Moreton Cheyney Receiver AVC System

 Having succeded in hearing broadcasts on medium and long waves, and having struggled to understand the automatic gain controls operating within the receiver, I decided to review and correct deficiencies in the circuitry. One problem I discovered is the condition of the Westectors at X1 and X2. So I could progress I replaced both of these with OA91 germanium diodes, but having considered the forward voltage characteristics of the Westector I need to further check for a better substitute. The W6 has six copper oxide elements and if each drops say 200mV when forward biased, this will amount to 1.2 volts. The OA91 will have a drop of say 200mV so the careful specification of the MC circuit will not do exactly what the designers intended. Basically the W6 requires enough AC to develop a rectified voltage of 1.2 and once this is passed will produce the intended AVC control voltage. Essentially this is what is referred to as "Delayed AVC"... or in other words, AVC action is suspended until a broadcast signal of sufficient strength is tuned. The OA91 may be so sensitive as to develop AVC action by rectifying mere noise.

However, the MC circuitry has components in place to provide a delay so the OA91 will not produce AVC control too prematurely.




 Above is one of two Westectors, marked "6" located inside the can of IFT4. Data on these is fairly sparse bit a WX1 has a rating of 6 volts RMS and a forward current of 100uA and is good up to 1.5MHz. The WX15 can handle 90 volts so a 6 element device will handle around 36 volts RMS.

 There are two types of volume control (nowadays referred to as gain control) within the MC receiver, normal AVC which is always active plus what they call QAVC which is intended to quieten the receiver when tuning between broadcasts. Both AVC and QAVC use a Westector, which I've labelled X2 and X1 respectively. As QAVC can be switched off, I'll first consider AVC, but as the two circuits are interlinked, what happens within the set when QAVC is grounded? V6 cathode is grounded via R23 and R24, X1 cathode is grounded and V8 is turned on slightly more than it was previously, but has no effect on the receiver. VR5 is disabled and the QAVC end of R55 is grounded, but having no measurable affect the AVC circuit.

 AVC is produced by X2 which is reverse biased by R49/R50, developing a little over 3% of the HT supply voltage, or 8 volts with 250 volts of HT. This means that AVC should not be active until enough RF is applied to X2 anode to overcome the delay bias plus the forward drop within X2, say 1.2 volts or a total of a little over 9 volts. With an OA91 at X2, this would be about 8 volts.

With no signal and with QAVC turned off, the AVC system should be inactive unless there is a lot of noise present, say from the aerial or general valve hiss etc. If you examine the circuit you'll find V1, V2, V5 and V7, are used for AVC, with broadcast signal amplification using V1, V2, V5 and V6. I can only assume this difference would allow for a different response, perhaps a less broad response for AVC (and QAVC). An interesting side effect would be to miss AVC action completely if IFT4 is mis-tuned by a few KHz.

As per usual practice AVC is applied to the RF valves, V1, V2 and V4 plus IF amplifiers V5, V6 but not V7 (the AVC/QAVC amplifier). This will mean that all those valves with AVC control have to be variable mu (such as the 6K7 type and V7 non-variable mu (such as a 6J7 type).
 Turning to QAVC. This is provided by X1 whose cathode is wired through R38 and VR5. This is a low impedance path so will need sufficient current to be generated to have any effect on the system. V8 is the QAVC amplifier and this can supply a decent current to drive the AVC circuit to a level sufficient to reduce the gain of the RF and IF amplifiers when a broadcast is not present. VR5 is available as a preset control to set the inter-station background noise level to a level which allows the user to hear stations and not to hear internally generated valve noise or general annoying electrical noise. How does the QAVC system work, other than modifying the AVC line via R55? Within V8 are a pair of rectifier diodes which are reverse biased through R23, R24, R38 and VR5 which together place a small positive voltage on V8 cathode. This is primarily governed by the cathode currents of V6 and V8 but adjustable via preset VR5. The bias voltage is stabilised or maintained by C13 and is added to the normal AVC voltage supplied via R82. Once the reverse bias is overcome by a signal of sufficient level present at the secondary winding of IFT3, demodulated audio passes through the low pass filter comprising C77/R72/C78. If the receiver is used for weak signal reception QAVC can be turned off at S1, the Gram/Radio selector switch.

 A quick check of some voltages revealed much as expected. With no aerial connected the IF amplifiers were getting around plus 0.5/1.1 volts of bias with the QAVC turned off/on. The AVC diode was cut off at about 8 volts, the QAVC diode was reverse biased at 22 volts and with QAVC off at -0.3 volts. Alignment should be performed therefore with QAVC off and at a test signal input level below that required to activate X2 (about 8 volts). Because of the high impedance of the audio output circuits an external audio amplifier will be necessary. Alignment of IFT4 would need to be carried out with a test signal of 465KHz and ideally a spectrum analyser to narrow the response of the coils compared with IFT1/2/3.

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