Strange 1176 Rev D MNATS Issue

[completelytorqued]

Working on a 1176 Rev D with an issue I haven’t ran into before.

Unit calibrates, works fine, but over time (aka, with heat) gain reduction goes away. Engineer who it belongs to complains that as the session goes on, the “compression” effect goes away and it just sounds like the “dry” effect.

Unit will operate within spec, but when I put a heat gun on the board (namely near the input amp section) I can recreate this issue. Interestingly, when the signal gen is disconnected DC voltages don’t seem to fluctuate much with heat. This makes me think what’s actually happening is heat is making the input level drop.

Leaky cap? Failing fet? Wanted opinions before I pull out the parts cannon as I’m feeling a little lost! Tested basically all caps and couldn’t find any leakers.

Also if anyone knows where the hairball Rev D test points correspond to the MNAT’s schematic, that would be super helpful.

 

[scott2000]

Leaky cap? Failing fet? Wanted opinions before I pull out the parts cannon as I’m feeling a little lost! Tested basically all caps and couldn’t find any leakers.

You're asking then telling you already tested?
Are you unsure of your cap testing? How did you test them?

 

[completelytorqued]

Tested capacitance with and without heat being applied and observed no change. I don't have an ESR meter, but should I repeat this test measuring resistance to get a rough idea?

 

[scott2000]

Tested capacitance with and without heat being applied and observed no change. I don't have an ESR meter, but should I repeat this test measuring resistance to get a rough idea?

I'm assuming you mean the capacitance is within spec and it doesn't change when heat is introduced?
Hard to not think they're ok but it's confusing when you mention that when the signal generator is disconnected, the dc voltages don't change with heat. With the signal generator connected, do the dc voltages change with heat?
Afaik, heat dropping the input level would affect more than just the sidechain? The complaint would include lower level?
Interesting..
Let us know what you find out. How are all of the voltages everywhere?