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Another Pioneer SX-780 repair thread

Discussion in 'Pioneer Audio' started by Odizzieus, Aug 5, 2018.

  1. Odizzieus

    Odizzieus Member

    Messages:
    69
    Hi, I have a Pioneer SX-780 that I got for free out of someone’s garage. It lights up, source lights come on, and the am/fm meter moves. There is no output, and I’ve tried resoldering the transistors already. None of the fuses are blown or have gone bad. I’ve seen mention of testing the power packs, but I was having trouble getting a voltage reading, so instead I tested for continuity between pins 2 & 3, and 8 & 9 on both packs. Both had continuity on both packs. When I switched to diode tester, I didn’t have a reading. Does this mean the packs are bad? Could someone explain In a detailed way how I test the voltage, more specifically where to put the ground probe?
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Welcome Aboard!

    Ground your meter to the unit chassis.
     
  3. Watthour

    Watthour Electron Rancher - JS3600

    As an initial check, measure the voltage at either end of R1. That's the 220Ω ceramic resistor mounted to the front of the heat sink. Expect to measure about 50VDC at one end and about 25VDC at the other. If that passes, locate the three TO-220 transistors mounted to stand-up heat sinks toward the left end of the main AF board. Those are the transistors whose connections should have been checked and re-flowed with solder, and they are Q20, Q19, and Q25 arranged front to back. Measure voltage at the collector tab of Q20 (the front one) and expect to read about -50VDC. Check the collector tab of the second one (Q19) and expect to read about +50V. Measure the voltage at the collector tab of Q25 (rear one) and expect around +25VDC.

    If those are intact, and the outputs are not damaged, the protection relay should engage about 3-6 seconds after powering on the unit. There should be an audible click of the relay armature. If not, further tests will e required.
     
  4. Odizzieus

    Odizzieus Member

    Messages:
    69
    I tried to measure the voltage at the resistor attaches to the front of the heatsink. My red probe was in the 10A slot of my multimeter. I attached the black probe to chassis ground and the red to one lead of the resistor and two fuses blew. What did I do wrong?

    Edit: the ones that blew were both the 1a 125v fuses
     
    Last edited: Aug 5, 2018
  5. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,551
    Location:
    uk.. the middle bit
    this is why ...
     
  6. Odizzieus

    Odizzieus Member

    Messages:
    69
    When I tried using the other slot, I wasn’t getting any reading and I’m not sure why, so I tried switching it over
     

     

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  7. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,551
    Location:
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    try measuring a known good cell battery or something to check your meter works .
    the amps setting is for amps .. it looks like a short circuit to whatever you connect it to . hence the blowing fuses .
     
  8. Odizzieus

    Odizzieus Member

    Messages:
    69
    I think my meter is broke, I’ll be back later once I buy a new one. Thanks for your patience
     
  9. Watthour

    Watthour Electron Rancher - JS3600

    It's likely that a fuse in the meter has opened, protecting the meter from the direct short created when attempting to measure voltage on a current scale.
     
    merlynski likes this.
  10. Odizzieus

    Odizzieus Member

    Messages:
    69
    Alright so I replaced the meter, just because it was a cheap one anyway and attempted to take readings again. On both terminals of the 220 ohm resistor, the voltage keeps jumping between 13 and 144 volts, and on the other terminal it jumps between 26 and 287 volts not sure if I’m measuring wrong, I have the black probe connected to chassis and I’m touching the terminals in the resistor with the red
     
    Last edited: Aug 5, 2018
  11. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,551
    Location:
    uk.. the middle bit
    did you replace the blown fuses ?
     

     

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  12. Odizzieus

    Odizzieus Member

    Messages:
    69
    Yes
     
  13. Odizzieus

    Odizzieus Member

    Messages:
    69
    Ok so I figured everything out and here are my readings

    220 ohm resistor
    23.7v
    47v
    Q20
    -47.8v
    Q19
    46.1v
    Q25
    23.8v
    Left STK
    1 -23.1v
    2 -37.2
    3 -11.5
    6 0v
    7 37.2v
    8 -12v
    9 37.2v
    0 -20.7v
    Right STK
    1 -1.2v
    2 -36.9
    3 0v
    6 0v
    7 37v
    8 0v
    9 37v
    0 1v
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,551
    Location:
    uk.. the middle bit
    if you remove the left pack the right should work by the looks of it
     
  15. Odizzieus

    Odizzieus Member

    Messages:
    69
    It works! COuld you recommend a place for me to buy a replacement stk 0050?
     
  16. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,551
    Location:
    uk.. the middle bit
    sorry i cant do that as they are obsolete as far as i am aware .. i believe some copied ones work if you get lucky .... there is a kit to build your own .. search here on AK
     

     

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  17. Odizzieus

    Odizzieus Member

    Messages:
    69
    Thanks for your help anyway!
     
  18. Watthour

    Watthour Electron Rancher - JS3600

    If the output module has failed, it is important to check some of the surrounding components. Specifically, the 0.22Ω 2W ceramic emitter resistors for that module. Check the resistance of each of them (in circuit) with the power off.

    As for the replacements, since the original Sanyo modules have been unavailable for many years, all that is left are several aftermarket devices of dubious quality. This may be informative, if not helpful:

    http://audiokarma.org/forums/index.php?threads/sx-780-output-modules.546514/#post-7145385
     
    merlynski likes this.
  19. Odizzieus

    Odizzieus Member

    Messages:
    69
    Thanks for your help, I’ll definitely do that!
     
  20. merlynski

    merlynski Curmudgeon Electronicist Subscriber

    Messages:
    432
    Location:
    High Plains, Eastern Colorado
    petehall347 likes this.

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