Repairing an old amplifier, working out what is dead

I have an old amplifier, that should be decent, but I was getting no signs of life so thought I would try a repair for fun.

I managed to trace down a dead IRF740 MOSFET in the power supply board and replaced it. I now get lights on the logic board and it is able to detect signals (auto-turn-on), however still no output from the amp.

After finding a circuit diagram and checking a few voltages I found that +VCC and -VCC seemed to be working in the power supply board, and hence assumed it must be the amplifier chip which is dead. A TDA8924 is connected via nothing but smoothing capacitors to +VCC and -VCC on the power board, and this chip seems a common failure. I dont have an easy way to test if the logic board is outputting an audio signal to the power board as I don't have a scope, but after the logic board it is basically just smoothing and TDA8924 before the output.

While that replacement is enroute I thought I would check a few more devices and was surprised to find some diodes (D3 and D13) not behaving as I would expect. I assume something about the transformer and capacitors are distorting the readings as I get mind boggling readings in either direction across both devices.

Can I ask what you would expect to measure for these? Seems the voltage slowly grows from about 0.1V on my multimeter (diode mode) over time (due to the caps).... in both directions, though stays at 0V for a good few seconds first.

Given the caps are big electrolytics would you expect any difference in opposite polarity? Is it possible to gleam anything useful from measurements without removing the diodes?

Thanks in advance, its been fun getting back to circuit repair!

 

Super useful, thank you! I will go and have a play!!

 

Sorry Im not sure I follow this step? I do have an auto ranging meter, I did find that the +/- voltages were virtually identical numbers (+28.43 and -28.41), so I think fairly symmetrical....? Just not sure about ripple

 

Haha, thats totally fair! An old in that its 15 years old, but not old in the grand scheme of things (or even old in my scheme of things

The amplifier is a subwoofer amplifier (Monitor Audio SD-12 for a BRW10) and is single channel output, though has dual channel inputs to the TDA8924 (that seem to be balanced across ground somehow - don't have that part of the circuit diagram and I didn't bother to trace it out). It does seem that a signal injector is needed, or I need to pick up my old (actually old Tektronix from 1970's) oscilloscope and see if that is still working (been in storage for 10+ years)!

Will report back when I have it, though I think I'll need to wait until lockdown ends first!

 

I replaced the TDA8924 and I'm pleased to report that everything is now working again!

It looks like the IRF740s failed somehow and took out the TDA, I guess, as they were the only failed components.