Pioneer SX-780 - Diagnosing a problem

Discussion in 'Pioneer Audio' started by thedude85, Jan 2, 2019.

  1. thedude85

    thedude85 New Member

    I am new to electronics and also new to AudioKarma. I'm learning as I go, but I'm motivated. I acquired a Pioneer SX-780. It turns on with lights, the receiver will tune into a station, but there is no sound from speakers/headphone jack, and also no click on the relay. I could use some AudioKarma wisdom and help diagnosing the problem.

    Here's my initial results with the DVM.

    STK-0050 - Left Channel
    0: -24.8 V
    9: 41.3 V
    8: -14.3 V

    3: -14.4 V
    2: -41.3 V
    1: -27.2 V

    STK-0050 - Right Channel
    0: 1.2 V
    9: 41.4 V
    8: 0 V

    3: 0 V
    2: -41.2V
    1: -1.3 V

    1: 13.8 V
    3: 0 V
    4: -0.6V
    5: 0 V
    6: -7.1 V
    7: 0 V
    8: 0.5 V

    I think the left channel STK is shot, and maybe that's why the protection circuit/relay isn't clicking? I'm not sure. Could the problem be from somewhere else? I appreciate your help. Thank you.
    rowan_mae likes this.


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  2. Watthour

    Watthour Electron Rancher - JS3600

    Welcome Aboard!

    Without taking my word for it, I think you'll discover that you've come to the right place. There is a LOT of support here for the venerable old 780.

    I would agree with your initial assessment. The Darlington pack in the left channel seems to have failed with E-C shorts. You can remove the pack, clean up any desoldering mess, disconnect the speakers, turn the volume to 0, and power the unit on to see if the protection relay closes. It may operate with one module removed, but don't load on any speakers or bring up the volume just yet.
  3. thedude85

    thedude85 New Member

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  4. sregor

    sregor AK Subscriber Subscriber

    If you remove the STK module, need to put resistors across "0" to "8" and between "1" and "3" or will not get relay click. Any vlu e between 330 and 1k ohms will work, and allow troubleshooting the rest of the circuits.
  5. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    High Plains, Eastern Colorado
    ^^This is good stuff, I have done it myself. Here is a quick schematic of the resistor connection for TS:
    Pioneer SX-780 dc-test.jpg
    thedude85 likes this.
  6. thedude85

    thedude85 New Member

    Thanks for the advice! I'm still waiting for some soldering/desoldering accessories I ordered. I also ordered the PCB's for replacing my burned out STK. In the meantime, I've been reading and educating myself on electronics and soldering technique. It's a deep rabbit hole I'm going down, but a fun one.


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  7. Watthour

    Watthour Electron Rancher - JS3600

    The STY replacement kits are a really nice package. A little deburring of the plates and cleaning the surfaces, and you can have at assembling the boards. I got a couple sets just to have and inspect, since none of the half-dozen 780/790s I have need replacements. If/when it happens, I'm ready. Of course, that next unit always seems to come along out of the blue.

    In the interim, if you can acquire a discarded unit, be it a radio, PC power supply, or something similar, you can get to practice your solder/desolder skills on something that you can't hurt.
  8. rowan_mae

    rowan_mae New Member

    I'm having the exact same issues with my Pioneer. I just joined this forum as I'm eager to learn more about diagnostics. I was wondering if you had managed to solve the issue yet and if part of the problem was with your delay / protection relay and that's why you didn't hear the click? Thx
  9. thedude85

    thedude85 New Member

    Yes, you've come to the right place. I just repaired my SX 780! I took the advice from the previous posts and desoldered the burnt out STK. I soldered in 2 resistors as suggested above, and click! The relay turned on.

    As to replacing the STK, I followed all the instructions from the link in post # 3 above. It is the more difficult path, but I wanted a bulletproof receiver. I had to read a lot into testing circuits with a DMM and soldering technique. I accomplished the task with a cheap 30W iron with a $7 'helping hands' stand. I ordered the parts from Mouser. I went ahead and rebuilt both STK's so they match. Finally, i did a thorough deoxit treatment of all the pots. Now she sounds great! So far no problems.

    A big thanks to @Watthour @sregor and @merlynski . I appreciate all your help.
    merlynski likes this.

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