[R-390] R-390a modified IF sub-chassis

[Jacques Fortin]

Larry and Gary,
I believe that the "mysterious module" is a reed relay.
The coil should be activated thru the 33k, 2W when the BFO power is on and the relay contacts exchange the AM detector and the SSB product detector outputs going to the limiter stage.
Something is still not clear with the schematic provided by Gary, because it cannot be seen how the AM or SSB audio goes to the limiter input... ??
Anyways...
Using a relay there prevent the mainframe/IF module wiring changes to carry the AM detector and SSB demodulator outputs to the front panel to be selected, as well as the required change of S101.
In other words, using a relay within the IF module makes the modified/unmodified IF modules interchangeable within the R-390A "stock" mainframes.
I implemented something similar in my own R-390/390A by using a small relay with a 24V coil (coil resistance 1.8 - 2.0k) connected in place of R531.

As we are there, using a 1000µF to develop the - DC bias source is overkill: a 100µF @ 40-50V is way sufficient.
If anyone still fears to introduce "hum" in the audio in SSB mode that way, a small negative 24V regulator (or more simply a Zener) can be used there.

73, Jacques, VE2JFE