Yamaha CR 2020 41Vdc on outputs!

Discussion in 'Solid State' started by Mid-Fi, Sep 12, 2017.

  1. Mid-Fi

    Mid-Fi AK Subscriber Subscriber

    Messages:
    1,326
    Location:
    Joplin
    This unit wouldn't come out of protection and I found big DC on R1 & L1. So looking into the service bulletins and inspecting the PS board sure enough, all the usual suspects. I ordered parts and swapped the resistors and caps and reflowed the joints and still...41Vdc on R1 & L1.

    This unit shows ~ +/- 59Vdc B+ and ~ 110Vdc b- which the manual shows to be pretty much spot on.

    I am no tech so this is as far as I know to go. Any help is appreciated. Thanks, Victor
     
  2. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    Seperate the pre-amp from the power amplifiers with the coupler switch on the rear panel. Then recheck your DC offset on R1/L1.
     
  3. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    Both channels ?
     
  4. Mid-Fi

    Mid-Fi AK Subscriber Subscriber

    Messages:
    1,326
    Location:
    Joplin
    Yessir

    Brown and yellow wires on the relay board. Welded the coil on a test speaker.
     
  5. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
     
  6. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    How did that happen ?
     
  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    Sounds like a power supply issue if its both channels.
     
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    If you burnt the voice coil in a speaker...
    1. Someone has jumpered over the protect relay.
    2. Or..Your protect relay is welded.
    3.Or.. the relay driver transistor is shorted.
     
    mbz likes this.
  9. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
    Melbourne, Victoria
    While avionic is getting a couple of hours sleep.

    First step is to identify problematic channel. LI and RI are
    connected by 2x47k resistors so a dc offset on one will kinda
    appear on the other. Important to know the sign of the dc offset,
    looks like positive 41Vdc. Please confirm.

    Measure dc voltage between RI(RED probe) and LI(BLACK probe), probably
    need mV scale, important to note sign pos or neg mV.

    positive xxmV indicates problem on right channel (yellow wire)
    Negative xxmV points to left channel.

    Man, you should be sleeping...
     
  10. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
    Melbourne, Victoria
    I don't have a good way of finding faulty TR, just slow voltage measurement, alternatively need to start
    testing transistors, start with TR611???
     
  11. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    Check your solder connections on regulator pass transistors TR712 & TR715
     
  12. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    Relay driver transistor .. just in case TR702
     
  13. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    Rust doesn't sleep.
     
    Ds2000 and DRecovery like this.
  14. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
     
  15. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
    Melbourne, Victoria
    take a look at R741, 2. They connect RI and LI
     
  16. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    To the protect circuit ? I suppose if they were both "open" that would disable the protect circuit and the relay would always "set".
     
  17. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    I suspect a power supply issue for the 41 volts of offset on both R and L channel.
     
  18. avionic

    avionic " Black Knights " Subscriber

    Messages:
    36,514
    Location:
    Fort Dodge, Iowa
    The burnt speaker is a secondary " protect circuit " issue.
     
  19. Mid-Fi

    Mid-Fi AK Subscriber Subscriber

    Messages:
    1,326
    Location:
    Joplin
    Great, I'll check all the above as time permits but keep the thoughts coming.

    To clarify, I fried the speaker because I did something stoopid by attaching the speaker leads to L1. :confused: At least I learned to test outputs for DC if the relay doesn't close. :whip:
     
  20. mbz

    mbz AK Subscriber Subscriber

    Messages:
    1,103
    Location:
    Melbourne, Victoria
    90% of the time, such high dc is caused by a failed component in the power amp stage.
    What's slightly confusing is both channels have same dc, this is explained since RI
    and LI are connected in the protection circuit via R741 and R742.

    First step, identify faulty channel by measuring voltages between RI and LI as per
    post 9.

    Once faulty channel identified you have a choice of a labourous hunt to track down
    the source of this voltage via voltage measurement or you can diode test key transistors.
    In circuit diode test will probably be ok, looking for a short. In this case I suggest
    diode testing starting with TR611, TR612
     

Share This Page