Yamaha M80 repair

Discussion in 'Yamaha' started by bobok7787, Dec 19, 2018.

  1. bobok7787

    bobok7787 New Member

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    Working on a friends M-80 that was re-capped and had the infamous glue completely cleaned and all local corroded components replaced. It was working fine until recently, when the left channel went out.

    First thing I did was unsolder all 8x output transistors to test out of circuit, all tested okay using diode test function on DMM. I also tested the other 8x power transistors on power boards 3 and 4, all tested okay. I also tested the driver transistors on the power boards, and the bias current transistors on the main board, all tested okay.

    Second thing I did was solder in 470 ohm 1/2 watt resistors across the bases and emitters of all output transistors per this awesome thread: http://audiokarma.org/forums/index....-if-you-are-servicing-a-yamaha-m80-85.591887/

    I then powered on using a DBT with a 100 watt bulb, it powered on perfectly! The bulb dims and all is well. I tested base voltages at all output transistors, which measure around 1 volt with out any load. Collector voltages are +/- 39.5 volts.

    Thinking I was clear for takeoff, I re-soldered all output transistors, powered on DBT and the bulb shows a dead short. If I remove the RP/LP power wires, there is no short and it powers on fine. I also tried removing one power feed at a time, both channels show a short. Should of left the working channel alone, I regret that decision!

    Any idea on where to start? I've inspected my solder connections up and down under microscope, I've tested for shorts between the collectors and ground. Could my meter be giving me a false sense that the output transistors are okay?

    Thanks in advance for your help, here are a few pictures to reference!
     

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  2. rottalpha

    rottalpha Yamaholic Subscriber

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    that is a mind bender!

    based on what I hear so far, you have a "lying" transistor, a backwords cap that finally geve up or another faulty component. Post some close-ups of the transistors.

    Look for fakes or NTE parts. It is possible that the unit was worked on for other reason than just to clean the glue and a recap ;) This of course it is just a hunch...I would wait for the mighty eagle keeper, our Yamaha resident expert to chime in.
     
  3. avionic

    avionic " Black Knights " Subscriber

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    Look over the main board ( both channels now) for burnt fusible resistors. Greenish in color mounted standing on end. Look for darkened bands around there centers. Check soldered connections on the 8 VAS transistors.( 4 of the 8 will have a fusible resistor soldered to the base lead.) Check the resistance of those 4 fusibles. (47 ohm)
     
  4. bobok7787

    bobok7787 New Member

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    Thanks avionic! This is on my to-do list tonight!
     
  5. bobok7787

    bobok7787 New Member

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    Finally back to this after the holidays and got to checking all VAS transistors and fusible resistors on the base leads... All checked okay!

    I did notice a few of the small stacked film capacitors around the 2x large caps in the amp section looked "blackened", I pulled C129 and C160 and both measured open. I checked them with a DMM and a component tester, both gave the same result. Could this be what's causing the short?

    Here are a few close-up pictures for reference. 20190101_233833.jpg 20190101_233839.jpg 20190101_233841.jpg 20190101_233847.jpg 20190101_233854.jpg 20190101_233926.jpg
     
  6. bobok7787

    bobok7787 New Member

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    I also checked D129/D131, and D130/D132 (in circuit). D129/D131 measured ~0.6 volts forward, and ~2.35 volts rearward. Interestingly enough, D130/D131 measure ~0.6 volts forward, and nothing rearward? Not sure what the correct measurement should be while these are in circuit, I will pull them tonight to check.
     

     

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  7. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    That second measurement has the scent of something in-circuit bypassing the diode.
     
  8. bobok7787

    bobok7787 New Member

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    Thanks EngineerNate! To clarify, the 0.6V forward/2.35V rearward is the scent of something in-circuit bypassing the diode?
     
  9. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    That's what I usually expect when I get some random voltage above 0.6V or so reverse biasing a diode. In my experience if the diode has really been cooked it'll either read open or short.
     
  10. avionic

    avionic " Black Knights " Subscriber

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    [​IMG]

    Mica caps not stacked film.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    D129-132 1SS82 -- BAV21
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

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    [​IMG] Did you install that TO-220?
     
  13. avionic

    avionic " Black Knights " Subscriber

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    Does this unit have a "legendary" sticker on the rear panel? It looks like Skip's work.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    Not a good idea.Need to be removed in + and - pairs if your going to remove them.It will slam the circuit to positive rail if you just remove the negative power lead or vice versa if you just remove the positive power lead.
     
  15. bobok7787

    bobok7787 New Member

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    My mistake!

    I think the light is reflecting funny off the top of that transistor in this picture, it is actually a TO-126.

    It does! It looks like he did re-did the binding posts too, very clean work.

    I did remove +/- pairs at a time, sorry for the confusion.
     
  16. avionic

    avionic " Black Knights " Subscriber

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    Was it in protect mode?
     

     

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  17. rottalpha

    rottalpha Yamaholic Subscriber

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    :whip:

    "Legendary Amps" work that needs re-work soon thereafter is now......Legendary
     
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  18. bobok7787

    bobok7787 New Member

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    No, from my understanding it was not in protect mode and could play from the right channel.

    I have new mica caps on order, hoping this does the trick!

    I also ordered some 1K/5W resistors to solder between the collector and the emitter of the power transistor pads to simulate the bias current, I will install these with 470 ohm resistors between base and emitter like I did before so I can rule out the output transistors.
     
  19. bobok7787

    bobok7787 New Member

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    Finally got around to working on this again. I installed the base/emitter and collector/emitter resistors in place of the output transistors. Powered on DBT and adjusted bias, no shorts! I re-installed the output transistors, powered on DBT, no shorts, relay clicks, and it doesn't go into protection! Powered on mains voltage, bias was easilty adjusted to 15 mV. Hooked up my phone and played a little music, it plays perfect.

    But, the right channel sounds noticeably quieter (both channels adjusted to the same volume). I checked output transistor voltages and found some weird readings in the right channel:
    OT voltages.PNG
    Checking upstream driver transistors against voltages from the working left channel, the emitter voltage on T126 is -329.8 mv, base is -1.51V (this is ~0.4V less compared to working left channel). T124 base and emitter are also ~0.4V less than the working channel. I checked back to T114, base and collector voltages match the left channel, but the emitter voltage is again ~0.4V less than the left channel. Would that be enough of a difference to cascade up to T126 not "turning on"? T126 tested just fine out of circuit previously, maybe I should re-check.

    Thanks so much for everyone's help, what an amazing community!
     
  20. amr2

    amr2 Well-Known Member

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    Bias must be adjusted with the final transistors, not with the dummy resistors. The dummy resistors are to check that voltages are ok, but leave the bias pot at 0.
     

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