Yamaha TX-1000U No FM Signal

Discussion in 'Tuners' started by mea2112, Apr 12, 2018.

  1. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,228
    Location:
    Lansing, Illinois
    I can pick up AM stations pretty well but FM is completely dead. I tried A and B antenna inputs and all the different front panel settings but nothing helped.
    Is there anything on this model that is known to go bad or fail? Is there anything logical I could check under the cover?

    Thanks!
     

     

    Please register to disable this ad.

  2. rcs16

    rcs16 Super Member

    Messages:
    3,404
    Location:
    Baldwin, Ontario, Canada
    It is hard to say where the FM problem maybe. if AM is working we can probably discount the PS. if AM locks to a station then mcu, LC7210 and that part of the PLL is operational.
    iirc the FM shares the same audio path to the o/p jacks as does the AM from its detector, it could be anything from the FM i/p to FM detector o/p.
    There is a bjt that switches the supply to the FM section, it could be that ckt. need a DMM to test that. It is labeled "FB" on the schematic, comes from Q75.
    LC7210-21 is the F/A switch line going to Q74 which controls Q75.
    It really requires someone to do some measurements and possibly trace the signal through the circuits to see what is going on.

    Good luck
     
    Last edited: Apr 13, 2018
  3. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,228
    Location:
    Lansing, Illinois
    Thanks rcs16. I figured there probably wouldn't be an easy fix.
    If I decide to keep it I'll be sending it to Mike. Unfortunately, I don't have the expertise to fix it.
     
  4. rcs16

    rcs16 Super Member

    Messages:
    3,404
    Location:
    Baldwin, Ontario, Canada
    If it is just Q74 or Q75 that is what I consider a easy fix. :)
    If you want to get ambitious, get a DMM, learn to use it, read the schematic in the service manual, find the component on the pcb, measure, replace(soldering iron, removal tools),. I guess I make it sound simple but that is it.
    Good Luck
     
  5. Blue Shadow

    Blue Shadow I gotta get me a new title

    Messages:
    19,365
    Location:
    SE PA
  6. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,228
    Location:
    Lansing, Illinois
    Thanks again for your help. I do have a DMM and know how to use it for the most part so I may try and check some voltages if I can make sense of the schematic.

    Thanks for that. Interesting thread! I doubt my tuner has any broken antenna parts but I'm going to look closer at it later when I get a chance.
     
  7. rcs16

    rcs16 Super Member

    Messages:
    3,404
    Location:
    Baldwin, Ontario, Canada
    Refer to p26 of 37 in the pdf service manual(big schematic) in the lower left corner, Q74,75 are to the right of IC12(LC7210)
    Refer to the parts layout drawing and locate IC12,Q74,75. R288 is above the big IC11.
    This drawing shows the copper/foil side view, you can access these points from the top.
    ground your probe using a alligator clip to any ground point on the pcb. A shield will work.
    Start with probing the DCV, using the DMM, for the F/A signal, on R288(47K,yel,viol,org), coming from IC12-21. One side of R288, the sig from IC12-21 will switch between ~5V(FM) and ~0V in AM mode. The other side of R288 will change between 0 and ~0.6~0.7DCV.
    If this does not change it is not the bjts, it is a control issue. if it changes, probe R291/Q75(collector), it should switch the +12VDC supply for FM op.
    Good luck
    Rick
     
    Last edited: Apr 18, 2018 at 11:40 AM
  8. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,228
    Location:
    Lansing, Illinois
    Thanks again rcs16! F/A signal is FM/AM signal correct?
     
  9. rcs16

    rcs16 Super Member

    Messages:
    3,404
    Location:
    Baldwin, Ontario, Canada
    the F/A signal from IC12-pin21 will switch between ~+5VDC(for FM) and ~0V in AM mode. Access at R288
     

     

    Please register to disable this ad.

  10. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,228
    Location:
    Lansing, Illinois
    Ok. Thanks again.
     

Share This Page