Yamaha A-1000 Repair and Info Thread

Discussion in 'Yamaha' started by tmsears, Apr 13, 2017.

  1. tmsears

    tmsears Active Member

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    B-E and E-B comparison tests so far:
    Output Transistors OK
    Driver Transistors OK
    Bias Compensators OK
    Predrivers: not so much. resistance for both NPN and PNP on the good channel is approximately twice that relative to the NPN and PNP on the bad channel..
     
  2. Hamish119

    Hamish119 New Member

    Messages:
    48
    Location:
    London, UK
    If you can't find the failure during passive (powered-off - the safest method) testing, get some sleep and when you're refreshed, measure the various supply voltages (with power applied, obviously) etc.

    The light bulb limiter must be used until the problem has been identified and corrected!

    Engage the 'Subsonic Filter' as this will/should remove any DC voltage (from an internal/external source) being fed into the power amp input.
    Set 'Auto Class A' and all other switches to off.
    Power on.
    Measure the plus & minus HB, LB and 22V (CRITICAL) rails. (These should always be documented)
    Measure the DC offset at the junction of R263 & R265. and the junction of R264 & R266.
    Switch off and check for hot semiconductors that shouldn't be hot. :yikes:

    Post the results.

    Good Luck! (And sleep well!)
     
    Last edited: Aug 20, 2017
    tmsears likes this.
  3. Hamish119

    Hamish119 New Member

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    Please disregard the above statement - I was looking at the A-700 circuit and block diagrams. :oops: :oops:
    The A-1000 SUBSONIC FILTER is NOT wired the same as the A-700 SUBSONIC FILTER, and according to the block diagram, will NOT remove any DC voltage induced by errors in the FLAT AMP and TONE CONTROL circuits from the power amp input.
    My humble apologies for the misleading and erroneous information. :oops: :oops:

    Good Luck!
     
  4. tmsears

    tmsears Active Member

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    No worries. I don't know how you pros keep these things as straight in your heads as you do. :biggrin:
     
  5. zaibatsu

    zaibatsu Active Member

    Messages:
    393
    That's good, means you found a problem. Something is amiss, have a good probe around on the resistors and diodes around those predrivers, as well as the same on any surrounding transistors. Did BDent get back to you on whether they stock the correct sub?

    Post the numbers if you like, might give some clues (which might tip Hamish / avionic off), and I can probably check mine to compare if needed.

    Hey off-topic, but thanks for the PM much appreciated, passed your link on to the owner.
    You have some heavy security settings so I can't reply (or view your profile, or anything).
    It's worth it to stop the evil audio-forum baddies I presume. :D
     
  6. tmsears

    tmsears Active Member

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    Update: B&D Replied to my email and apologized for the error. They say that they do indeed have the c4793 in stock and that it was a packing/shipping error. The fellow said he would ship the correct ones out that day at no cost. :rockon:
    No tracking number, but I won't complain.
     
  7. tmsears

    tmsears Active Member

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    Well back from an absence of personal nature, and I am back in the mood to look at this thing.

    I have been testing diodes and resistors in circuit, comparing the readings with the good channel, but they are all the same, so far.

    I have pulled a diode or two and tested them with my little chinese doodad but nothing there either.

    Not had tunes the "Audiokarma way" for nearly 6 months or more now, and it I don;t get this resolved sooner than later, I may put it aside or sell it off, although I will never get back what I put into it so far. :(

    One a side note: the correct transistors from B+D arrived by mail a whole day, maybe two, after they responded back via email, so go B+D!
     
  8. zaibatsu

    zaibatsu Active Member

    Messages:
    393
    • Is the bulb dimming normally after you turn the amp on? (couldn't see this mentioned)
    • Are the correct drivers installed now on both channels?
    • Did you re-measure the B-E and E-B resistance across both predrivers afterward?
    Please post the values in both directions (for good and bad channel), and also post the transistor TR numbers you're measuring to confirm.

    You should take the offset measurements Hamish suggested too (while on DBT), and post them.

    The remaining problem will be something quite small, I'm sure we'll figure it out if you're patient and post measurements.
     
  9. OMGCat!

    OMGCat! AK Subscriber Subscriber

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    The last M-60 I went through was giving me fits until I checked literally every singe component and it turned out a few off value resistors were giving me wacky voltages. You might need to sit down and compare between channels and see if anything is off. Going between channels means that in circuit testing is ok since whatever may effect readings of, say, a resistor should be the same on the other channel.
     
    Last edited: Sep 4, 2017
  10. tmsears

    tmsears Active Member

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    OK, I tested all the diodes and resistors I could think of and they all match the good side...so much for that.

    OK, there is nothing at the junction of the resistors on either side, which I figured is normal since the relay won't click.

    As for the rail voltages, forgive this amateur, but where do I test these at? I mean, they are everywhere.. Do you mean at the power board that the giant caps are on? I measured at the BBQ board and the numbers weren't right at all for the 22v, but then again, I don't know what the voltages for the others are supposed to be or where to measure them at..
     
  11. tmsears

    tmsears Active Member

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    See post #241, outside of that, nothing is awry, but then again I barely know what I am doing so there is that.
     
  12. avionic

    avionic " Black Knights" Subscriber

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    At the collectors of the output transistors.
     
  13. avionic

    avionic " Black Knights" Subscriber

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    Or.. At the big capacitors.
     
  14. tmsears

    tmsears Active Member

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    With negative probe attached to ground, set to VDC?
     
  15. tmsears

    tmsears Active Member

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    While I waiting on an answer on how to measure the rails using the collectors by @avionic , I connect the voltmeter to the negative and positive of on of the big capacitors, it measured 33.8 VDC, 74.5 AC, with the dim bulb tester. didn't know if AC or dc readings was needed. have forgotten which runs through the system. I thought capacitors blocked dc, but I dunno right now.

    I apologize for my stupid questions in advance.. I have had a mildly traumatic last few weeks, and since my "electronics" knowledge has not been burned into my brain by rote, I have forgotten half of what I have learned so far :(
     
  16. avionic

    avionic " Black Knights" Subscriber

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    Correct..
     
  17. tmsears

    tmsears Active Member

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    The good side left the right side is what blew up. The transistors are not soldered on, the "test" resistors are on base and emitter instead:
    LP+ 33.5
    LP- 33.5
    RP+ 49.2 <---- I know enough to know that this ain't right, but as to why, I know not..
    RP- 33.5
     
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  18. avionic

    avionic " Black Knights" Subscriber

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    That could definitely give you about 16 volts of DC offset.
     
  19. avionic

    avionic " Black Knights" Subscriber

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    Stand-by..................
     
  20. avionic

    avionic " Black Knights" Subscriber

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    Hmm delimma. According to Hoyle.. The RP+ is +45vdc being target value. Unless theres and error in the SM schematic. :idea: Definitely have a power supply issue. If your still on the dimbulb probably explains the low voltages. ie +/- 33.5vdc

    Possibly D164 is the culprit.
     
    Last edited: Sep 7, 2017

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