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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
    Hello AK friends.
    One of my buddies recently picked up a CR-620 which turns on normally, however, we get no relay click and no sound. We swapped the bulbs too since we figured why not.
    Following merrylanders advice, we tried swapping the 2SD234 transistor out for a 2SD476 (if I recall correctly), which did help some (the 234 was definitely dead after testing it). However, after the swap, we still get no output. I went ahead, in my frustration, and tested the DC voltage at the two large coils as per another user's advice, and no we're getting a 16V DC reading on one coil (the one closer to the transformer), I assume this 16V DC isn't supposed to be here, and the expected (less than 4mV DC) on the other coil.
    At this point, we're both frustrated and looking for any advice that may help, we'd both like to get this working as its a fairly cool amp (and my friend doesn't want to be out his money).

    Thanks in Advance!
     

     

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  2. avionic

    avionic " Black Knights " Subscriber

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    Did you check both output transistors on the side that has 16volt or just the one 2SD?
     
  3. goldswimmerb

    goldswimmerb Member

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    I haven't actually checked anything else on the side with 16V.
     
  4. avionic

    avionic " Black Knights " Subscriber

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    Check both of the outputs on the 16v side. I'm not sure if the D234 is a compatible sub for the D476.
     
  5. avionic

    avionic " Black Knights " Subscriber

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    No problem there.
     
  6. goldswimmerb

    goldswimmerb Member

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    Which parts would those be, if you know?
     

     

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  7. avionic

    avionic " Black Knights " Subscriber

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    Output transistors 2SD234 and 2SD476. I checked on their compatibility. Should be no problem.
     
  8. goldswimmerb

    goldswimmerb Member

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    88
    The 2SD234 was the one we already replaced
     
  9. avionic

    avionic " Black Knights " Subscriber

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    Correct---
    Have you tested its complement the 2SB yet?
     
  10. goldswimmerb

    goldswimmerb Member

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    I think we may be referring to different transistors?

    The 2SD234/476 is the transistor with the small silver heatsink that's known to die in these.
    I think you're referring to the 2SB531 and 2SD371 pairs, yes?
     
  11. goldswimmerb

    goldswimmerb Member

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    88
    Okay so weird update, after poking around with the multimeter at TR-701 and TR-703 (703 seeming to have 15v for no reason) we got the relay to flip, with a DC bias measured at 30mv-100mv, it seems to be all over the place and the audio is very poppy, could one of those be bad?
     

     

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  12. avionic

    avionic " Black Knights " Subscriber

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    You mean DC offset or idle current/bias?
    Probably. These would be the output transistors mounted to the large heatsink. I take it you initially were talking about power supply pass transistor.
    From now on reference transistors with there TR #.
     
  13. avionic

    avionic " Black Knights " Subscriber

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    15v where ? Emitter,base or collector? Need to be a little more specific.
     
  14. avionic

    avionic " Black Knights " Subscriber

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    Measure the voltages around TR802 (2SD476 now I assume)
    B----?
    C----?
    E----?
     
  15. goldswimmerb

    goldswimmerb Member

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    So TR703 Was giving us
    B- 15 (or 7.5) (I took a break a bit ago so the number here is fuzzy in my mind, my apologies)
    E - 7.5
    C- -38
    (I may have the order wrong, I can check later, my apologies)

    It's been a while, but recalling checking that transistor right after replacement, the TR802 was within +/- 10% of the voltages on the schematic from hifiengine.

    What's weird, now, is the amp is working after poking around (all of a sudden the relay clicked and now it's working seemingly fine?) On startup the DC at L701 (I think, may have been L702) is around 40mV and under use it was dropping down towards 20mV. Right after startupthere's a lot of popping from the speakers, but it calms down after a few minutes with only occasional light pops.
     
  16. avionic

    avionic " Black Knights " Subscriber

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    TR703 expected voltages
    E) .6 v
    B) .08v
    C) - 39v
     

     

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  17. goldswimmerb

    goldswimmerb Member

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    So as I think i said earlier, out of the blue, the relay clicked and the amp has output now.
    I think that transistors voltages dropped back to within 10% of spec.

    Now, the DC at the coil varies from 100mV down to 20mV, and there's popping as the amp warms up and randomly on occasion during playback. The popping is not related to the volume. I went in a reflowed a bunch of joints since i think poking around must have fixed a cold one. Any thoughts as to what may be causing the popping?
     
  18. avionic

    avionic " Black Knights " Subscriber

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    Could very well be a cold solder connection.
     
  19. petehall347

    petehall347 the brandy coffee man Subscriber

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    failing transistor can cause popping
     
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  20. avionic

    avionic " Black Knights " Subscriber

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    Can't rule that out as well.
     

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