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CR-620 Woes

Discussion in 'Yamaha' started by goldswimmerb, Nov 6, 2018.

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  1. goldswimmerb

    goldswimmerb Member

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    88
    The 371s are matching up with the voltage supplied on the schematic
     

     

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  2. ConradH

    ConradH Addicted Member

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    Not saying it's the PS, but there seems to be a generic problem with the entire power side board, like they cleaned them with something that caused early cap failure. The ones I've worked on had the same basic problems, and nothing's going to work right unless the caps are dealt with. As for throwing parts at a problem, you know full well I don't work that way, but in this case, unless one has the equipment to test 'em, those caps can be assumed bad. Ditto the two transistors I mentioned. Odds are very high they're a source of erratic behavior. Not to say there aren't 37 other problems, but those are low hanging fruit.
     
  3. avionic

    avionic " Black Knights " Subscriber

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    Post a photo of the heatsink so I can see the output transistors labels.
     
  4. goldswimmerb

    goldswimmerb Member

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    88
    So this resistor (I think) (the one covered in the tube) Just popped and made a nice little fire for a bit.[​IMG]
     
  5. goldswimmerb

    goldswimmerb Member

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    88
    Also here's the transistor pic

    [​IMG]
     
  6. avionic

    avionic " Black Knights " Subscriber

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    [​IMG]

    I can't read the transistors.. Are they from left to right 2SD 2SB 2SD 2SB ?
     
    Last edited: Nov 8, 2018

     

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  7. goldswimmerb

    goldswimmerb Member

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    You got it.
     
  8. avionic

    avionic " Black Knights " Subscriber

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    If so measure the voltages around(E,B&C) all four and post results.
     
  9. goldswimmerb

    goldswimmerb Member

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    88
    After the resistor blew I'm not sure it safe to turn on
     
  10. goldswimmerb

    goldswimmerb Member

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    However before it popped the E B & C on the 2SD371s we're within tolerance. It was the 2SBs that we're showing 38v on the EB and C
     
  11. goldswimmerb

    goldswimmerb Member

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    88
    But not sure why only L701 would be showing rail voltage in that case
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

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    Looks like a 4.7Ω 1/4 watt. What were you doing when this burnt up?
     
  13. goldswimmerb

    goldswimmerb Member

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    88
    Nothing actually, it was between measuring voltages.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    I have to crash. Its midnight here.
     
  15. avionic

    avionic " Black Knights " Subscriber

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    Because....

    both of the of the 2SB531 are showing C- -37.2 B- -37.2 E- -38.1v


    Remove the 2SB's and test them out of the circuit.
     
  16. goldswimmerb

    goldswimmerb Member

    Messages:
    88

    So just an update, pulled both the 2SBs and tested them (just using continuity/diode mode on my multimeter). One of them tested fine. The other showed a dead short between the Collector and Emitter. Going to guess that was our problem.
     

     

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  17. avionic

    avionic " Black Knights " Subscriber

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    That will solve one problem.
     
  18. goldswimmerb

    goldswimmerb Member

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    Okay so, we removed the two bad transistors, and powering the amp on with the good ones in yields a click (In the TR-723 and TR-725 spots). However, doing so in the TR-726 and TR-724 side yields wildly varying negative voltages. And using a known good set of transistors to replace the bad ones puts us back to the beginning with there being rail voltage at L701 again, and caused our replacement resistor to pop.
     
  19. avionic

    avionic " Black Knights " Subscriber

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    TR723 ---- 2S ?
    TR725 ---- 2S ?
    One 2SB and one 2SD?
     
  20. goldswimmerb

    goldswimmerb Member

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    88
    Yup, blown pair.

    725-2SB531
    723-2SD371
     

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