Repairing an old amplifier, working out what is dead

[cjcharles]

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!

 

[Harald Kapp]

I assume something about the transformer and capacitors are distorting the readings as I get mind boggling readings in either direction across both devices.

Measuring componnets in circuit is tricky and often leads to unexpected results due to other components that influence the measurement.
Remove one leg of the diodes from the circuit so you can measure the diode alone. That is the surest way to check them.

Given the caps are big electrolytics would you expect any difference in opposite polarity?

No.

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.

Electrolytics are prone to loss of capacity with age. What you observe looks like such a failure accompanied by excessive leakage. Even if they show no obvious signs of defect like bulging caps. Changing the electrolytics is often the simplest way to revive a power supply. You can do a rough test of the quality of +V_CC and -V_CC by measuring the voltages with your multimeter in AC setting- Make sure the multimeter doe not recognize DC when set to AC (some do, some don't). If the multimeter registers DC while set to AC, add a series capacitor of 1 µF or so between multimeter an V_CC. You should now get a rading of the AC ripple. If that is significant, it is a good indicator of lost capacity.

 

[cjcharles]

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

 

[73's de Edd]

Since that is a balanced + & - supply, why not place you metering in DC mode 50VDC range . . . unless auto/ranging . . . and then probe the outer ends of the 8.2 K bleeder resistors and see, how close to 0VDC that your reading is being.
If not being so, that then gives you a clue as to which supply is being "lazy" .

 

[cjcharles]

Since that is a balanced + & - supply, why not place you metering in DC mode 50VDC range . . . unless auto/ranging . . . and then probe the outer ends of the 8.2 K bleeder resistors and see, how close to 0VDC that your reading is being.
If not being so, that then gives you a clue as to which supply is being "lazy" .

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

 

[Bluejets]

I usually find a simple signal injector useful in situations like this.
Try starting multimeter measurements at the speakers and work your way back.
Is it mono or stereo..?
Any amp model or details?

 

[73's de Edd]

O.K. . . . . .on the balanced output of the supplies . . . .we just awaiting for the brand and model number of unit now . . . . .

 

[Ylli]

I have an old amplifier,.......I now get lights on the logic board and it is able to detect signals (auto-turn-on),

A disconnect here. If it is an 'old' amplifier, it wouldn't have a 'logic board' or auto turn-on. Those would be features of a relatively new amplifier. An 'old amplifier' would have tubes, or germanium transistors, of at least no digital parts. J/K

 

[cjcharles]

A disconnect here. If it is an 'old' amplifier, it wouldn't have a 'logic board' or auto turn-on. Those would be features of a relatively new amplifier. An 'old amplifier' would have tubes, or germanium transistors, of at least no digital parts. J/K

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!

 

[cjcharles]

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.