiter control kills the audio and the cross-mod control seems to do nothing. I'd guess bad diodes are the reason.


  Click the picture to see a better picture of the tuning mechanism.

One of the annoying things about this example of the B40 is imprecise tuning. According to the manual you should not see backlash of more than +/- one division at the logging scale. I've found a sort of pulling effect in the tuning which I've come across before in a few receivers. It can be due to either stiffness in the tuning mechanism or oscillator pulling. The latter would be due to the local oscillator shifting, perhaps on account of a voltage change as the set is tuned to a strong signal. Also possible is a shift in the IF response resulting from a significant change in AGC voltage. The latter be checked by switching off AGC, and IF pulling due to a strong signal can be tested by reducing the RF gain. (see later for explanations)

Hopefully poor lubrication is the problem because this can be sorted out.

 I decided to bite the bullet and dismantle enough of the 62B to access the tuning mechanism to see if anything was wrong, or indeed if everything was as the designers left it. To start I removed the knobs so I could detach the front panel. Here, I found a major problem. The wavechange knob is secured by two grub screws. The smaller one came loose, but the second larger screw refused to turn. I think it may have been previously attacked because the slot was damaged and my only recourse was to drill it out. Fortunately the grub screw (about 0BA) was not hardened steel and after a pilot drill had penetrated to the square part of the spindle a second and a third larger drill removed the old grub screw and the knob slipped off. I tapped the hole for M6 and I'll refit it later. After removing the screws holding the front I pulled it away. The various tuning gears were completely dry so I removed the tuning flywheel and log disk and applied WD40 to the various gears followed by cleaning and then grease. I also found the dial window assembly was seized in place. After oiling the securing nuts this was movable, but I might return to this later to make it less stiff. After greasing I found the dial moved much more freely and without backlash. The latter is dealt with by a split sprung gear which I'd also greased.


For those unfamiliar with the B40, the tuning mechanism is really clever. As the tuning knob is turned clockwise the dial carrying the frequency markings rotates, and simultaneously the curved plate carrying the apertures though which the markings are visible rises, so that only the correct markings are visible. This is similar to the method used in the AR77 except each of the six wavebands on the B40 has its own aperture, making six apertures compared with only one for the AR77, which switches into place to show the selected waveband. There's a long chain that drives the tuning gears and this is fitted with a tensioner so that backlash is minimised. Also coupled to the tuning knob is a heavy flywheel and a disk carrying 0-100 for logging. A clever mechanical gizmo rises at each end of the tuning range to act as an endstop and between the flywheel and the main tuning shaft is a friction clutch set by a large locknut to prevent mechanical damage if the knob is spun rapidly. Click here or the picture above to see some detail.



 I fitted a temporary tuning knob having a mounting bush that fitted over the original nut used to hold the flywheel and turned my attention to the tuning condenser. I noticed that a tuning direction change would result in a slight knock coming from insde the screening cover mounted over the tuning condenser. I found this cover is held in place by four 6BA scews, just about accessible. I removed these using a magnetic screwdriver and tried to lift off the cover. I found it's only detachable when the rear of the tuning spindle coupler is precisely vertical. The tuning condenser is 4 gang and held in place by what?? there are wired connections to the turret tuner and earthing braids to the chassis, but nothing securing the frame to the chassis other than three loose screws mounting it to springs. When the spindle turns one way the various wires stretch to their limit before the vanes move, then reversing spindle rotation, the frame rotates, compressing the connections, again before the vanes rotate.

 The right hand end carries the flexible coupler but the left hand end has nothing coupled to it. As you can see the centre shaft is a ceramic material. The left end of the tuning condenser frame is secured by a single 4BA screw into a springy mount. The right end has two 4BA screws and all seem to be very loose. For some reason the cover has a pair of shims (you can just see the RH one) which I must ensure are in place when I refit it. I need to tighten the loose screws, lubricate/clean any moving parts and confirm the thing is steady then confirm there are no broken wiring connections to the stators.  


 These are the loose screws. At the left is a springy mount onto which the upper screw should be a tight fit but its so loose the whole frame can be lifted. On the right I can't see anything securing the frame to the chassis. The bracket held by the RH screw is not secured to the bracket held by the LH screw.All it does is prevent the front of the frame fifting more than an eighth of an inch. Clearly the whole assembly was designed to be relatively flexible to prevent damage from mechanical shock.


 So there it is.. the reason for severe tuning backlash and the difficulty in tuning 40m SSB!

Fixing the problem was easier than I thought because in the rear panel directly behind the loose screw was a hole through which you can insert a screwdriver to tighten the wayward screw. Clearly someone had recognised the fault in earlier examples and a quick fix was possible. Refitting the screen which I'd detached to investigate the problem was tricky. Each end of the screen is supported on metal plates which might be shims fitted to raise the height of the screen. There's only just enough room to use a long thin magnetised screwdiver to replace the 6BA screws. The front was easier and I loosely fitted the first two screws, but in order to align the rear of the cover you need to use a mandrel, such as a thin phillips screwdriver, to line up the holes through the cover, the shim and the chassis. Once this is right you can line up the other holes and fit a screw. Once this is loosely fitted, extract the mandrel and fit the last screw, then tighten all four.

I can now tune SSB much more easily, but there is still a little backlash due the the action of the chain used to turn the tuning condenser, although further work to lubricate this and its tensioner might improve things. I also looked at the S-Meter. Taking it apart revealed some black flakes stuck to the cylinder around which is the coil. I found that blowing through the coil area cleared the needle from sticking. The black flakes had come from the back of the magnet whose paint was in poor condition.

Looking back to the second of the two tuning problems. The pulling effect is now resolved having tightened one loose screw, but the strange effect in the IF response curves seems to be from the "Miller Effect". This was suggeseted by Michael G8MOB, then Pete G4GJL sent me an excerpt from the G2DAF receiver write up which describes the effect in detail, below. The two increasing peaks that appear when the input test signal is increased are the result of extra bias from the AGC circuit affecting the careful settings of the IF transformers. This effect also appears when tuning across a strong broadcast signal such as Radio 4 on 198KHz when a long wire aerial is used. In this case the signal is spread across the range 192 to 200KHz instead of peaking properly at 198KHz. My guess is its just a foible of the B40 and should remain so for authenticity. In fact, as AGC is also applied to the second RF amplifier I'd expect the S-Meter to get fooled as well. The solution, especially when encountering very strong SSB is to switch off AGC and reduce the RF gain control. I say this because I noticed strong 40m SSB was tricky to tune correctly with a rubbery feel to the tuning, but as resolving SSB wasn't on the cards when the receiver was in the design stage I can at least not blame the designer for this failing. Click the excerpt below to read the whole book.


 I now need to carry out a final check on alignment after reassembling everything because I noticed that after fixing the tuning condenser the dial no longer read correctly. This might be partly explained because I applied switch cleaner to all the moving parts and probably slightly altered the local oscillator current paths. The other reason is that I freed up the cylinder carrying the dial markers. This was seized and possibly off-centre. There's also the noise limiter to fix because it kills audio when switched on.

A word about the response curves shown below.Firstly, these do not show actual power levels. For example the first curve shows a peak of -75dBm and a baseline of circa -104dBm. The difference between these readings is purely indicative, which is really what we're after. The spectrum analyser is feeding a swept signal (50KHz) centred about 500KHz at a level of 0dBm from a 50 ohm source via a 220pF capacitor into the plug connecting to C201 and hence the tuning coil at the grid of V201. This is easier than feeding the anode of the mixer which would be slightly better but awkward to do as it's at the opposite side of the chassis from the IF Unit. Output to the spectrum analyser is taken from the anode of the detector V204b via a 20pF capacitor anchored to an adjoining tagstrip, but the signal goes through a high impedance probe. This is necessary for three reasons... (a) to isolate the 50 ohm input of the spectrum analyser from any inadvertent high voltage, (b) to avoid damping the high impedance receiver output and (c) to minimise any detuning of the IF circuit due to the capacitance of the probe. The probe has an indeterminate loss of about 30dB, but will remain at a constant level during measurements. The tracking generator 50 ohm output feeds the IF Unit via 220pF, again to avoid damping the input, and at 500KHz this will have an impedance of about 1.5Kohm, so you can see the 0dBm at 50 ohms (=225mV) translates to something like 7mV at the input to the IF amplifier. This equates to a loss of about 30dB, so the effective tracking generator output will be circa -30dBm.

Summarising.. the IF input is -30dBm and the IF output is seen as -75dBm. The latter figure appears a lot smaller than it actually is, by the loss through the probe (30dB) and the loss due to the 20pF capacitor. Although this capacitor has an impedance of about 16Kohm at 500KHz the probe input impedance can be assumed at least 5Mohm. The detector anode will be recorded as 1.25mV referenced to 50 ohms, but if the detector circuit impedance is for example 5Mohm, the anode voltage will really be circa 400mV. Very roughly (and this ignores the state of the probe batteries which could easily increase its loss by a further 10dB) the actual numbers involved will be 7mV IF input and (at least) 400mV IF output, which makes sense...

Receiver B/W Setting 1KHz

Horiz: 5KHz per division

Response: +/-1KHz


Receiver B/W Setting 3KHz

Horiz: 5KHz per division

Response: +/-2.5KHz

Centre: 500.05KHz

Receiver B/W Setting 8KHz

Horiz: 5KHz per division

Response: +/-7.5KHz

Centre: 500.05KHz

Receiver with BFO on

Horiz: 5KHz per division

Frequency: 500KHz


Low was 498.13KHz

High was 501.13KHz

The curves show that with modern test equipment the old receiver can be set up to produce excellent results. The initial basic (best) results using a signal generator and wattmeter were pretty poor, with multiple responses from strong signals and hopeless crystal filter performance, all no doubt aggravated by interactive effects of signal strength and detuning (Miller Effect).

During the tests I did not bother to disable the local oscillator and therefore the receiver was actually trying to work on its lowest waveband. Using older test gear it was customary to disable the local oscillator but I'd rather leave it running to permit realistic noise levels to be seen. Whilst this doesn't materially affect the shape of the response curves, disabling the local oscillator would certainly reduce the level of the noise floor (measured at about -104dBm) to possibly -130dBm or better. Comparing with historic response curves, the results above could therefore be significantly better than the 104dBm-75dBm= 29dB indicated, perhaps by at least another 20dB, to something like 50dB.

The front of the case is now back in place and the knobs all re-fitted. Intially I'd tweaked the IF alignment as this may have changed following the inevitable bumping around and slight earth current chages around the tuning condenser from tightening and the use of switch cleaner on its moving parts. 

After the final tweaking, the IF was centred nearer to 500KHz (about 500.05KHz) than previously (501.67KHz) and the IF amplifiers appeared to develop 29dB above the noise floor rather than 25dB previously because I used 100Hz rather than 1KHz for RBW/VBW. The figures for bandwidth match the numbers in the B40 manual. The centre BFO frequency (used in TUNE) should ideally be dead accurate for zero-beating a signal and conveniently around +/- 1.5KHz (CW, HIGH and LOW) for resolving USB and LSB but these are not critical.


 Return to Reception