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Repairing an old amplifier, working out what is dead

Discussion in 'Power Electronics' started by cjcharles, Nov 24, 2020.

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  1. cjcharles

    cjcharles

    13
    0
    Sep 10, 2016
    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!

    upload_2020-11-24_15-49-47.png
     
    Last edited: Nov 24, 2020
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,833
    2,442
    Nov 17, 2011
    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.

    No.
    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 +VCC and -VCC 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 VCC. You should now get a rading of the AC ripple. If that is significant, it is a good indicator of lost capacity.
     
    cjcharles and Martaine2005 like this.
  3. cjcharles

    cjcharles

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    Sep 10, 2016
    Super useful, thank you! I will go and have a play!!
     
  4. 73's de Edd

    73's de Edd

    2,999
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    Aug 21, 2015
    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" .
     
  5. cjcharles

    cjcharles

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    0
    Sep 10, 2016
    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
     
  6. Bluejets

    Bluejets

    4,424
    938
    Oct 5, 2014
    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?
     
  7. 73's de Edd

    73's de Edd

    2,999
    1,261
    Aug 21, 2015
    O.K. . . . . .on the balanced output of the supplies . . . .we just awaiting for the brand and model number of unit now . . . . .
     
  8. Ylli

    Ylli

    346
    97
    Jun 19, 2018
    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
     
    Martaine2005 likes this.
  9. cjcharles

    cjcharles

    13
    0
    Sep 10, 2016
    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!
     
  10. cjcharles

    cjcharles

    13
    0
    Sep 10, 2016
    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.
     
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