Sansui G-7500 Fried Resistors F2980 Board

Discussion in 'Exclusively Sansui' started by Young46, Oct 10, 2017.

  1. Young46

    Young46 AK Subscriber Subscriber

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    Houston,Texas
    So, can you verify what happens with the Bias when you turn the amplifier on. If the amplifier is oscillating I would expect the bias to be normal for a few seconds, and then suddenly shoot up...
    Can you connect the meter, and try this...
    [/QUOTE]

    When I turn the amplifier on, it starts climbing immediately.
     

     

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  2. kevzep

    kevzep Its all about the Music Subscriber

    How quickly does it climb to 29V?
     
  3. Young46

    Young46 AK Subscriber Subscriber

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    It always begins around 27-28V and climbs to 29V in about 15-30 seconds. Then it will continue to climb at around the same pace. Seems to slow down at 31V. Highest voltage I’ve see is 31.14 but I always shut it down when it gets up around 30-31V. Also when it gets to around 30V the voltage starts to fluctuate down then up.

    The comments about the burned traces are typical of my board. As I stated before, this board had been worked on before. Several pads are lifted or completely gone. I have bridged the connections but maybe (probably) that’s a big part of the problem.

    Thanks for the advice on the scope.
     
  4. kevzep

    kevzep Its all about the Music Subscriber

    At this point, I don't think it's oscillating........I think you are going to have to go meticulously through every trace on that board and verify them. I am thinking this is the problem, until we do this, anything else is just an assumption.
     
  5. Young46

    Young46 AK Subscriber Subscriber

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    Yeah, I went through it carefully but not meticulously. I need to do that and will spend next week doing so. What are your thoughts on bridging? Best method for bridging? I wish I had a way to repair the traces but I don’t. Would it be of help to list what components have been bridged? I will report back ASAP, probably toward the end of next week.

    Thank you for your help and patience.
     
  6. kevzep

    kevzep Its all about the Music Subscriber

    There are many different ways to repair traces, just depends what works for you.
    You always have to scrape the green stuff off, then, you can either use a small piece of a leg from a resistor or diode, chop it to length and solder it over the crack/gap. I mostly do it this way rather than trying to flow the solder over the gap.
    If you bridge the gap with solder, it may well crack again as the solder doesn't really have enough tensile strength for that.

    I keep a bunch of off-cuts from components for that kind work work...
    Use flux as well, it helps the solder flow nicely and make a good bond to the surfaces.

    Just go over the board paying attention to the area where the STV joins the rest of the bias circuit.

    I am sure you will find something.
     

     

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  7. rcs16

    rcs16 Super Member

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    When i repaired this F-2980 before I used some 24awg solid telephone wire, stripped the insulation, formed the wire as was the original trace, make small circles for eyelets (wrap around the intended lead diam to form) so the leads can poke through. Solder the ends first and then the middle parts. you can use the stripped insulation bits and slide back on, to use as spacing or insulation from other comps. Can glue if needed.
    Follow the pcb pattern and the other channel.
    While you are at it, you should do an exact 1:1 ohm readings comparing the good channel vs the bad channels. Follow paths like from a supply to a semi junction to compare 1:1.

    I re-ran simulations for this design, using modern day parts,
    As you see I have the DC ecap(C15 in the sim) installed in the FB loop, you should do the same, as it helps to tame the amp down and may fix your problem.
    I believe this design has too much open loop gain for a DC amp.

    Good luck
    Rick
     

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    Last edited: Jul 8, 2018
  8. Young46

    Young46 AK Subscriber Subscriber

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    Location:
    Houston,Texas
    Thank you for the information and encouragement.

    I mentioned an oscilloscope in a reply. I’ve never used one but would like to learn. I enjoy this hobby so it could, as kevzep said, be a good investment.

    Frank
     
  9. kevzep

    kevzep Its all about the Music Subscriber

    Some good stuff there Rick, I am actually in the middle of rebuilding my G7700, I might just apply some of this whilst I am in there.....
     
  10. kevzep

    kevzep Its all about the Music Subscriber

  11. Young46

    Young46 AK Subscriber Subscriber

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    Thank you, I will. I took a chance on the Hitachi V223. It powered up and I could adjust on xy axis. It is in very good condition and had two new probes.
     

     

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  12. kevzep

    kevzep Its all about the Music Subscriber

    Nice one Frank, take your time with it and look at the tutorials, basic scope operation is not that complicated.
     
  13. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    well done
    This series from Uncle Doug are very clear (just in case)
     
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