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Someone talked me into repairing a Pioneer SX-950. need replacement output transistors

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  • #1

    Someone talked me into repairing a Pioneer SX-950. need replacement output transistors

    I didn't even want to take this job on, but here I am. Without boring you with troubleshooting my way down the circuit. The Last bit of work (fingers crossed) is replacing the output transistors in the faulty side.
    I was wondering if these would be suitable replacements:?

    The NPN pair are 2SD427s

    The PNP pair are 2SB557s

    I have some MJ15003s (NPN) and MJ15004s (PNP). Any reason these would not be okay to replace all 4 output transistors in this channel?

    Click image for larger version Name: pioneer-sx-950-poweramp_10003.jpg Views: 204 Size: 443.5 KB ID: 978147
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.
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  • #2
    I also have some NEC 2SB600s and 2SD555s, which are over spec’d, but match up better on some of the other parameters.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    • #3
      Minimum hFE of the MJ types might be too low.
      - Own Opinions Only -

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      • #4
        Your datapage link does offer up the 2SD555 as a cross for the 2SD427. Oddly, it does not offer up the 2SB600 for the 2SB557, but I think that cross is equally valid.
        Agree with Helmholtz that min. HFE should be maintained.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        ​

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        • #5
          Audiolabs of GA does't have them but they list a NTE280 as a replacement.
          nosaj
          soldering stuff that's broken, breaking stuff that works, Yeah!

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          • #6
            This thing suffered a pretty substantial fault in the lower channel (when looking at the schematic). The first thing I found was the 2 complimentary drivers, 2SD381 & 2SB536, had shorted and completely smoked the 150R emitter resistors. Along with that, I discovered that One of the 2SD427s had shorted, offsetting the output and burning out the 100R bias trimmer. This one was a tough one to track down all the issues. It was time consuming because with all the failed components, I couldn't really apply power for testing without causing a more components to fail.
            I ended up installing the NEC 2SB600s and 2SD555s and rewiring the power amp. Powered it up, monitored the mains current while measuring the voltage at the output, looking for any offset. The mains current looked normal, power supply and output normal, and protect circuit engaged the relay indicating all things are in safe operation. I wanted to stop there for the night and take the first small victory.
            I'll power it back up tomorrow, set the bias, and make sure it is passing signal. Then I'll load the output and test the audio.
            If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.
            • Likes 2

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            • #7
              Fixed!
              If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.
              • Likes 3

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              • #8
                I also have a couple of vintage Pioneer stereos to repair as well.....and I also have a Marshall Origin 50 and a 1965 Fender Super Reverb for repair.....it is all original so it should be interesting..

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