Summit Audio MPE-200 EQ output popping

[Nanitlig]

Hey Everyone:

I'm looking for some help finding a gremlin in a Summit Audio MPE-200 Mic Pre/EQ.  The second channel EQ output has a problem with periodic noise and popping.  It almost sounds like a static build up that begins to fizzle and get louder until it finally pops, as if it's discharging.  After it pops it is fine for a while until it starts to charge up again.  There's no rhythm or timing to it.  Sometimes it happens right away when it's cold.  Sometimes I turn it on and it's fine for a couple hours.  This does not happen out of the Mic out, so I believe the problem is in the EQ or output amp section.  All power rails are clean and at the correct voltages.

 

[pucho812]

IIRC the unit has some discrete opamps in it. Hasve you tried swapping them to see if the problem follows the doa. It's a 990  IIRC

I would also look for a cold solder joint or anything out of place. Since you have a working channel you have one to compare it to.

 

[Nanitlig]

Thanks!  I swapped the EQ Ch. A/B output transformers and swapped the op amps.  The static/pop switched sides.  I swapped op amps back.  I think it may be the output transformer.  Does that seem like a symptom of a transformer on the fritz?

 

[pucho812]

static pop and switch would not be a transformer at all. Possible cold solder joint or bad opamp...

 

[Nanitlig]

I swapped all the opamp cards and filter cards from Ch.B to Ch.A and the noise followed.  Swapped each piece back one at a time until the noise went back to Ch.B.  It ended up coming from the shelving EQ filter card.  Compared measurements against the Ch.A card and found three open caps.  Replaced those and it's been running for a few hours with no pops or fizzles.  I think I found the gremlin.  Thanks for your replies!