SX-780 - Counterfeit Darlingtons?

Discussion in 'Pioneer Audio' started by clk1981, Jul 18, 2017.

  1. clk1981

    clk1981 New Member

    Messages:
    25
    I have a Pioneer SX-780 that was blowing a main fuse (#4 - 4A I believe). I removed the power wires going to the Darlington outputs, replaced the fuse and the unit powered on and lit up AND the speaker relay clicks. Great, right?

    So, I went to eBay and bought some replacement STK-0050's ($15 each). These weren't the cheapest ones so I figured I had a better chance of getting genuine parts.

    I touched up several solder joints on the voltage regulators since I knew it's an issue with these units. I seem to have all voltages properly.

    I replaced the bad STK's with the new ones from eBay, the unit now powers on WITH the STK's wired, fuse doesn't blow, but I have no sound, headphones or speakers, and NO speaker relay clicking. I can remove the power going to the new Darlingtons, turn it on, and the speaker relay engages and I CAN get sound out of the headphone jack, although it doesn't sound good, kinda distorted sounding.

    I'm at a loss. Do you think I got a bad/counterfeit pair of STK-0050's???

    Thanks!
     
  2. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    You may have swapped out the darlingtons, but did you check the 0.22 ohm resistors for open circuits?
    Or the other components around them?

    You don't even mention setting the DC offset voltage to 0.000v on each channel.
     
  3. clk1981

    clk1981 New Member

    Messages:
    25
    The emitter resistors all seemed fine. They all measured fine and none were open. I have not tried anything with the DC offset voltage.
     
  4. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

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    DC offset nulling (0.000v target) is IMPORTANT!!!! Use insulated screwdriver shaft to avoid shorts.
     
  5. clk1981

    clk1981 New Member

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    25
    Ok, I checked the DC Offset. The Right Channel is 0.000V and the Left Channel is -36.5V ? Any ideas? Adjusting VR5 had no effect
     
  6. rcs16

    rcs16 Super Member

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    Obviously something is wrong with this, first thing to do is check the STK to see if it is the cause. If it is the STK, it is probably a shorted device. You can use the DMM in ohms mode to check the R from the neg power supply input to the o/p.
     
  7. clk1981

    clk1981 New Member

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    25
    I just tested voltages on each Darlington, and they were both very similar.

    1) -40.35V
    2) -41.75V
    3) -35.58V
    4) NC
    5) NC
    6) -39.9V
    7) 42.0V
    8) -35.58V
    9) 41.56V
    0) 42.0V
     
  8. rcs16

    rcs16 Super Member

    Messages:
    2,814
    Location:
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    Having -35V at the o/p's (pins 3,8) is a problem. So it is either the STK's themselves or the driver ckt is causing the problem. With power off and the big ecaps DCV charge bleed off (measure with DMM first), measure the resistance between the -ve supplies (pins 2,9) with respect to (pins 8,3).
    Also check those emitter 0.22 ohm R's.
     
  9. clk1981

    clk1981 New Member

    Messages:
    25
    Resistance between 2 - 9 was fluctuating a lot. But sometimes it was in the M Ohms
    Resistance between 3-8 was 0.7 Ohms on both

    Emitter resistors were ok (.4 ohm), but everything I have tested so far has been in circuit
     
  10. clk1981

    clk1981 New Member

    Messages:
    25
    Whats odd is DC offset was OK on one of the channels. It was only one channel with the -36V.
     
  11. tsd71

    tsd71 Gittin Some Head!

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    There are currently zero authentic original Darlingtons on eBay, they pop up from time to time but for the most part they are all fakes.
     
  12. clk1981

    clk1981 New Member

    Messages:
    25
    I tested these voltages with the black lead on the chassis. Should I put the black lead of the meter on the - suppy (Orange) wire powering the STK's instead?

    With Black Meter Lead on Orange Wire (- suppy to STK's) I have

    1) 1.5V
    2) 0V
    3) 5.5V
    4) NC
    5) NC
    6) 1V
    7) 83.8V
    8) 5.5V
    9) 83.8V
    0) 83.2V
     
    Last edited: Jul 19, 2017
  13. mxthunder

    mxthunder New Member

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    21
    I am in the exact same boat. In the process of replacing 1 of my darlingtons in my 780. after measuring voltages on the darlingtons, one appeared to be bad - bought a $15 one from ebay, installed in unit, but still getting ~40V on pins 1 and 0. I found one of the .22Ohm white resistors was giving me an open reading. Trying to figure out if I can replace with mouser # 71-CP0003R2200JE14

    Is there any drawback to using the counterfeit darlingtons besides lack of quality/consistency?
     
  14. clk1981

    clk1981 New Member

    Messages:
    25
    Hopefully we can figure this out! Do the emitter resistors have to be tested out of circuit? All 4 of mine tested .4-.5 ohms while in cicuit.
     
  15. mxthunder

    mxthunder New Member

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    21
    Not sure - i could not get consistent readings while mine were in the circuit, so i took them out and measured just to be sure. i was getting .4-.6 on the ones in my good channel while in circuit so that might be about right.
     
  16. zebulon1

    zebulon1 Into Phase Linear's lately Subscriber

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    You can swap the modules, see if the railed one swaps with it.
    Mark the modules so you know what was where before you make the swap.
    It's not a good idea to run without the modules installed. A potential bad module is better than no module.
    Fasten them in like they will stay there. They need the heatsink to stay cool. If you leave them unattached, they will cook themselves in a short order. They get extremely hot very quickly.
     
    Last edited: Jul 19, 2017
  17. petehall347

    petehall347 the brandy coffee man Subscriber

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    why is it not good to run without a module in circuit ? i never considered this as a problem . am always learning .
     
  18. clk1981

    clk1981 New Member

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    25
    I haven't swapped them, but I have disconnected the power going to them, one at a time. No matter what, the speaker relay does not click. With BOTH unpowered, speaker relay clicks.
     
  19. petehall347

    petehall347 the brandy coffee man Subscriber

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    is it running on dbt ?
     
  20. clk1981

    clk1981 New Member

    Messages:
    25
    Dim Bulb Tester? No, the unit powers on fine with the new replacement STK's, lights light up, I just have no sound.
     

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