Help me with Rockford 500bd

[valterdaw]

I got myself a new toy - Rockford Fosgate 500bd. And, as usual it happens with all Rockfords and me, total disaster.
Amp in protection mode. Power supply works, on twisted pair is ~75 V, on half of outputs - full 75V, on other half - zero. All outputs seems to be fine and no shorts in driver IC's
U17 illuminate protection LED, it's pins are at:
pin2: 2mV
pin3: 1mV
pin6: -0.4V
And it seems to be I miserably fail to find out why pin3 is that high
I appreciate any advice! Thank you!

[valterdaw]

Kind of fixed it
Q120 is a protection LED (and shutdown signal) control, taking signal from U21. U21 pins 5 & 6 were different by absolute value in 1 volt Next, U16 is under suspect... due to not having anything to replace it with, just soldered 2k resistor between pins 5 & 6 of U21, I think desensitizing or even disabling protection at all. Amp is playing... But need further thinking...

[Perry Babin]

Measure the DC voltage on C51. Place your black meter lead on the ground terminal of the amp. If either terminal is below 0v, change U21. You will need to remove the resistor you added.

[valterdaw]

That's amazing! It was far below 0 on C51. Replaced U21... and amp good as new!!! When turning it on, protection light blinks, then goes away.
Thanks a lot, Perry! You saved me once again... What is your PayPal??? I need to know!

[Perry Babin]

That's a common problem. The negative power supply leaks to the inverting input of the op-amp (inside the op-amp).

If you work on lots of the BD amps, you'll likely see it again.

[valterdaw]

Three days ago and today was not that warm outside, and my poor Fosgate showed very similar problem:
When it's colder outside, it turns on, and then shutdowns into protection after 1 - 10 seconds of working. When you restart it (off/on on turn-on wire), it will do it again, working fine for 1-10 sec... If you restart it many times, it finally starts to work, and when completely warmed-up will never go into protection. In warm days it always works terrific. Help!

[Perry Babin]

Are you sure that you don't have a bad solder connection on the op-amp you changed?

If you can't get the amp to misbehave on the bench, throw it in the freezer for a while to cool it down to the temperature where the problem will show up.

I'll assume that you checked the voltage on the op-amp you changed and there is no negative voltage on the capacitor (as there was before).

[valterdaw]

You were right, Perry! Problem was related to carbon, resulting from overheated iron while soldering op-amp. Actually, I had a similar case before, but completely forgot about it. Cleaned everything with alcohol and needle, and it works! Thanks again!