QX-949 Power supply issue

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

  1. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Yes, I've done most of that. Already replaced all 2SA726, 2SC1451 (power amp boards and control board). Replaced all transistors on power supply. Replaced all e-caps on power supply, power amp, protection boards. Since this receiver does not have pre-in/power-out separation, I've checked power amp output on each channel with input from control amp grounded. Does not seem to make a difference, so static issue seems to be on power amp.

    Here's the current situation. I can get the channels on the B board down into the right range (0V) using the trimmers (though it is still too variable), but not the A board channels, AND the A board channels are stuck in negative voltage. Voltages at each pin:

    5B (rear right): 0.001±0.01
    23B (front left): 0.0007±0.009
    5A (rear left): -0.066±0.02
    23A (front right): -0.05±0.1

    Protection seems to be working, as it will click in and out if voltages on the channels get too high. What's left on the power amp are Q5 & 6: 2SC869, and Q9-12. Have replacements for all of those, so that is likely the next step. I'm not expecting this is an issue with Q13-16, at least I hope it isn't.
     

     

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  2. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    Ok, so the regulated power supplies are rock solid now (especially +B3 and -B3? And you see fluctuation on B1 and B2?
    On the back there is this 2/4 channel switch. Did you clean it already?
     
  3. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Yes, that’s correct. B3 voltages vary by 0.04V at most, while others are around 0.2V.

    Did clean the 2/4 channel switch. Did so again yesterday. Also replaced ALL transistors on both power amp boards. Some change in output voltages, but still static in some of the channels. Pin 5B drifts up to about 0.4V over time putting the unit in protection.

    Last thing I did yesterday was put the back 2/4 switch in the 2 setting. Voltages improved on the power amp board output pins, but unit did not come out of protection. Will do more testing today. My next step will be the control amp board. Already replaced the 726s on there, but have not don the 1312s or any of the caps. That may take me a while. Only have time to work on this intermittently.
     
  4. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    Right. The 2/4 channel switch basically sets the B1 voltage to 45V in the 2 channel position. Apparently this leads to a state where the protection kicks in. You tested with the inputs of all amps connected to ground, and still had static on all channels. Common denominator would still be the power supply. How is your -B3 doing (-42V)? Is it solid? Did you also replace the trimmers on the power amp boards?
     
  5. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Yes, tested with all inputs on all four channels grounded, and static is unchanged.

    Here are power supply voltages in 4 channel mode:

    +B1: +38.2±0.5
    -B1: -38.2±0.5
    +B2: +38.1±0.1
    -B2: -38.1±0.5
    +B3: +31.0±0.05
    -B3: -44.4±0.02

    Did not replace the trimmers, but adjusting them does not affect the static. Static is mainly on rear right channel, some on front right channel, and only weak/intermittent on left channels.

    When I move the rear switch to 2 channel, static is drastically reduced to almost non-existent.
     
  6. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Is it worth checking the ceramic capacitors on the power supply B board? I have not done that yet.
     

     

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  7. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    Your power supplies are ok, there is something wrong with the B amplifier board. Can you measure voltages on all pins of the B board? Also worth doing a good visual inspection. Any bad joints, especially on components that tend to get hot, like R49-R52?
     
  8. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    B amp board voltages, some quite variable, indicated by ~

    1: ~0.26
    2: ~0.2
    3: -38.1
    4: ~0.2
    5: ~0.5
    6: 0
    7: 0.002
    8: -44.3
    9: -44.3
    10: 0.002
    11: 31.7
    12: 31.7
    13: 38.0
    14: 37.9
    15: 31.7
    16: 31.7
    17: 0.047
    18: -44.4
    19: -44.4
    20: 0.003
    21: 0
    22: ~ -0.02
    23: ~ -0.03
    24: -38.0
    25: -0.03
    26: -0.02

    27: ~ -0.64
    28: ~ -0.08
    29: -38.1

    30: ~0.05
    31: ~ -0.01
    32: 38.0

    Will have to remove the board to inspect joints.
    33: 38.1
    34: 0.12
    35: ~0.73

    36: -37.7
    37: ~0.09
    38: -0.51
     
  9. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    Ok, no suspicious voltages.
    Yes, it is painful to remove the board, I know. But we need to be sure.
    As a next experiment, let's remove the output transistors from the suspicious rear amplifier. You can do that by removing pins 33-38 from the B board.
    Do not connect a speaker to that amp, but check if the static is still present on the other channels.
     
  10. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    25,747
    Location:
    uk.. the middle bit
    1: ~0.26
    2: ~0.2

    4: ~0.2
    5: ~0.5

    4 and 5 should read the same . ..
    dc offset is high . see if it will adjust closer to zero volts .
     
  11. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    It won't, this is one of the problems. I can only get one of the channels close to 0V with the trimmers.

    OK, so then do I check for static with headphones only?

    I think the fact that the static is very minimal when I set the back 2/4 channel switch to 2 channel says that there is definitely something wrong with at least one rear channel, which from my observations says it must be the rear right. THAT is definitely where almost all the static is. DC offset cannot be adjusted below 0.2V on that channel and it jumps around a lot. Since only resistors and coils have NOT been replaced on the power amp boards, it does seem to point to the large output transistors.

    Will disconnect those pins when I get a chance.
     

     

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  12. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    The 2/4 channel switch does 2 things: It sets the B1 voltage for the front amps (in 2 channel mode a bit higher so you have more output power), and disconnects the speakers of the rear channel when set to 2 channel mode.
    You observe that the static is less in 2 channel mode, a state where the rear channels are functioning, but are not delivering current to the rear speakers, and more static when in 4 channel mode (probably only with rear speakers connected, and less when disconnected).

    With the outputs disconnected on the rear right channel, check for static with speakers connected on front channels and rear left, but no speaker connected to the rear right channel. By doing so, I hope we will be able to find out if the static has something to do with the outputs or the driver stage of the rear right amp. With the outputs removed, you will again measure the voltage at pin 4/5. Is it still jumping around? then it is probably something in the differential or driver stage of the amp, if it is rock solid, then we have to look at the output stage.
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    25,747
    Location:
    uk.. the middle bit
    losing track here ..
    ok so dc balance is out of control .. is idle current bias in good order ?
     
  14. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Yes, I have been able to set the idle. Only channel that is stable and I can set to zero is 22/23B.
     
  15. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    OK, pulled pins 33-38 on the B board. All symptoms the same, no change. Some static on Right front channel, none on Left front or rear. Voltage on pin 5B has not changed, about 0.2V and quite variable.
     
  16. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    25,747
    Location:
    uk.. the middle bit
    bad something around input differentials .. anything else it should adjust in theory .. or is over loaded by feedback .
     

     

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  17. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    Ok, so you changed all electrolytics and transistors on the amp boards. Does adjusting VR1 (dc balance) change average dc on the output of the rear right amp?
     
  18. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Yes, but it does not go below about 0.2V.
     
  19. Unjoy

    Unjoy New Member

    Messages:
    46
    Location:
    Apeldoorn, Netherlands
    Ok, is pin 7 properly connected to ground?
    My first suspicion is VR1. Can you measure voltages on all pins of VR1? Top end should be 0,07V and bottom -0,12V, per the service manual.
     
  20. KeithD

    KeithD Active Member

    Messages:
    458
    Location:
    La Crosse, WI
    Yes, pin 7 is connected to ground, 0.33 ohm resistance between pin 7 and ground.

    On VR1, I'm getting -0.095 to +0.12V on the two ends of the variable resistor.
     

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