Diagnosing heat-related problem with amp

[valterdaw]

Amp in my own car (PG Titanium, 4-channel) yesterday developed a problem with shutting down right rear channel after amp reaches "normal" operating temperature. It starts producing very quiet and extremely distorted sound with infrequent loud pops. All other channels works as they should. If you let it cool down, this channel will start working normally until it heats up.
I have two questions about that:
- what is the right way to diagnose this problem?
- what elements are likely to be in charge for this behavior - semiconductors, or it can be a capacitor?

I appreciate your help. Thank you!

[Perry Babin]

If you disconnect the right rear channel (at the amp's speaker terminals) and connect a different speaker (not using the wire that was used for the RR speaker), does the amp have the same problem?

[valterdaw]

Originally posted by Perry Babin
If you disconnect the right rear channel (at the amp's speaker terminals) and connect a different speaker (not using the wire that was used for the RR speaker), does the amp have the same problem?

Tried today connecting only left rear speaker to this channel, and it behaves the same Heat gun/hairdryer? What parts are most commonly responsible for this?

[Perry Babin]

Many things can cause this but the most common are...

A leaking output transistor will act like a resistor tied to one of the rails. This tends to pull the output to the rail. Since the amplifier uses negative feedback, it compensates by driving the other output transistor until there is no DC offset. This causes excessive power dissipation and therefore excessive heating.

Another possible problem is a bad connection on the biasing transistor.

Either of these problems should cause excessive idle current.

Oscillation can also cause overheating. The frequency of oscillation may be above audibility so you'd need to check the output with a scope to see if it's clean.