Thread: Moreton Cheyney
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Old 22nd Oct 2019, 1:12 am   #84
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Moreton Cheyney

Thanks for posting the full circuit.

Looking at the AF side, and starting at the output end:

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V10 is a cathode follower feeding the tone control.

VR3 might be a preset gain control.

The tone control itself, VR1 treble and VR2 bass, is a Volkoff/James passive type of the era (see: https://www.vintage-radio.net/forum/...1&postcount=62), although with an unusual feature.

At first glance one would say that instead of combining the outputs of the two tone control potentiometers in the usual passive way, MC has used electronic recombination, the slider of each potentiometer feeding its own triode stage with recombination thereafter.

But that idea fails because the output of the bass triode, V12, goes nowhere except to ground. As its stands. R51, at the anode of V12, with the 4700 pF capacitor, probably forms a low pass filter to stop any treble going via this route, bearing in mind that whilst the bass tone control network around VR2 ensures that this potentiometer has no effect on treble, it does not actually block treble. One might expect the top end of R51 to be connected to a load that is common with that of V13. The likely candidate, R59 fails, because its bottom end is bypassed to ground via C29.

The treble alone gets out, from the anode of the treble triode V13. In this case a high pass filter is not needed, as the network around the treble potentiometer blocks the bass.

It is difficult to imagine that this was the original circuitry. Rather, as Tanuki has said, it has been seriously reworked. In that case, knowledgeable intent seems unlikely, or at least if so, the purpose. Is very obscure. Accidental mis-rewiring seems more likely.


Cheers,
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