API 525 intermittent issue, seems to go away on warm up

[hereforever]

I have an older 525C module that is exhibiting strange intermittent behavior.  It came with a +/- 15 V psu that is putting out the proper power.

No issues when compression is turned off.  Assuming I am reading the schematic correctly even when compression is off audio runs through the input and output opamps as well as the output transformer.

When I first turn on the unit the meter starts wildly pegging, and the audio cuts in and out as if it were gated.  I assume there is some issue in the compression circuit (either the compression opamp or some other component), but not really sure where to start.  Curiously, if the module sits with power on the issue often seems to go away after 5 min or so.  If I turn it off and back on again it will perform normally, but then if it sits cold the issue returns until warmed up.

Any idea where to start looking?

 

[Bo Deadly]

There are some pretty high value resistors around the release time switch. I see 20M, two 15M, a 4.7M and 1M. That's high enough that some dirt or residue in those paths might cause weird slow behavior. If you have some 99% isopropyl you might try cleaning those above and below. Use a new tooth brush to agitate and a little squirt bottle to rise if you can. But it MUST be 99% isopropyl. Anything with too much water and it might into a crack somewhere and not evaporate away before causing corrosion. Then place it somewhere warm and dry and wait at least 24 hours before trying it.

 

[Whoops]

Curiously, if the module sits with power on the issue often seems to go away after 5 min or so.  If I turn it off and back on again it will perform normally, but then if it sits cold the issue returns until warmed up.

Any idea where to start looking?

Your problem is probably a broken of Dry Colder joint

Retouch and Re-heat all the solder points

 

[Walrus]

I've had 2520's that have shown that sort of behavior.

 

[hereforever]

I've had 2520's that have shown that sort of behavior.

Aha, this is sort of what I was waiting to hear....

I did try cleaning with alcohol, that didn't resolve the issue

signs are pointing to the 2520 in the compression circuit

 

[mjrippe]

More common for the problem to *appear* on warm-up.  You are fortunate if it goes away