SX-950 Relay Bypassed and Burnt Resistors

Discussion in 'Pioneer Audio' started by Pageplayer, Jul 12, 2018.

  1. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    Hey All,
    As I briefly mentioned in this thread Here, I happen to find a SX-950 at a OK price knowing it would need cleaning and a recap.
    What I didn't expect is that when I opened it up before powering it up, I found that the relay assembly has been bypassed with a wiring hack, and I also noticed a couple burnt resistors on the power amp board.

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    So I guess I'll need a relay which I'll order with the recap BOM. I seen a list of Pioneer relay equivalents here on AK a couple weeks ago, but now I can't find it. o_O Can someone point me in that direction?

    Seeing as how I'm pretty new to this hobby, I'll take any suggestions on how to approach the repair.

    Yikes!
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Hold off on ordering the relay until a more complete bill of material can be compiled. Get ready with your meter to begin checking components.

    Precisely which resistors have overheated? That should provide a reasonable indication of what circuit(s) went over current.
     
  3. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    I can visually see the insulated sleeve on R40 is burnt and R38 is charred like a burnt hotdog.
    Is it ok to test voltages with it plugged into my DBT?
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    I would be wary of powering on the unit with anything but the smallest wattage lamp, maybe 40W as a start. That could still cause heating, so monitoring everything on the questionable channel would be advisable.
     
  5. markthefixer

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

    Messages:
    21,011
    Location:
    Bensenville,Illinois
    NO power for a while. Parts will have to be removed to clear the problem.

    GOOD job on identifying the components, just be sure that you haven't been fooled by mirror images,
    edit - you were - those are R37 & R39 - the tip off is looking at the clusters of the pots. The image you used was of the FOIL side - while the picture shows the component side. It gets us all at first....

    D7, D9, Q9, Q11 are probably all damaged,

    Check R43, R45, R47, R49 with one leg each disconnected - these MAY have saved your outputs - the output transistors will have to be removed and tested.

    R51, R53, R55, R57 all need to be tested, ALSO with one leg each disconnected. Failure here will be open circuit, not shorted.

    VR3 (and VR4 on the other channel) need to be new.

    Mark each part with it's position when removed for testing or replacement.

    There are also other caps and transistors to replace - known troublemakers - since you are in there.
     
  6. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    Ok. Sounds like a plan, I will follow to a tee. I will try and get to it this weekend.

    Thanks!
     

     

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  7. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    Sorry for the delay.. Busy week.
    I had parts come in.. I'm going to pull the amp board, replace the charred resistors and start taking measurements of the others. I'll also check transistors with my Dynamic Transistor Checker..
    153226606234490403005.jpg
     
    Last edited: Jul 22, 2018 at 9:04 AM
  8. rcs16

    rcs16 Super Member

    Messages:
    3,655
    Location:
    Baldwin, Ontario, Canada
    It depends on your budget, but it might be wise to just rebuild the whole amp pcb and when you order parts buy the parts to re-build the power supply at least. If you can afford to do a complete rebuild it would be a good idea. Sounds like this unit has had a hard life :)
    We know all the faulty bits so it is just a matter of time, money like most things.
    Who knows the resistor beneath the fibreglass cover maybe okay, some one replaced the R already and used the old cover, you have to isolate and measure.
    Comparing one channel to the other is a way to find faults.
    Usually charred resistors mean a semi failure(s), starting from the bjts on the big heatsinks. I first blew mine back in the 70's, finally used the newer parts which are so much better. The NTE drivers are still working :)
    Still good experience to fault find, so following Mark's advise, it is a way to learn from the experts methods.
     
  9. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    Here's what I have so far...
    Left Channel
    R37 = 20ohms.. Should be 150
    R39 = Was Open
    R41 = 6.84K Should be 5.6K

    I replaced all these with new on both channels.
    Starting to look at the other now....
    Stay Tuned
     
  10. kevzep

    kevzep Its all about the Music Subscriber

    Never seen a resistors drop in value like that, the usual failure mode is they go highZ.
    Maybe have another look at that R37.
     
  11. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    What am I doing wrong :(
    20180722_160003.jpg
     

     

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  12. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
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    And this one was pulled from right channel..
    20180722_160436.jpg
     
  13. kevzep

    kevzep Its all about the Music Subscriber

    Okay, I guess there's a first time for everything, thats a new one to me....
    No harm in double checking though, you're not doing anything wrong, carry on!!

    Looks like you have quite the mission to get this one running, should be a good project...
     
  14. Pageplayer

    Pageplayer AK Subscriber Subscriber

    Messages:
    23
    How do I check Output transistors?
     
  15. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
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    Location:
    uk.. the middle bit
    someone may have put wrong value resistor in there .. going off that lovely work around the relay anything might have happened .
     

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