Fisher 800T No FM

Discussion in 'Fisher' started by Hi Fi and I, Jan 10, 2019 at 10:51 PM.

  1. Hi Fi and I

    Hi Fi and I Member

    Messages:
    71
    Hello,
    I am having an issue with 800T.
    AM and AUX work well.
    FM had worked first with a hum (I was able to tune to any station), then FM just stopped working and hum disappeared as well.
    Any
    suggestions ?

    Thank you!
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    22,014
    Location:
    Glen Burnie Md.
    Trace the PWR for the Tuner section back to the power supply. Verify Voltage at the pwr supply and then again at the Tuner board. Sounds like you lost B+ to the tuner section.
     
  3. Hi Fi and I

    Hi Fi and I Member

    Messages:
    71
    Thank you! That is what I will do.
    I used a laser temperature gauge to measure lytics temperature. The 2 x 200uf (power supply caps "B+") had higher temperature than the rest.
     
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    22,014
    Location:
    Glen Burnie Md.
    Finally got around to looking at the manual. On the 800-T and 500TX (They are functionally identical except for the transformer) The AM and FM front ends are separate boards. However the IF/muting/mpx board is a combination board. Lack of FM but having AM and AUX indicates a PWR SUPPLY PROBLEM. On the POWER SUPPLY BOARD, check Pin "GG" for 15V. "GG" is the supply tap for the FM. If it's non existant it could be a couple of things. 1st i would check is the 15V 3W Zener Diode (CR977). Also if the PWR SUPPLY board HAS NOT BEEN RECAPPED, DO SO. It sits upside down and like the later Pioneer SX-1010 holds in the heat from components and bakes everything.


    Replace the Electrolytics with higher capacitance and voltage units. All of the following caps are 105*C rated and are in use in my 500-TX Since 2010. So they are proven to work well.

    Part #'s are Mouser #'s and the prices are current as of 1-12-2019
    C975 & 976 - 4700uf 63V 647-UVZ1J472MRD $4.19ea
    C974 - 1000uf 100v 647-UPW2A102MHD $2.83ea
    C979 & 980 - 470uf 50V 647-UPW1H471MHD $0.87ea
    C982, 984 & 985 - 220uf 50V 647-UPW1H221MPD $0.56ea
    C983 - 2200uf 50V 647-UPW1H222MHD $2.62ea
    CR977 & 978 15V 3W Zener Diode 863-1N5929BG $0.36ea
    CR976 30V 1.3W Zener Diode 78-1N4751A $0.30ea
    So for roughly $18.50 plus shipping (use the cheapest iteration) you've got basically a rehabbed pwr sup. (Minus any burn't resistors). On the 500-TX and 800-T this is the 1st board to rehab. Get the voltages here right and everything else will fall into line as far as correct voltages at the rest fo the boards.

    Note that there may or may not be what looks like a leg on the TOP of the C975 & 976 caps. This is a NON-FUNCTIONAL stabilization leg IT IS NOT internally connected to the cap. Attach the leads to the board at the end of the leads (you'll need the slack when you lay them down). Hot glue the caps on their side.
     

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