QX-949 Power supply issue

Discussion in 'Pioneer Audio' started by KeithD, May 6, 2018.

  1. petehall347

    petehall347 the brandy coffee man Subscriber

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    corrected my post now
     

     

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  2. KeithD

    KeithD AK Subscriber Subscriber

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    Ok, replaced all transistors. Replaced Q1-3, no change. Voltage on pin 8 (+B3) improved a bit after replacing Q4. Did the rest of the transistors, no change. Replaced D2, 3, 6, 7. Still no change. Here are the readings:

    Pin: voltage
    1: 46.8VAC
    2: 46.8VAC
    3: 7.3VAC
    4: 38.4VAC
    5: 38.4VAC

    6: 9-12VDC (should be 13.5V)
    7: 0
    8: 13.8±3VDC (+B3, should be 30V)
    9: 0
    10: 0
    11: 9.7VDC
    12: 9.7VDC
    13: 0
    14: -44.4VDC (-B3, should be -42V)
    15: 0
    16: -38.7VDC
    17: -13.3VDC (-B4, should be -13V)

    So something is still not right on the circuit fed by pins 4, 5 and out put at pins 6, 8. All that's left are a couple of ceramic caps and resistors.
     
  3. petehall347

    petehall347 the brandy coffee man Subscriber

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    or as i thought but never mentioned it something might be pulling the voltages down further on .. need to look for the current limiter resistor on the high side and measure voltage drop across it ... its 2.16am here .. will check back in the morning .
     
  4. KeithD

    KeithD AK Subscriber Subscriber

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    I will test transistors on the power supply board tonight. Looks like R6, 7 ,8, and 9 should be tested. Will lift one leg and see if reading is ok. Anything else to look for?
     
  5. KeithD

    KeithD AK Subscriber Subscriber

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    Checked R6, 7, 8, 9: all good.

    So, that leaves me with what Pete mentioned about something down circuit pulling the +B3 voltage down. Looking at the schematic, YIKES! +B3 powers every board in this unit! The thing to do may be to disconnect each board one by one from +B3 and see if the voltage restores to +30V. Sound like a plan? Can't really see any way around it at this point... :(
     
  6. john stumpf

    john stumpf AK Subscriber Subscriber

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    yeah it sure does. two colors of wires too red and a pinkish grey. the source is pin 8. you could un do the wire wrap and use your meter and check the draw in milliamps its only supposed to be 150 total. its all yes/no and following the daisy chain
     

     

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  7. KeithD

    KeithD AK Subscriber Subscriber

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    OK, disconnected all three wires from pin 8 of the power supply board. Readings are still around 12VDC and variable. So, it appears the problem is on the power supply board, but where? All electrolytic caps replaced, all transistors replaced. Replaced D2, 3, 6, 7. What's next?
     
  8. Unjoy

    Unjoy New Member

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    Location:
    Apeldoorn, Netherlands
    Strange. Could you measure voltages on all legs of Q3 again and post a picture of how the board looks now?
     
  9. KeithD

    KeithD AK Subscriber Subscriber

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    Q3: E, C, B

    8.6±2, 8.1±4, 10.5±1.5VDC

    As you can see voltages are quite variable.

    QX949PwrSupp5-15-18.JPG
     
  10. Unjoy

    Unjoy New Member

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    Location:
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    It's a bit difficult to see, but is Q3 installed correctly?
     
  11. petehall347

    petehall347 the brandy coffee man Subscriber

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    voltages say its not right .. have been working out what might happen if q3 was removed for now .. i first see emitter of q1 voltage should be higher than normal if the rest of the circuit is good .
    q2 base needs to be about 1.2 volts higher than q1 emitter to count out q1 q2 problems .
    so long as nothing connected to pin 8 pulling q3 should be fine for testing purposes .
     

     

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  12. KeithD

    KeithD AK Subscriber Subscriber

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    Yes, Q3 is in correctly. Matches schematic and printing on the board. Repacement KSC1845FTA is ECB configuration.

    Actually have another QX-949 to compare to with a good power supply (it has a bad channel on the rear power amp board). Q3 voltages on that board are (E, C, B): 13, 33.1, 13.7, with pin 8 at 30.8VDC.

    Will measure some other voltages for comparison and see if I can narrow down where the discrepancy is.
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

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    q3 e to c voltages are like they should really be b to e .. b to e in most cases should be 600mv ish . if you remove q3 and check pin 8 again it should rise substantially if i am correct .
     
  14. KeithD

    KeithD AK Subscriber Subscriber

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    Did as you said, pulled Q3. Voltage at pin 8 still 13 volts.

    Measured voltages at all four transistors before pulling Q3:

    E, C, B (expected values from good power supply in parenthesis):

    Q1: 15 (31), 50.5 (47.7), ~14 (32.8)
    Q2: ~14 (32.8), 50.5 (47.7), ~14 (33.3)
    Q3: ~7 (13), ~8 (33.1), ~10 (13.7)
    Q4: 13 (13.1), 35.5 (33.7), 13.7 (13.7)
     
  15. petehall347

    petehall347 the brandy coffee man Subscriber

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    q2 appears shorted ..sort of doesn't make sense though .
    check r5 r4 .. and maybe check all other resistors .
     
  16. Unjoy

    Unjoy New Member

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    With Q3 removed, what happens if you also pull C10? If no change, then also pull C9 and see if that changes anything.
     

     

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  17. KeithD

    KeithD AK Subscriber Subscriber

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    R3, 4, and 5 good.

    Found the trouble, disconnected the wire from pin 6, and pin 8 came up to 30V and unit came out of protection. :)

    Now I just need to trace back what pin 6 is powering. Something back there is shorting pulling too much power, I suspect.
     
  18. petehall347

    petehall347 the brandy coffee man Subscriber

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    tuner is powered from pin 6 . maybe q7 ha1142 took a dump or q6 ha1137 or even q8 ha1138
     
  19. KeithD

    KeithD AK Subscriber Subscriber

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    Oh, no! :( Are there replacements for those??
     
  20. petehall347

    petehall347 the brandy coffee man Subscriber

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    best figure out if it is any of those first . or a simple trapped wire or something .
    try lifting r40 tuner board . or first try switching fm to am or visa versa .
     

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