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Pioneer SX-434 issue

Discussion in 'DIY' started by elitopus, Jul 16, 2017.

  1. elitopus

    elitopus Super Member

    Messages:
    2,125
    Location:
    Saint Augustine
    I picked up a Pioneer SX-434 from a friend. I took it knowing it had an issue with one channel.

    It plays music fine on the left channel, and has 20VDC on the right channel.

    Where should I start with this?
     
  2. Binkman

    Binkman Addicted Member

    Messages:
    5,687
    Location:
    Raleigh, NC
    I'd start with editing your post by explaining what you mean it has 20vdc on right channel.
     
  3. elitopus

    elitopus Super Member

    Messages:
    2,125
    Location:
    Saint Augustine
    Lol. When I hooked up my test speakers (just an old pair of generic 4" woofers), one of the cones sucked in pretty hard, and the magnet got real hot.
    At the binding posts I have 10mvDC on one channel and 20vDC on the other.

    The channel with the 20vDC does play music, but much quieter than the other channel. Could be because the woofer is being zapped with DC while trying to play music :)

    This is why I use some throw away speakers as my intial test speakers, and then step up to some Minimus 7's
     
  4. Binkman

    Binkman Addicted Member

    Messages:
    5,687
    Location:
    Raleigh, NC
    Your 'throw away speakers' are not a good idea... IMO you need to up your testing methods versus plug in and see if it works.
     
  5. elitopus

    elitopus Super Member

    Messages:
    2,125
    Location:
    Saint Augustine
    I was thinking this could be an easy fix. I knew one channel was out. I cleaned the pots and switches, and then hooked up the test speakers to see what would happen. As soon as the woofer sucked in I knew I had DC on that channel.

    What do you recommend for a better testing method?
     
  6. markallen

    markallen Luddite Tendencies

    Messages:
    1,119
    Location:
    Bismarck ND
    If you plan to work on vintage audio amps and receivers even occasionally, a "dim bulb" tester (DBT) is a cheap investment that will allow you to eliminate use of speakers to identify DC on the speaker outputs. There are plentiful entries on AK detailing how to build and use one. If you have the interest and ambition to get as far as you have on your SX-434, you likely have the ability to build and use a DBT.

    If you've got 20vDC on one channel, chances are good that one or both of the main output transistors on that channel have failed. I've seen situations where that's all that is wrong. Other times you'd find other components on that channel of the amp section have failed as well. I'd start by removing these outputs on the failed channel and using your DMM to test them. It's easy enough to identify these transistors on a small receiver like yours: they'll be fastened to a metal surface that serves as a heat sink. You can find the schematic free on Hi-Fi engine, but if you're not already signed up, you'd have to do that.
     
  7. elitopus

    elitopus Super Member

    Messages:
    2,125
    Location:
    Saint Augustine
    Good advice. I do have a dim bulb tester. I always thought to use it when the unit in question was blowing fuses because of too much current draw.

    The main fuse, as well as the 4 fuses inside test good. I will remove the output trabsformers and test them.

    Thanks
     
  8. OMGCat!

    OMGCat! AK Subscriber Subscriber

    Messages:
    1,532
    Location:
    San Diego, CA
    Sounds like blown outputs for sure. I'd get the SM and look it over before you go in there just to familiarize yourself with the circuit.
    Pull and check the outputs and the emmiter resistors and then the drivers. If you're lucky it will just be the outputs, if not then you are looking at a handful of other components. Either way, definitely fixable.
     
  9. elitopus

    elitopus Super Member

    Messages:
    2,125
    Location:
    Saint Augustine
    I'll study up on the SM and then take the receiver apart this weekend.

    I assume if I replace the outputs on one channel I should replace the other channel too.
     
  10. slimecity

    slimecity Super Member

    Messages:
    1,282
    Location:
    New Zealand
    Yup, fraid so.
     
  11. elnaldo

    elnaldo Addicted Member

    Messages:
    6,379
    Location:
    Buenos Aires GMT-3
    20V DC at the output, without blowing fuses, could mean also another fail not just a shorted output. Probably some fuse resistor is blown , and that's the reason the main fuses are not blowing under a shorted output. In different kind of amps, I've seen problems at the differential pair transistors or even at the preamp causing high DC at the output.

    I'd remove the output transistors and check the DC with the transistors uninstalled. If zero, or just some mV, you could install new output transistors. Test resistors or fuses between the blown transistor (if you find one) and the power supply. If the transistor is shorted, some component connected to it is probably blown due excessive current flow during the short.

    You don't need to replace both channels, if one channel is still good, I´d let it alone by now. Fix the fail first. Your good channel is your reference, don't touch it or swap components, you can end with 2 blown channels.

    A "better testing method" is simply measure DC at the output before connecting speakers.
     
  12. Bubba57

    Bubba57 AK Subscriber Subscriber

    Messages:
    360
    Location:
    Apache Junction, Az
    Although these all are excellent responses, you may consider having the Mod's move this to the Pioneer forum. Just a thought. Good luck with it.

    Bubba57
     
  13. avionic

    avionic " Black Knights" Subscriber

    Messages:
    34,139
    Location:
    Fort Dodge, Iowa
    No.Its not necessary.
     
  14. KKnight

    KKnight Active Member

    Messages:
    318
    Wrong forum- move to Pioneer forum.
    OP- there are a ton of 434 threads over the past decade. Research them and start with those.
     

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