Returning to the analogy I started with, this circuit is the bear's lair, where it drags its kill and munches leisurely. I took the schematic and removed nonrelated portions to isolate it.
The purpose of this little circuit is to generate a quadrature (90º phase-shifted) version of the sinewave test signal, from 10Khz to 10MHz in 1-2-4 sequence. No small feat. (Nowadays, with a fast-enough DAC, you'd probably just do it digitally.) The operation of this A2 modulator board (especially at high freqs) depends on it generating a precise 90º shifted version.
I do not understand how this little circuit works. I've never worked with the 1496 modulators before. The manual doesn't describe the circuit in detail, but says to verify that the output at TP3 (lower right) is precisely 90º offset from the 0º input at TP5 (center top) at all freqs.
And it isn't. It's more like 150-160º, not quite inverted, over the whole range. So the circuit is generating a constant phase offset, just not the correct one.
In case of error, the manual says to check U1 (a 1458 opamp) and U2 (a MC1496 mod/demod). Since I have those parts, I just replaced them rather than figure out if they were good or not. Unfortunately, that made no change.
Before going further, I checked all active devices (except Q10) and capacitors and saw nothing wrong.
Overview: The test signal enters upper left "Osc in", is xfmr-isolated and goes through an AGC leveling amp. This is the in-phase reference signal. It feeds the 0º detector and phase-tracking, and is the input to the 90º shifter. It is buffered and drives xfmr T3. One side of T3 has two capacitors that are switched in by comparator U10 depending on the freq range, which is controlled by logic coming from another board. I verified that this part works properly.
Within each decade range, the signal coming from U2 & U1 controls the phase offset via the mosfet Q10. I do not know how to debug this. But it appears to me either T3 or Q10 are the faulty parts. If I see opposite-phase signals on either side of T3, it should be OK. Will check it today.
But the main suspect is Q10. This part is made by Sony and marked as "429D". The only info I find about it is:
www.ozvalveamps.org/repairs/circuits/stomps/tx-429d-datasheet.pdfIt appears to be unobtanium.
I welcome any suggestions.