Marantz 2325 Amp Offset

[Rambling Man]

Howdy!

Covid has finally given me the time needed to re-cap my 2325.
I had a great time combining my soldering skills with the joy this receiver brings to me.

After re-soldering/re-capping each board I would check all features for functionality.
My only issue arose with final adjustments. The bias for both channels is all over the place. The voltages were set on the power supply and amps after a 30+ minute warm up; set to Aux and volume at zero.

Offset readings range from 4.3 mV DC to -5.4 mV DC averaging .01 mV. I settled for the best average I could manage. When an amp gets “dialed in” the voltage tends to shift further negative over time.

Is it common to see such fluctuations?
Should I start by replacing the transistors at H701 and H702?
My main goal is to not replace every part as that’s a rabbit hole I prefer to not travel.

The only modifications made was to the PE-01 tone control board. The BA312 IC’s were swapped to the Sparkos Labs BA312-M modules... sacrilege I know!

These readings were read with a Fluke 87V on the min/max setting.

Thoughts?

Thanks,
Nick from Texas

 

[elnaldo]

Hello. Offset of 5mV is fine. Positive or negative. Fluctuations are also normal in many circuits. That's a kind of "servo" that reacts to DC at the output and tries to correct it. It should stabilize at some point, but I've seen those fluctuations very often in different amplifiers. Seems it's the time needed by the circuit to react to small changes.

The problem you describe, DC going to the negative side once set, makes me think is an imbalance at the differential pair transistors or related resistors. Seems the circuit is "correcting" the offset but biased a bit to the negative side. You could start swapping the differential pair, just swap the position of those transistors. Perhaps one of them is conducting a bit more, and swapping them the offset is corrected.