Pioneer SX-434 issue

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

  1. elitopus

    elitopus Super Member

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    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

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    I'd start with editing your post by explaining what you mean it has 20vdc on right channel.
     
  3. elitopus

    elitopus Super Member

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    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

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    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

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    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

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    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

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    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

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    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

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    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

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    Yup, fraid so.
     
  11. elnaldo

    elnaldo Addicted Member

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    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

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    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

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    No.Its not necessary.
     
  14. KKnight

    KKnight Active Member

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

    elitopus Super Member

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    I pulled the outputs. I have 4 different ones. This unit must have been worked on in the past. They are
    Sanyo B507D
    Sanyo B507
    Sanyo D313D
    No make D313

    So they are the correct number according to the schematic.

    If I remember how to test transistors correctly, i use my meter in diode mode, and go from base to collector, and then base to emitter. Depending on what type of transistor it is, it will read correctly with the positive lead on base, or the negative if its the other type
     
  16. elnaldo

    elnaldo Addicted Member

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    You are right, you'll measure a voltage drop of around 0.6V from base to emitter and from base to collector, only in one sense, and no continuity in the opposite way, and no continuity between emitter and collector.. Usually, a shorted transistor measures a short from collector to emitter, or to base. A good transistor won't have continuity from collector to emitter or from emitter to collector.

    Also, you need to check DC offset with the outputs uninstalled, and +V and -V reaching the output transistors. BE CAREFUL. If you see high DC at the output without the transistors, something else is shorted.
     
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  17. elitopus

    elitopus Super Member

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    Took some voltage readings. Lets see if I get this correct.

    First of all, there are two test points.
    TP1= -21.3Vdc
    TP2= 0Vdc

    Next I measured voltage on the pads where the transistors go. I put down measurements are collector, base, emitter. Measurements were taken with the negative lead on chassis ground and positive touching the pad

    Q22=
    C=-21.5Vdc
    B=-24.1Vdc
    E=-22Vdc

    Q20=
    C=-21.6Vdc
    B=23.4Vdc
    E=-20.8Vdc

    Q21=
    C=0Vdc
    B=23.4Vdc
    E=0Vdc

    Q23=
    C=0Vdc
    B=23.5Vdc
    E=0Vdc
     
  18. elnaldo

    elnaldo Addicted Member

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    something is not OK here. You should have +24V at the collectors on Q20 and Q21, -24V on the collectors on Q22 and Q23, no voltage at the emitters.

    TP1 and TP2 should be near zero.

    I'd remove the driver transistors Q16-17-18-19. I'm sure some of them are shorted. Q20 , 21, 22, 23 with + or -24V means shorted drivers.

    Bias diodes Q14 and Q15 could be damaged too due to a short in a driver transistor.

    Having just a few transistors in the output, I'd replace all of them, including the differential pair Q10-11-12-13. That could be a last fix after the unit is working.

    Check the resistors connected to any shorted transistor you find.

    Voltages are detailed in the schematic, identify the points and check they match your unit.
     
  19. elitopus

    elitopus Super Member

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    I agree that something amiss with this receiver. I will take your advice and pull the other transistors for testing. As well as other parts that might have been fried.

    Its interesting to me that there is so much wrong inside, yet all the fuses are still intact.

    I did notice on the SM that the voltages were listed. That should come in handy.

    Once I find my problem(s), I plan to recap the PS before I put the cover back on and put this unit into use.

    It has been a while since I have done any in depth, audio related, troubleshooting. I appreciate the help.
     
  20. Binkman

    Binkman Addicted Member

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    testing after the fact.. check the rail voltages after the rectifier. that will at least assure near even voltage on the rails. The fuses not blown is because there is no short to blow them. Probably a driver transistor in bad channel already (and perhaps that output transistor(s) and good channel is intact, So you seem to have channels to compare. for general testing may need to find a good chassis spot for that neg on the dmm that repeats well. If you as have a soldered on ground cable to chassis assure that is good, these chassis are plated, I usually replace with a lug and screw. if AC power cord is 'flimsy'? replace. (new lug power cords are the small prong is power and the wide prong is neutral.)

    Main caps good idea but also rail service caps may need replacing. anything 10v range rated? usually need replacing.. 30v range usually still serviceable. up to you.

    Avionics link to test transistors: with pics.
    http://audiokarma.org/forums/index.php?threads/testing-transistors.201303/#post-2362176

    hope it helps.
     
    Last edited: Jul 26, 2017

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