Moreton Cheyney RF Repairs

General circuit tracing and RF fault finding

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.

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.

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.

See the results of the repair to PA2

 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....

 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?

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

BANDWIDTH SWITCH SETTING	1	2	3	4	5	6
BANDWIDTH IN Kc/s	5	8	11	15	18	TRF

 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. Later, I connected the workshop audio amplifier to the receiver and the results were really good. Everything now works but I'm not sure about RF alignment. It's difficult to check image reception because of the vagueness of the dial which is designed to give the user the location of broadcasts rather than exact frequency settings. To complete the alignment requires a chart to supply dial settings as my signal generator doesn't use wavelengths. The number of degrees is the angle from the dial end to the first calibration mark with 90 degrees for a vertical pointer.

 I spent an afternoon experimenting with the receiver and discovered a number of basic problems. The coils for long, medium and first shortwave bands are difficult to align with the dial markings and I suspect that it is virtually impossible to track the local oscillator to the dial calibrations. It seems also impossible to align the RF amplifiers. Because of the large amount of gain it may not be too important, other than undue sensitivity to images. It's quite llikely that the dial was marked up from a prototype receiver and little by way of adjustment carried out in the factory except perhaps to check the dial settings for the major British stations such as Droitwich. Oddly, when scrutinising dial markings I noticed two different markings included 15Mc/s, these being 20m and 30m. The later of course should have been marked 10Mc/s. The local oscillator was so far out on some bands I needed to use a portable receiver to identify the local oscillator frequency. As an example, with the dial set at 60m or 5Mc/s the local oscillator coil could be tuned by its dust core from 4.8 to 5.1MHz. The correct frequency should have been either 5+0.465MHz=5.465MHz or 5-0.465MHz=4.535MHz so it seems impossble to set the local oscillator to either frequency. In fact the dial markings appear to show the local oscillator frequency. A solution would be to add a small trimmer capacitor across the coil to then permit the local oscillator to track below the RF tuning. At the 5Mc/s marking the oscillator could be tuned to 4.535MHz.

The two highest bands are very difficult to adjust because of the positioning of the coils, but when I checked, the local oscillator frequencies at various dial settings seemed to be approximately OK.

I was able to roughly adjust the longwave dial settings by setting the Irish station on 252KHz to just below 1200 metres. Because of the very wide IF response combined with high overall gain and potent AVC tuning is exceedingly broad for the strongest broadcasts. Medium waves also were not too far from their markings accommodating a range of 1600Kc/s to about 550Kc/s, although dial readings are rather vague at their ends.

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

Review of Moreton Cheyney AVC System

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 Now.. for the matching amplifier/power supply

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