SX-950 - Power supply pin 15 is 7VDV when it should be higher.

Discussion in 'Solid State' started by EvanF, Feb 13, 2018.

  1. EvanF

    EvanF AK Member

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    Recapped the board and tested all resistors and transistors. Where to next?
     
  2. mbz

    mbz AK Subscriber Subscriber

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    I assume you are reading minus 7Vdc, +7Vdc would indicate other problems.
     
  3. glen65

    glen65 Member

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    According to the manual it should be -19V, regulated at Q6. Did you check to see if -50 V is present at the other side of the transistor?
     
  4. mbz

    mbz AK Subscriber Subscriber

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    Suggest you ask a moderator to move this to the Pioneer forum, greater visibility to their experts.
    Zener ZD12 is a candidate. Will need dc voltage measurements for Q2 and Q4, base, emitter, collector
    to ground/chassis, so 6 measurements.
     
  5. Tom B

    Tom B AK Subscriber Subscriber

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    Was this problem there before the Cap replacements?? If no, check the cap polarity and verify all connections. If the same problem exists, I suggest you try the following
    Verify you have -62V on the collector of Q2 the 2SB507 green transistor on the heat sink. If OK, make sure you have -51 volts on pin 14 of the regulator board. If pin 14 is low, suspect Q2. If you have the -51v on pin 14, then check for -50v on the collector of Q6. If low, suspect the 150 ohm resistor. If OK suspect a bad Q6 transistor. (measure voltage on emitter to base. If it is above 0.7v then the transistor is open. ) If the voltage B to E appears correct, I suggest you disconnect the wire on pin 15 to see if it now goes up to -19v. If it does, then suspect a leaky cap on the equalizer board dragging down the voltage.

    As a bit of experience, the design of the SX-950 and the placement of this regulator supply board leads to a lot of issues. The heat builds up on this board and there isn't any place for it to go. So the transistors really cook on the heatsinks, and connections fry, and the board discolors. I always replace the TO220 transistors with higher rated ones as a precaustion because they have typically overheated. I have had issues with the 2SB507, opening under load, causing low voltage on pin 14 and pin 15.

    Good luck
     
    Last edited: Feb 14, 2018

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