Bad DC offset on my PC-4002m.

Discussion in 'Yamaha' started by frisouille, Mar 16, 2017.

  1. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    Hello Friends,
    I'm actually recapping my pc-4002m. The in circuit board and the two main boards are finished. So when i'd check the bias (set ok) end dc offset, one channel have high offset.; The SM give 0-20mv per channel. Mine have 5.3mv on the left and 245mv on the right channel. What do you advise to check?
    Thanks for your help.
     
  2. Mr. Yamaha

    Mr. Yamaha Yamaha Junkie Subscriber

    Messages:
    433
    Location:
    Haarlem, The Netherlands
    Isn't there a DC offset pot per channel so that you can adjust it?
     
  3. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    I haven't looked at the manual
    Most likely not. Probably a DC servo. I'll have to look over the schematic.
     
  4. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    No DC servo like many yam pro. IC chips seems doing the job. I think that for this one it's a ti NE5532P.
     
  5. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    I don't have a scat. But I'd be looking at the input differential pair.
     
  6. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    Thanks Avionic. Waiting for your analyze.
     
  7. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield


    Don't hold your breath.. I don't have a manual or a schematic diagram.
     
  8. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    I have one. Lucky i am!!
     

    Attached Files:

  9. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    Q201 on the right channel.
     
  10. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    2SK389BL would be my first suspect.
     
    frisouille likes this.
  11. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    Thanks Avionic. I'll test him tomorrow. If it's shorted, which equivalent will go for it?
     
  12. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    Euh, after looking, i don't know how to test it!!
     
  13. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    2SK389BL -- If it was shorted it would probably trigger the protect circuit.
     
  14. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    The protection circuit seems to run fine.
     
  15. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    And sometimes the offset change and be ok for a few seconds.
     
  16. brutal

    brutal YamaHoarder

    Messages:
    4,449
    Location:
    Parker, CO
    Thing about those n-Channel JFET is they will "work" but can be squiffy.

    Basic test here:

    You can build a test rig and watch this video for further insight.

    Around 8:45 he shows the schematic for the test rig and connections.

    Disclaimer, I haven't built this myself or put a JFET under test like this so you might want to find a sacrificial N-Channel JFET (doesn't have to be the dual JFET in your amp) to experiment on first.
     
  17. frisouille

    frisouille French menber

    Messages:
    158
    Location:
    South of France
    2 questions. If one 2sk389 is out, should i have to change the two channels, cause i've see that this j-fet are matched by pair?
    I have a mk-168 transistors tester, does it can hlp me for testing the j-fet?
    Thanks.
     
  18. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    Your matched pair is on the single chip (2SK389). So, no you would not need to mess with the other channel.
     
  19. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    I don't know. I'd have to read the owners manual.
     
  20. avionic

    avionic " Black Knights" Subscriber

    Messages:
    33,143
    Location:
    Fort Dodge, Iowa cornfield
    If its anything like my DCA. About all it will tell you is its a n-channel or p-channel jfet. Or if its shorted or unidentifiable component.
     

Share This Page