Quote:
Originally Posted by Craig Sawyers
It is not such a bad idea - pin 2 goes high on mono, resulting in about 1.4mA of collector current in TR101. Quad fitted several types of transistor depending on availability, with BC550C being an equivalent. That has a beta of 400 - 800. So it will pull a pretty insignificant 3.5uA to 1.75uA from pin 2 on mono.
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I was of the impression – perhaps wrongly so - that the separate mono output, via emitter follower Tr101, was independent of anything that was happening in the stereo decoder IC (MC1310) - available regardless as it were. And that the DC conditions at pin 2 (MC1310 input) were not conditional upon what mode the decoder was in.
Quote:
Originally Posted by Craig Sawyers
Having met the guy who designed the FM3 (the late great Mike Albinson) he would have thought that through long and hard before deciding on that way of doing mono.
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I had wondered who had designed the FM3. Was it Mike Albinson’s first major job at Quad? I think that Quad had been without a designated RF person since John Collinson had left, which might be why the FM3 was somewhat late in appearing.
In the original version, the independent mono output was obtained by summing the decoder L and R outputs (taken ahead of the MPX filters) in an emitter follower. Muting was done in the decoder, from the externally generated muting signal. In the second iteration, it was taken from just ahead of the decoder input, and after the muting transistor Tr100. I’d guess that the use of a separate muting transistor allowed the muting to work on mono as well as stereo. The muting circuit itself was quite intriguing in that it was a noise muting circuit in tandem with the tuning indicator circuit, with the latter also providing deviation muting.
Cheers,