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Pioneer SA-940 - mystery

Discussion in 'Solid State' started by Janochytron, Dec 8, 2018 at 5:25 AM.

  1. Janochytron

    Janochytron Member

    Messages:
    93
    Hi,

    I got this nice amp as faulty, the description was, shortly tested, one set of speakers only working.

    I opened it up before plugging it in for a quick visual. This amp has the power transistors not soldered but plugged in sockets. So I took out the heat sink with the 4 power transistors out, tested them, all good. I turned the amp on without them, all good, relay clicked.
    Now I put them back in, turned it on but the bulb tester lit up like it was Christmas. Full on. And the bulb is 150 watts. I did not test it before removing, so can't be 100% sure it was not me doing something, but the heatsink has 2 screws and can be put in in only one way. I have not removed the transistors off the heatsink at this stage either, so couldn't be put in wrong way around.

    Because it's easy to swap the transistors around I have tried different combinations. It draws too much current whenever a pair of them is plugged into any channel (or both).

    Without them, voltages are good and it plays into headphones fine.

    Now the manual lists pair of 2SC252/2SA1075. But the amp has 2SC3546/2SA1389. But they don't look to have been replaced, they look to be put there in the factory. From what I could find they are compatible (correct me if I'm wrong here)

    As it's happening on both channels independently I was suspicious of power supply. I checked the big capacitors, even replaced them just in case. tested smaller capacitors. the rectifier bridge looks ok. Also all of these work fine without the outputs in.

    I checked for bad solders, there were few loose ones, but fixing them did not make any difference.

    Any ideas what could it be?

    service manual is here:

    https://www.hifiengine.com/hfe_downloads/index.php?pioneer/pioneer_sa-940_service.pdf

    here are some pictures of how the heatsink assembly looks and works

    IMG_20181203_165420.jpg IMG_20181203_165420.jpg IMG_20181206_215003.jpg IMG_20181206_215024.jpg
     

     

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  2. waggs098

    waggs098 Super Member

    Messages:
    1,238
    Location:
    Amherst, NE
    How did you test the finals? I have heard that sometimes if a transistor is tested with just a multimeter it can seem ok but when installed and under higher voltage it will go to a failed state.
     
  3. Janochytron

    Janochytron Member

    Messages:
    93
    Hi, I have one of those small Chinese component testers. All 4 transitors tested fine with HFE from 80-120.
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    The 2SC3546/2SA1389 spec about the same a the devices on the schematic. It was probably a production change which was not altered on the diagrams. They are probably original.

    Verify that Q1/3 and Q2/4 are in the correct positions. The service manual shows the board artwork and their locations. This would be on the odd chance that someone pulled them for testing and got them back in the wrong positions.

    If you find failed outputs, viable options might be MJL3281A/MJL1302A or FJA4313R/FJA4213R. Of course, if an output has failed, it would be essential to check the surrounding resistors and the driver transistors for damage as well.
     
  5. Janochytron

    Janochytron Member

    Messages:
    93
    Hi, thanks for your reply.. I did check the pnp npn position and also the correct position, All checks out. They are not blown either. I tried pulling out 317 or 319 or 305 and 307, the current draw drops down as well (same as if I take out the power pair).
    I tested the components in the power amp. And all works fine without the power transistors.

    I still think something has to be off in power supply or some maybe protection (something that the two channels have in common as it's happening on both channels exactly same way. Or bad solder or broken track or short where there should not be. But I can't find any.
    Farnell has the MJL3281A/MJL1302A so I might order a pair just to eliminate the outputs are the cause of the failure.

    I was also wondering if the rectifier bridge can behave badly when under load.
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,070
    Location:
    uk.. the middle bit
    check for continuity from output transistors to heat sink . there shouldn't be any .
     

     

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  7. Janochytron

    Janochytron Member

    Messages:
    93
    There is not. Also now I am testing without the heatsink altogether, just plugging in the transistors.
     
  8. century tek

    century tek Super Member

    Messages:
    3,427
    Location:
    The Evergreen State
    Don't do that! They need their heatsinks. If you don't have a power transistor mounted to a heatsink, you will get thermal runaway and blow the transistor immediately.
     
  9. Janochytron

    Janochytron Member

    Messages:
    93
    I only have it on for few seconds with them in as the light bulb tester turns on fully, so I don't leave it on. Once I find the problem and the bulb won't shine (after the caps charge) I will mount it to heatsink before leaving it on more than few seconds.
     

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