The Fisher 250TX - just noise on FM section

Discussion in 'Fisher' started by havols, Aug 1, 2018.

  1. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    Hi all
    I have a little SS receiver The Fisher 250TX
    I did the complete recap work on power supply, preamp, eq / tone and power amp boards
    Now, the amplifier section sounded very nice. I really like this sound.

    But it stills with one problem. The FM and AM section have issues.
    The FM section had just a continuous noise, like a schhhhhhhhhh (when FM muting is OFF). If FM muting is on, there is no one sound signal (noise or other).
    I tested the normal FM, and tune-o-matic, in all diferent positions. Nothing Change.

    I have the scheme
    When can I start to find the problem?
    I would appreciate some help from you guys.
    Regards
     

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  2. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    When = Where
     
  3. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,582
    Location:
    Cleveland, Ohio
  4. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    Thanks for reply Dr Audio
    I appreciate your help
    I cleaned the controls with contact cleaner. I have not deoxit here.

    At first, I prefer do not clean the tunning capacitor. I do not have the proper tools to do this. there is a high risk here!!!!
    A photo of the Tuning capacitor is attached. It looks in good condition.

    Measured the voltages on the 2021-9 board (AM, FM, IF, MPX, Meter, Tunning).
    Pin 4C (11V on scheme)
    Pin 4E (15V in scheme)
    The voltages measured were 12.1V on 4C and 15.2V on 4E.
    At the 3K pin (15V on scheme), I measured 15.3V when the FM Section was selected.

    Are there other possible sources of problems?
    Regards.
     

    Attached Files:

  5. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    I can't listen AM stations too. Just a noise, with interferences in some frequencies, when i rolling the tuning
     
  6. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,582
    Location:
    Cleveland, Ohio
    Do you have an antenna connected to the FM antenna terminals?
     

     

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  7. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    Yes, i have.
    I connected a double wire (4 meters = 13 feet long).

    Sometimes, right after turning on the receiver, I can tune it in one station. But about 10 seconds later, the tuning disappears, and I do not hear anything, except the continuous noise, like schhhhhhh
     
  8. dr*audio

    dr*audio Fish fingers and custard!

    Messages:
    18,582
    Location:
    Cleveland, Ohio
    This is something that requires specialized rf test equipment to troubleshoot.
     
  9. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    Thanks Dr. Audio
    I'm sorry to hear that. I really wanted to fix this 250 TX under problem.
    Anyway, thank you for your help.
    Regards from Amazon, Brazil
     
  10. previd01

    previd01 Idaho Spud

    Messages:
    26
    Location:
    Twin Falls, Idaho
    The 250 TX has a Varactor FM front and and a normal AM front end. The tuning capacitor you see is only for AM. The FM tuning is done by varying the tuning voltage to the FM front end, using the potentiometer on the back of the AM tuning capacitor assy. or the tone-o-matic controls. This voltage can be seen at test point 9b on the tune-o-matic board, and should vary from around 3.5 to 27 vdc depending on the dial position (see service manual). The only common thing I see for both AM and FM is Q 301 on the I.F. mpx board. I just did a repair/restoration on a 800tx for a customer and had an issue with poor FM performance. Turned out to be a bad IC, Q902, ua703, 10.7Mhz I.F amplifier. There are 2 of these in your unit (IC301,302) which are labeled tr09005 and tr09006 on the schematic, but I suspect they are ua703's. Good luck.
     
  11. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    Thanks for reply previd01
    O 800TX should be a fantastic receiver
    Well. On 250TX.
    I find one AM station ... with a lot of interference. At least it's some sign and life.
    The tuning meter worked too, fine, in AM mode.
    When selected FM, I can not find anything.
    I put a screwdriver on the pin 3B of the 2021A board (the base of the Q301).
    When I back up, I can see the tuning VU move a little, and the noise increases.
    I do not know if it's important information.

    I measured the voltage on 9b pin of tune-o-matic board: 3.8 to 27.7.0 VDC, in the minimum to maximum position.

    What can i do wow?
    Wish i have to replace the Q301?
    I have to find one equivalent here...

    Regards
     

     

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  12. previd01

    previd01 Idaho Spud

    Messages:
    26
    Location:
    Twin Falls, Idaho
    If it's constructed like the 800, the I.F.-mpx. board is a bear to get off. If you haven't already done so, plan on re-capping this board. Make good notes, drawings, pictures etc. so you know where every wire goes. You will have to cut the ground connections and re-do them when you put it back in. Before you do that, try this. Back when they used to use the ua703's for I.F amplifiers, there would sometimes be 3 or 4 of them on the I.F strip. In order to find the bad one, you can take a small value capacitor, say .01 uf or so, and connect it to pins 3 and 7 of the ua703. Any increase in noise/volume/signal meter indicates a bad IC. Not sure if it will work on this unit, but it's worth a try. Personally, I would plan on replacing both of them when you re-cap.
     
  13. havols

    havols Active Member

    Messages:
    217
    Location:
    Manaus, Brazil
    Great Previd
    Thanks again
    I will do a complete and careful recap.
    Should I replace only the electrolytic (bipolar) capacitors?
    if the problem persists, I will do the ua703 tests, with the small cap.
    Then post the results here
    Regards
     
  14. previd01

    previd01 Idaho Spud

    Messages:
    26
    Location:
    Twin Falls, Idaho
    Yes, just the electrolytics. Note; NTE 703/703a is a direct sub for the ua703. Good luck
     

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