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The first strangely failed STV diode I found.

Discussion in 'Pioneer Audio' started by zebulon1, Aug 11, 2018.

  1. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    A set I found in SoCal I bought to flip. It was broke and in crusty shape.
    It had been repaired once before but this time both amps were railed due to a bad PS.
    I got that sorted and rebuilt the amps with new silicon and caps.
    The left amp would not show any idle adjustment. Checking everything again I looked at the Bourns trimmer values as I did have some base voltage at the pre drivers and they responded to the trimmer movement. The bad amp was only half the 1.5v pre driver base voltage of the good amp. Half? WTHeck I looked at the traces and ohm'ed out each trace (Which doesn't take as long as some would think). They all checked good.
    If I cranked the idle trimmer full CW I could get a mV. So I went to the trusty STV and found this:
    Its a STV-4H (Should read 1.835v)
    Not that this failure is really rare but it is the first time I saw one reading almost half of the correct value.

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    had a bias diode do that recently .not an stv but should have measure 3x diode drop . only measured 2x . put it down to one diode shorted .
     
  3. kevzep

    kevzep Its all about the Music Subscriber

    As these components age more and more, I think we'll see more of these sort of things popping up.
    I had that partially shorted ceramic cap in the SX1250 Stabiliser circuit...now this....

    How are you going to get around this issue?
     
  4. zebulon1

    zebulon1 Building a new bench. Finally! Subscriber

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    I had spares. You see what they go for on eBay?
    Figures Pete would of seen this sometime:D
    EW and MTF have threads on using the 4148's in series and some have used the work-a-round with success.
     
  5. petehall347

    petehall347 the brandy coffee man Subscriber

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    Last edited: Aug 11, 2018
  6. kevzep

    kevzep Its all about the Music Subscriber

    Thats convenient to have spares.
    Yes, easy to find STV3, not so easy to find the STV4, and when you do they are no cheap!!

    I have studied those threads with the 4148 and it looks like they'd work just fine if there was no other option.

    There's always a way...
     

     

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  7. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    The REALLY fun ones are those that meter OK but don’t work right.

    I’ve seen some do that crap!
     
  8. sregor

    sregor AK Subscriber Subscriber

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    Just a thought - I wonder, given the price of STV-4s, if it were a counterfeit part previously put in.,..

    Good catch
     
    Last edited: Aug 12, 2018

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