SX-636 Repair Help

Discussion in 'Pioneer Audio' started by revision-88, Jan 10, 2019 at 7:07 PM.

  1. revision-88

    revision-88 New Member

    Messages:
    19
    Hey guys,

    I picked up a Pioneer SX-636 in good exterior condition a while back and have been working on repairing the thing for quite a while now. When I got it, it barely worked, so I recapped all of the important parts of it, including the large coupling caps, which had bloated like nothing I have ever seen before. After this, it worked in a sense, however it would send loud crackles to the speakers, which I found to be short bursts of DC, about 5 to 10 volts or so. I tested the main output transistors (2SB531/2SD371) independently and out of circuit, and found them to be good. I then replaced the known-bad 2SA726 and 2SC1451 transistors with KSA992F and KSC3503 transistors. However, after these replacements the speaker terminals show a near-constant 30 volts, only occasionally dropping down to near zero for short bursts. Returning the original 726s and 1451s brings back the original bursts, but noticeably worse, spiking to around 12 volts more frequently and for longer. Any advice on this issue would be greatly appreciated.

    Thanks.
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    How do the power amp circuit voltages compare to this?
    SX-636_AWH-034.jpg
     
  3. revision-88

    revision-88 New Member

    Messages:
    19
    Well, it doesn't look good. If I measured right, the 1509 that should be seeing 1.2v at base is seeing 30v. The 777 is seeing 28v at base. The 3503 (replacing the 1451) is seeing 28v at collector. And the 992s (replacing whats labelled as 763s in the diagram, mine had 726s) are seeing 17.5v at emitter. As for the output transistors, they are seeing 30 at all pins, except the collector of the 531s, which is seeing -32.
     
    Last edited: Jan 10, 2019 at 8:39 PM
  4. Watthour

    Watthour Electron Rancher - JS3600

    Since there are new devices replacing the old, and duplicates across the channels, it is easier for us to follow along if the components are identified by position rather than device numbers. It is a convention that has worked well for us. For example, listing the voltages at a transistor by its designation, then the E-B-C voltages as follows:

    Q7
    31.0
    30.0
    30.0

    Q9
    31.0
    30.0
    -32.0

    That keeps us all aboard, regardless of what new devices are use to replace the originals, and helps differentiate between the two channels.

    We sometimes need all the help we can get.
     
  5. revision-88

    revision-88 New Member

    Messages:
    19
    My apologies, here's the voltages again:

    Q1
    0.7
    0.12
    -41.2

    Q2
    0.7
    0.12
    -41.2

    Q3
    0.7
    9.7
    -47.2

    Q4
    0.7
    9.7
    -47.2

    Q5
    -47.2
    -41.2
    28.8

    Q6
    -47.2
    -41.2
    28.9

    Q7
    29.6
    30.2
    32.7

    Q8
    29.6
    30.2
    32.8

    Q9
    29.5
    28.8
    -33.3

    Q10
    29.5
    28.8
    -33.3

    Q11
    29.5
    29.6
    33

    Q12
    29.5
    29.6
    33

    Q13
    29.5
    29.5
    -33

    Q14
    29.5
    29.5
    -33
     
  6. Watthour

    Watthour Electron Rancher - JS3600

    HO-LEE CRAP! Yes, you have high voltages everywhere. I am frankly surprised that the output transistors tested "good" since something there is pulling everything up to the supply voltages, just like an E-C short in an output transistor. The other explanation for that is IF (somehow) an output transistor got installed in the wrong position, and the NPN in a PNP circuit is simply passing all that voltage.

    I would mark the positions, and pull the outputs again, then test the circuit voltages once more. I suspect that the readings would be markedly different.
     

     

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  7. revision-88

    revision-88 New Member

    Messages:
    19
    Alright, pulled the output transistors again. A few interesting points: my voltage measurements in the circuit without outputs were exactly the same. I checked the location of the output transistors and they were in the right places. Outputs checked out good once again. Finally, the positive speaker terminal was still showing 30v with the outputs out. I have no idea whats going on.
     
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    Describe exactly how you are measuring these voltages please.
     
  9. revision-88

    revision-88 New Member

    Messages:
    19
    I have my multimeter in voltage mode, ground lead clipped to the chassis. Turning the amplifier on and touching the red lead to the leg of the transistor/speaker terminal/whatever I'm measuring.
     
  10. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    This is the preferred
    way to list voltages.
    Q2
    E 0.7
    B 0.12
    C -41.2
     
  11. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    OK...Thats exactly correct.
     

     

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  12. revision-88

    revision-88 New Member

    Messages:
    19
    My bad, all measurements are in that order. I assumed the order was implied. Sorry for any confusion.
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,246
    Location:
    uk.. the middle bit
    check transistors pin outs .
     
  14. revision-88

    revision-88 New Member

    Messages:
    19
    Already did that a couple times, will do again though.
     
  15. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    Both channels acting identicle?
     
  16. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    You mean the two main power supply filter caps? I'm not seeing any large coupling caps in the schematic.
     

     

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  17. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,246
    Location:
    uk.. the middle bit
    where were the 2sa726 ?
     
  18. revision-88

    revision-88 New Member

    Messages:
    19
    Yep.

    Yeah thats what I meant, my bad.

    Q1-Q4 in the diagram posted by Watthour.
     
  19. avionic

    avionic " Black Knights " Subscriber

    Messages:
    45,470
    Location:
    Fort Dodge, Iowa
    Were you aware that the pin outs of the 2SA726's are BCE and the KSA992's are ECB?
     
  20. revision-88

    revision-88 New Member

    Messages:
    19
    Yeah, I checked their datasheet and aligned the transistors with the diagram printed on the PCB before soldering.
     

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