BAE 1028 - BA283 amp card makes the input attenuator pop!

[Golgoth]

Hi guys,

I swapped the stock BA283 card in my BAE 1028 for a vintage Neve one.

When switching through input gain positions with the attenuator, the preamp is okay with the stock BAE card but I'm getting very very loud pops with the vintage one (converter is clipping).

I took DC measurements at a dozen points (trimmer, 3055, most caps, transistors) and compared both cards, the values are very similar.
The vintage Neve card sounds just fine and like the stock one so I guess it's working pretty much correctly.

Where should I start looking?

 

[MagnetoSound]

Leaky input cap?

 

[Golgoth]

Would that be C1?

 

[MagnetoSound]

Would that be C1?

Yes, pad U.

 

[Golgoth]

Yes, pad U.

That fixed the issue for the "line" steps, the problem is strangely still present on the "mic" steps..
Impedance issue somewhere that could be unrelated to the card but showing as a symptom there?

The preamp has been behaving strangely, for instance the output pot gives a major pop in the first 1/4 section when turned, even on an "off" position of the input switch..

 

[MagnetoSound]

Use the same logic to find the input pops at MIC gain settings.

If the 1028 conforms to standard Neve gain configurations, there is another input cap for the MIC gain stage, and a feedback shunt cap (to ground via the GAIN switch) for the higher gain settings. Chances are these might be leaky too.

(I am not familiar with the unit in question, so the output pops might be something similar, or not.)