Mainline distortion after coming out of storage [resolved]

[dd076990]

A few months ago I had to pack my Mainline amp into storage since I was moving. Amp was sealed in a box + bag and the box didn't have any visible moisture or drop damage. I recently took it out to discover the amp giving off small amounts of distortion when it has signal, kinda like a guitar amp going into a slight crunch. Same sound on all music types. Dead silent with no signal with the fine and coarse controls at 0 (or any other position).

Things I already tried:
1) replaced the original tubes with new ones - no change
2) checked all the resistance measurements - passed all of them
3) tried moving the amp to a different power socket - no change
4) tried feeding it signal with various DACs and PCs - no change
5) tried different power & RCA cables - no change
6) re-checked the voltage and bias measurements - passed & output below

• +275vDC on PSB = 273.7v
  +6.3vDC on the Power Supply Board = 6.29V
  IA on the A side C4S Board = 273.1V
  IA on the B side C4S Board =273V
  Breg Regulator Board (both sides) = 219.8V & 221.5V
  -reg Regulator Board (both sides) = 0.4mv & 0.3mv
  Kreg Regulator Board (both sides) = 10.58V & 10.67V
  
  T20 = 144.6V
  T30 = 144.8V

7) Tried with variety of headphones & all possible balanced/unbalanced & impedance combinations - no change

Any ideas?

Pictures in case you need them (can't link directly) -> imgur.com/a/2hn1c

[Doc B.]

Does the distortion get worse as the music signal gets higher? Or is it more or less the same at any level?

[dd076990]

The distortion gets worse as the signal gets louder. Very slight but still there on the low parts of a classical music piece for example, but glaringly obvious on the loud parts and anytime vocals are involved.

[Paul Birkeland]

In the process of moving the amp and storing it, you've likely disturbed a solder joint that wasn't soldered or maybe was a cold joint to begin with.

If the distortion is on both channels, then I would be looking at all the black ground wires and inspecting all of those solder joints.  A flaky joint will allow the DC voltages to all look right but will do strange things to signal current.

[dd076990]

Just re-soldered pretty much every ground connection I could get access to. Seems to have fixed the distortion Thank you both for the help.