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Yamaha TX-1000U No FM Signal

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

  1. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    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!
     

     

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  2. rcs16

    rcs16 Super Member

    Messages:
    4,070
    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:
    1,311
    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:
    4,070
    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

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  6. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    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.
     

     

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  7. rcs16

    rcs16 Super Member

    Messages:
    4,070
    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
  8. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    Location:
    Lansing, Illinois
    Thanks again rcs16! F/A signal is FM/AM signal correct?
     
  9. rcs16

    rcs16 Super Member

    Messages:
    4,070
    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
     
  10. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    Location:
    Lansing, Illinois
    Ok. Thanks again.
     
  11. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    Location:
    Lansing, Illinois
    Finally got a chance to do some probing. It looks like the problem is a control issue. One side of R288 showed 5.33-5.35V. The other side showed 0.66V. Neither side changed even after 30 seconds or so.
    So, what is a control issue and what should I do next? I'm guessing replace R288?
     
    Last edited: May 1, 2018

     

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  12. rcs16

    rcs16 Super Member

    Messages:
    4,070
    Location:
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    It is not R288, since it is passing the control DCV on through to turn on the transistor(bjt) with 0.66V measured.
    I assume that the spot where you measure 5.33-5.35V, it does not change level, when you switch from/to AM/FM?
    So what does the control? the front panel switches and the remote? The mcu scans the keyboard switches for closure and interprets the IR remote codes. I assume that neither the remote nor F/P switches can change AM/FM modes?
    There is logic between the MCU and the K/B switches, it could be that, they need to be checked out. There is charts/tables in the schematic showing the logic states, as an aid to troubleshooting
     
  13. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    Location:
    Lansing, Illinois
    Thanks rcs16. I went back and checked again since I realized I forgot the most important part, switching between AM and FM.
    So, one side of R288 does indeed change from 5.3V to 0V when switching from AM to FM and the other side of R288 changes from 0.66V to 0V when switching from AM to FM.

    Moved on to R291 and I got readings of 0.66V on the right side and 11.88V on the left side with tuner set to AM.
    Then 12.39V on the right side and 12.44V on the left side with tuner set to FM.
    Q75 collector reading was 12.43V for FM and 0.66V for AM.

    Based on these readings and what you've said, everything with R288, R291 and Q75 seem to be good right? Where do I go now?
     
  14. rcs16

    rcs16 Super Member

    Messages:
    4,070
    Location:
    Baldwin, Ontario, Canada
    I can pick up AM stations pretty well but FM is completely dead.
    When in FM mode, what does it do?
    Is there anything displayed?
    Can you put in fine tune mode and change frequency on the display?
     
  15. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    Location:
    Lansing, Illinois
    In FM mode I cannot pick up any frequency from one end of the dial to the other. The Signal Quality meter won't show even one bar anywhere on the dial.

    I can put it in Fine Tune mode and change the frequency on the display. In Fine Tune mode it shows for example 94.00 instead of 94.0. When Fine Tune is off it shows 94.0.
     
  16. rcs16

    rcs16 Super Member

    Messages:
    4,070
    Location:
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    That is the way it is supposed to work. So you can change the freq in fine tune mode? but receive nothing. Lot of possibilities.
    Measure DCV tuning voltage (VT as marked in the schematic) at 88 and 108MHz as set on the display? It should be close to ~25V at 108M and ~>5V at 88M. This will test the PLL ckts in fine tune mode.
     
    Last edited: May 2, 2018

     

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  17. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
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    Location:
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    I see VT on the schematic but I don't see anything on the PCB (top-side). Is it in the MPX PLL area or CSL CONTROL area?
     
  18. rcs16

    rcs16 Super Member

    Messages:
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    Location:
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    I assume that there is no label on the pcb for VT. It is the junction of Q83,84 collectors. Connects to L16 ... I see Q83,4 to the left of IC12 on the pic in the manual.
     
  19. mea2112

    mea2112 AK Subscriber Subscriber

    Messages:
    1,311
    Location:
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    No, no label on the pcb for vt. I see L11, 13 14 and 17 all near the copper-shielded area to the far right but no L16. Could it be under the copper-shielded area?
     
  20. rcs16

    rcs16 Super Member

    Messages:
    4,070
    Location:
    Baldwin, Ontario, Canada
    Measure at Q83,84 collectors, they are labeled on the pcb. To the right of the LC7210 IC?
    Post some pics of the circuit boards so we can identify parts and I can compare against the docs.
     

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