Yamaha M4 troubleshoot help

Discussion in 'Yamaha' started by Komodo, Feb 4, 2018.

  1. Komodo

    Komodo New Member

    Messages:
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    Report:

    -DBT constructed.
    -Inserted correct fuses in correct slots.
    -removed all connection from L and R amp sides.
    -Out of sheer paranoia and the dream I had, I pulled the main caps, checked everything (was fine), and reset. Neater this time, those huge lugs are an artform. Polarity was/is correct. Added 2.2uf bypass caps, cause I'm in the why not crowd.
    -Measured the bridge rectifiers (two round black things on right main board, correct?) with diode setting on DMM, and got what looked like good diode readings but this out of my knowledge range.

    I'm posting two vids.
    First vid is either 1st power up or 2nd. (tired lost count)
    Second vid is 2nd and 3rd power ups, or 3rd and 4th.

    On the second vid the relays kicked on which is interesting.
    I'm pretty sure the new OT are toast on the L channel again, probably lost with the 10A fuse. I've got more. I'll test thoroughly tomorrow.

    Should the DBT bulb be completely out?


     
    Last edited: Feb 5, 2018
  2. avionic

    avionic " Black Knights " Subscriber

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    Location:
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    Now plug in one of the amplifier assemblies. And repeat the DBtest. No guts no glory..:biggrin:

    Might as well see if both amps are mucked up.
     
    Last edited: Feb 5, 2018
  3. Komodo

    Komodo New Member

    Messages:
    47
    In each attempt, the main bulb flare is less bright than the previous. Is this a result of the new main caps juicing up?
    I think I'll check the OTs on the left channel and plug in that one tyvm. I'll be protecting the remaining Toshibas with my life!

    (side note: the DBT is in a "shield" box as I never really know what will blow up anymore, and I'm kind of over it now.) :(
     
  4. Komodo

    Komodo New Member

    Messages:
    47
    Before I fire up the DBT with L channel amp, I tested new OTs (in sockets). Toast. Do they have to be removed to test?

    Also, I put DMM on ohms and ran through all resistors and ceramic caps on the drive board just for a rough gauge of life. I found 5 suspects. R441 read 20 ohm when it should be .47k. Three ceramic caps showed OL, and one of the clear and silver foil caps. Is this a useful test?

    I’ll source and replace these before power again as well as check the predrive and all R channel again.
     
  5. petehall347

    petehall347 the brandy coffee man

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    Location:
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    resistors fail high ohms . so i guess in circuit testing was done . that leaves something shorted like semi conductors .
     
  6. avionic

    avionic " Black Knights " Subscriber

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    Location:
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    The most accurate test is with them removed. Goes for all components.
    These are polystyrene capacitors. Very easy to damage just by soldering them in or out of the PCB.
     
  7. petehall347

    petehall347 the brandy coffee man

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    Location:
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    r441 emitter resistor ...0.47 ohms . not k .. emitter resistor . you have a bad r441 and shorted transistor/s . driver and output NPN . might be wrong but logic says that to me .
     
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    38,105
    Location:
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    [​IMG]
     
    petehall347 likes this.
  9. Komodo

    Komodo New Member

    Messages:
    47
    I guess I misread it. Here's the suspect.



    My thought with the impromptu test was that even if I wasn't measuring capacitance, if I got something vs nothing it would be sort of a continuity test on the ceramic caps. Quick snap of what might be wrong?
     

    Attached Files:

  10. Komodo

    Komodo New Member

    Messages:
    47
    That polystyrene is hard to find. The SM says it’s a 500v.

    I know pulling is best for testing, but also recommended to not put heat on the board if you don’t have to? ie. pulling every component to test.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    Location:
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    A little caution note : Polystyrene will not tolerate much heat from soldering/unsoldering. I us a nifty aluminum hair clip as a heatsink when unsoldering/ soldering these little gems.
     
  12. Komodo

    Komodo New Member

    Messages:
    47
    Thank you. Will use heatsink for sure. I’ve managed to source a small amount, so I have backups.

    Sourced the emitter resistors, and transistors, they’re on the way. This weekend I’ll go through the whole stinking thing again part by part to check for fails.
     
  13. Komodo

    Komodo New Member

    Messages:
    47
    UPDATE:
    My parts finally came. In the LEFT channel drive board I replaced:
    -burned FR407
    -bad emitter resistor R441
    -bad transistor TR421 (new but blown). All three of the others tested OK.

    To be safe:
    ceramic caps C413, C435, C407 and polystyrene C405

    DBT with just LEFT connected showed full light bulb.
    DBT with just RIGHT channel showed dimmed and relay clicked on.
    ——————————
    EDIT OOPS - went back with a fresh head, pulled L drive board and measured OTs again.
    Both MJ21193 were blown. Unless they went out when I powered up again?
    Can replace tomorrow and try again.
    ——————————
    So, there is something else. Or, is it possible the other three output transistors could be damaged but still checking OK using simple DMM test of NPN PNP on diode setting? I do have more new ones, so I could try them - but my gut says there is something else. I'm unsure how to proceed at this point.
     
    Last edited: Feb 20, 2018 at 11:40 PM
  14. Komodo

    Komodo New Member

    Messages:
    47
    Three things:
    1) So, I pull the two MJ21193 and check them directly with the DMM with diode setting and now they check good?! Last night I measured the same method but they were mounted in the sockets, so I was using the soldered terminals on the backside - and they measured not good. [POS to C and NEG to B got .004]. Will recheck everything fresh tomorrow.

    2) Also rechecked TR413 and TR415. I hope my little BCE-pos/neg diagram makes sense.
    TR415 [2SA968] is giving me:
    B C E
    n – p .555
    n p – .555
    – p n .000 (not OL, it jumps to .000)

    Is that bad?

    3) On the pre-drive board, I'm not getting a reading from D203. SM calls it a Naristor MV-12. Strangely, the other two red/black spider-egg D207, D209 aren't even listed in the SM.
    Web search turns up NOTHING. What is this component? Is this bad too?!
     
    Last edited: Feb 21, 2018 at 11:37 PM
  15. avionic

    avionic " Black Knights " Subscriber

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    Location:
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    MV-12's as well . Its in the schematic. They can be troublesome.
     
  16. avionic

    avionic " Black Knights " Subscriber

    Messages:
    38,105
    Location:
    Fort Dodge, Iowa
  17. Komodo

    Komodo New Member

    Messages:
    47
    Thank you. I've got datasheets for everything and all of the TO3 check OK.
    The .000 reading threw me, as all other bad ones have been .004 or similar value.

    Also, anyone know of a source for MV-12?
    I've read they are same as VD1212, and that two 1N4148 in series can replace, but also read that this is incorrect.
     
    Last edited: Feb 22, 2018 at 7:24 AM
  18. Mr. Yamaha

    Mr. Yamaha Not so much Yamaha lately... Subscriber

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    Location:
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    That'll work just fine.
     
  19. Komodo

    Komodo New Member

    Messages:
    47
    - Pulled both TR413 and TR415. They measure the same as when installed, but I'm sure TR415 [2SA968] is bad. New one coming.
    - Will replace the MV-12 with a 1N4148 series pair.
    -All L channel TO3 check good.

    Closer?
     

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