[Synchrodyne]

Originally Posted by allan

I've just added a note on the full circuit to the effect that VR3 is ganged with VR4.

OK, so the volume control was done by tandem potentiometers at two points in the circuit. Sometimes this was done to steer clear of overload on the one hand and poor signal-to-noise ratio on the other.

Originally Posted by allan

VR5 is some sort of preset gain or AVC depth?

It might also be to set the muting threshold for the QAVC system.

Originally Posted by allan

I suspect the anodes of both V12 and V13 originally went to the rear connector via 2700pF capacitors, one of which is now grounded.

Possibly the two outputs were simply combined after the DC blocking capacitors.

Then maybe the 4700 pF capacitor at V12 anode was not part of a low-pass filter. It could be that V12 and its load served in place of the standoff resistor that was normally used in this kind of tone control in order to avoid the bass control having any material effect on the treble. V12 and V13 would in part have made up the loss of 14 dB or so incurred in the passive tone control networks.

Originally Posted by allan

I had assumed that the outputs were in push pull but maybe the external amplifier has the combining stage followed by a phase splitter. There are two EF39s and two KT66s on the amp chassis.

Possibly one EF39 as driver and the other as phase splitter?

Originally Posted by allan

Whoever made the modifications didn't possess a decent soldering iron as many wires fell off during the circuit tracing and wafer 3 of the bandwidth switch may have had its solder tags mis-identified. See Bw3/2 which shorts out the anode resistor. Can this be right?

With R46 shorted out, V10 anode is still looking into R48, the anode resistor of V9, which adds the outputs of the two valves. The purpose of R46 appears to be to reduce the amount of the V10 output that is added to the V9 output at certain selectivity settings. I am not sure about C32 – with the anode resistances it would seem to form a low-pass filter, but presumably with a sufficiently high turnover point not to adversely affect the signal.

Originally Posted by allan

I'd have expected a ground connection in the circuitry around the cathode of V10 but the resistance to ground is too high for one to be (now) present.

V10 cathode looks to be connected directly to ground. But there is no apparent DC return path for the grid.

I have the impression that the MC designers may have had more RF than AF expertise, perhaps not unexpected if their background was WWII military radio. With AF they may have used circuits from various available sources. For example, treble and bass tone controls with electronic summing had been introduced by Sound Sales by the beginning of 1946. And I think volume expansion circuits had been published in Wireless World.

Be that as it may, it looks as if whoever modified the circuit might have gotten lost in the complex wiring. It would be difficult to develop a sound rationale for taking the output from V13 alone.


Cheers,