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Yamaha B2 driver board diagnosis

Discussion in 'Yamaha' started by belgianbrain, Nov 4, 2018.

  1. belgianbrain

    belgianbrain Super Member

    Messages:
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    Location:
    Greater Toronto Area, Canada
    I am going to start a new thread for this, since this discussion could be long and protracted.

    I ran into a snag trying to adjust the driver boards in my B2 rebuild. Basically, I replaced all resistors in the signal path, many fusible/fire resistant resistors, and all capacitors I had polyproylene replacements for. I also changed the big electrolytic cap with Nichicon Muse.

    I have unit assembled with VFETs removed. I have a 10k resistor between PG and NF and 10K again between NG and NF to simulate presence of VFETs for rough adjustments.

    DC balance can be set to zero with VR101, but bias remains at zero regardless of VR102.

    Front power meter for one installed driver board stays elevated at about 0.01W (this one has me a bit perplexed as how does signal escape FET board with no FETs installed?).

    I took some readings and I get the following on the first driver board:

    +B2=85V
    -B2=-85V
    +B=60V
    -B=-60V
    PG=15V (a tiny bit high, should be 13.5)
    NG=-1.3V (very low, should be -13.5)
    NS=0
    NF=0
    PS=0

    On second driver board, NG measured at -45V on this one and then I quickly realized that R168 was burning up. R149 also looks charred on this second board now. I also heard a loud pop at some point.

    I haven’t tested transistors yet, but I’m thinking I will just replace all of TR 121, 122, 123, 124. Maybe I should just do TR 117, 118, 119, and 120 while I am at it too. Thoughts? I would also check and replace, as necessary, resistors and such as I work.

    I do wonder why I am having a problem in the same area of both driver boards, though. I can’t see that I have altered anything on this boards. Both driver boards seem to have a similar problem, which really has me scratching my head. I've rebuilt these driver boards before in another unit without issue. I'm also careful to measure each component I remove and measure each replacement part just to make sure it's a match. Yet, if the issue is on the driver board, it seems I made the same mistake on both driver boards. :dunno:

    As an aside, what is the purpose of the studs that stick up and contact the TR 121, 122, 123, 124 heat sinks? They appear to just be for stabilization, but these studs look to be coupled to signal pads, which I find weird.

    I'd appreciate any insights as I work through this unwanted excitement.
     

     

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  2. belgianbrain

    belgianbrain Super Member

    Messages:
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    Anyone have recommendations for replacing TR 121, 122, 123, 124? I found replacement parts in other threads, but nothing that is actually available on Mouser.
     
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
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    How about these new products from Toshiba TTC004B/TTA004B
    https://www.mouser.com/datasheet/2/408/TTC004B_datasheet_en_20180223-1102342.pdf

    https://www.mouser.com/_/?Keyword=TTC004B,Q
    https://www.mouser.com/ProductDetail/Toshiba/TTA004BQ?qs=/ha2pyFadujkBBywZDMUAZwqtCoiCpEUnT%2bKJIUMYe4=

    Heatsinks would be needed most likely.
     
    Last edited: Nov 4, 2018
  4. avionic

    avionic " Black Knights " Subscriber

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    Last edited: Nov 4, 2018
  5. belgianbrain

    belgianbrain Super Member

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    Location:
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  6. belgianbrain

    belgianbrain Super Member

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    Does anyone have a B2 parts unit they are willing to sell parts from?

    I need a bridge board that connects the driver board with the VFET circuit board. One of mine cracked.
     

     

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  7. avionic

    avionic " Black Knights " Subscriber

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    How badly did it crack?
     
  8. belgianbrain

    belgianbrain Super Member

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    Right in half. I slipped with my probe and shorted +/- 60V. It blew the traces right of the bridge board and cracked it in half.
     
  9. avionic

    avionic " Black Knights " Subscriber

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    :needpics:
     
  10. avionic

    avionic " Black Knights " Subscriber

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    That" dog bone" board should be very easy to duplicate.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Probably wouldn't be that tough to repair. Granted it probably wouldn't be pretty.
     

     

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  12. belgianbrain

    belgianbrain Super Member

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    20181111_154333.jpg
    Repair this? Do tell. :)
     
  13. avionic

    avionic " Black Knights " Subscriber

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    If I can repair this......
    [​IMG]
    Yours would be a walk in the park.;)
     
  14. belgianbrain

    belgianbrain Super Member

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    Care to share the details? How do you bond the board together? Can you repair the traces or do you have to do point to point wiring?
     
  15. avionic

    avionic " Black Knights " Subscriber

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    Epoxy
     
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  16. avionic

    avionic " Black Knights " Subscriber

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    For the traces that haven't burnt and peeled-Simple jumpers across the crack -- I would use small pieces of solder wick. For the two that were blown off the board. P2P with solid coated wire. Held in position with a couple dabs of epoxy.
     

     

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  17. avionic

    avionic " Black Knights " Subscriber

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    Every time I replace a light ballast at work. I keep the trimmings. Its 20 gauge solid copper. Great for this app.
     
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  18. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    How is it possible to brake a PCB like that ?

    One AK member offered this some time ago ( october 2013 )

    NAO6753 Holey Cow.JPG
     
  19. avionic

    avionic " Black Knights " Subscriber

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    Like I said. It would be easy to duplicate.
     
  20. avionic

    avionic " Black Knights " Subscriber

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    Last edited: Nov 11, 2018

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