MTX 1501d - U115 blew within a second

[valterdaw]

My friend managed to kill MTX 1501d amp one more time. I repaired this amp before (rebuild power supply, badly burnt and outputs), but this time it's something different. Amp powers on and plays but distorts badly. U115 has a hole in it All surrounding resistors fried too. I replaced all that resistors and new U115 still emits magic smoke a second after turn-on. Please, give me advice where to look at. I have no idea what U115 suppose to do Thank you!

[Perry Babin]

Which IC is U115? The designations are under most of the ICs so you can't see it until you pull the IC off of the board.

[valterdaw]

Yes, that's the case. Place for it is in the center of the following pic. Thank you!

[Perry Babin]

Can you confirm that pins 4 and 8 are directly connected to pins 4 and 8 of the op-amp near the RCA jacks?

What's the DC voltage on pins 4 and 8 (black lead on chassis ground terminal)?

[valterdaw]

Voltage is -15.6 and +16.5; also capacitor c119 get's hot. And 4 & 8 connected to corresponding pins of opamp near RCA's. I noticed that two bloated output(?) capacitors close to speaker terminals are getting warm too. I replaced these, unfortunately with 20% less capacitance, as I don't have needed ones in hand.
After that, U115 still runs hot, but would not burn Amp works, but has mighty distortion in output. I appreciate any help!

[Perry Babin]

When the capacitance is too low on the output filter, the voltage of the square wave on the output of the inductor can be higher than the voltage driven into it. I've seen more than double the rail voltage on the output of the inductor in some amps when the caps were damaged.

U115 and the surrounding components are part of the compressor circuit. The output signal is fed back into the comparator via C114 and R114. Since the circuit is not designed to operate with anything higher than rail voltage, it caused it to fail.

You'll need to replace the caps with the correct value (higher value caps would be OK for testing). to determine if there's another fault. The caps are in series/parallel so you need to replace all of them.

C119 should not be running hot unless the output of the op-amp is high (near the 15v regulated supply voltage). If the output is high, you should replace the output filter caps and confirm that there is not more than ~1v of high frequency ripple across the speaker terminals before you do any more troubleshooting in this circuit.