Yamaha CA-810 repair

Discussion in 'Yamaha' started by bberkom, Sep 8, 2018.

  1. bberkom

    bberkom AK Subscriber Subscriber

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    Picked up a really nice condition CA-810 for a couple bucks on craiglist today. Advertised as not working, and by the description from the seller, it sounded like blown output transistors. He said that it blew the main fuse one day when he turned it on. He replaced the fuse, and it powers up, but there is now no relay click and no sound from the power amp. It passed the dim bulb test fine. I tested the preamp section with an external power amp, and it sounds great.

    I pulled all four output transistors, and both on the left channel were shorted. The relay clicked in fine after a few seconds with the outputs removed. The output transistors don't match the schematic. I have a 2SD425 and a 2SB555 on the good channel, and a 2SD551 and 2SB555 on the bad channel. The 551 also had a different-looking metal for its case than the other three. It didn't look like anyone had been in there before, but I'm not sure now. I think the 425 and 555's could be originals, but the 2SD551 has been changed I bet. For replacements, I plan to order the following from mouser:

    2SD425-----------MJ21194G
    2SA679 (2SB555)-----------MJ21193G

    Does anyone have advice on how to get to the power amp board? I can't see anything that is obviously damaged just looking in with a flashlight, but I want to inspect it a little better, and it's buried under the function circuit board, which I may need to remove. What other transistors are suspect before I install the new outputs, or does the relay click with the outputs removed indicate that new outputs should fix it?
     

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

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    i would try headphones to see if it works without distortion . if good i might try new outputs with dbt inline .
     
  3. bberkom

    bberkom AK Subscriber Subscriber

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    Good call. It sounds crystal clear with headphones with the outputs pulled. No distortion. I'll go ahead and order new outputs and insulators and do as you suggest with the DBT in line when I fire it up. Hopefully that will get it in working order and I can go from there.
     
  4. onwardjames

    onwardjames Hoardimus Maximus Subscriber

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    Can't tell ya how to fix it, but I will say that if you want minimalism, but still world-class sound, hard to beat that specific model Yamaha.

    Had it, sold it, and miss it. Great all around performer.

    Good luck.
     
  5. bberkom

    bberkom AK Subscriber Subscriber

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    Thanks for the well wishes. I have been running a Yamaha combo in my main system for a while now, and I have become a big Yamaha fan. Wasn’t really looking for another project or amp, but i couldn’t say no.
     
  6. petehall347

    petehall347 the brandy coffee man Subscriber

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    just be sure of idle bias setting after putting new outputs in ..
     

     

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  7. bberkom

    bberkom AK Subscriber Subscriber

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    Yep I plan to set all of the voltages per the service manual when the new outputs are in. I’ve seen some replacing fusible resistors when recapping these too, but that’ll be round two if I get around to it.
     
  8. petehall347

    petehall347 the brandy coffee man Subscriber

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    be sure to use current limiting lamp . DBT .
     
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  9. avionic

    avionic " Black Knights " Subscriber

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    Be thankful your tone board doesn't look like this.. DSC04223.JPG
     
  10. bberkom

    bberkom AK Subscriber Subscriber

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    Yikes! The amp I picked up hasn’t seen anything close to that kind of abuse.
     
  11. drumbum

    drumbum Super Member

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    So that's where it went.:thumbsup:

    Had my sights trained on it.
     
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  12. bberkom

    bberkom AK Subscriber Subscriber

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    Yeah, it was only up a few hours and I had to have it. Hasn’t been much good cheap stuff on the Nashville CL lately, but this should clean up nice.
     
  13. bberkom

    bberkom AK Subscriber Subscriber

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    Well, this is still giving me issues after installing the new output transistors! @petehall347 @avionic

    I installed the right channel output transistors first since I knew that channel's originals were still good. I fired it up and it passed the DBT and the relay clicked. I checked the bias, and although a little low at around 16mv, I left it for now. I hooked up a speaker and got good, clean, undistorted sound from the right channel.

    I installed both of the left channel output transistors, and it will not pass the DBT and the relay doesn't click. I get good output from the headphone jack from both channels with the left output transistors removed, but there is still a short somewhere with them installed. Visually, there is nothing burned up on that channel, but there is some of the brown glue around transistors TR423 and TR409 and some surrounding resistors and diodes. I need some guidance on what to pull and test next to continue troubleshooting.
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    test the insulation on the outputs and be certain they are installed in the right places
     
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  15. avionic

    avionic " Black Knights " Subscriber

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    If they are installed correctly. Then pull and test TR421 and 423 drivers for "shorts"
    Might check the emitter resistors as well. They sometimes burn open when outputs short out.
    R469 and 471 - .47 Ω 5watt.
     
    Last edited: Sep 16, 2018
  16. petehall347

    petehall347 the brandy coffee man Subscriber

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    :idea:
     

     

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  17. bberkom

    bberkom AK Subscriber Subscriber

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    I checked installation locations, and they are correct. I also loosened and reseated the screws, making sure they are centered in the holes in the heatsinks and not shorting out. I also measured the continuity between the cases and the heatsink and got in the neighborhood of 0.8 to 1.0 M-ohms on all four output transistors, left and right channels. So I think the installation is fine. The wire-wraps on the transistor sockets and amp board all look fine as well.

    I am going to pull them and confirm again that the output with headphones is undistorted. Would clear output from the headphones rule out bad driver transistors, or should I pull and test them anyways?
     
  18. avionic

    avionic " Black Knights " Subscriber

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    Remove all traces of the glue and inspect for corrosion damage.
     
  19. avionic

    avionic " Black Knights " Subscriber

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    Is the mode switch in mono or stereo?
     
  20. bberkom

    bberkom AK Subscriber Subscriber

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    I believe it was in stereo last time I checked, but I will confirm that again once I pull the outputs and test with headphones.
     

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