Fisher TFM-300- Replacing Selenium Rectifier

Discussion in 'Fisher' started by artsybob, Feb 11, 2018.

  1. artsybob

    artsybob Member

    Messages:
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    Location:
    Northern Virginia
    I have already posted this in the General Audio thread, but I figured I'd repost it here as well. I'm in the process of restoring a Fisher TFM-300 tuner. It's been advised that I replace the selenium rectifier in the unit but I've never replaced one before. I bought this full-wave bridge rectifier as replacement: https://www.alliedelec.com/nte-electronics-inc-nte5314/70215806/ . Is that a suitable replacement or should I use something else? Would I need to to install additional resistors? If so, where do I place them? Thanks for the help!

    Schematic: http://www.fisherconsoles.com/non console manuals/fisher tfm300 beg 10000 sm.pdf
    0114181737.jpg
     
  2. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Location:
    Glen Burnie Md.
    It's fine. Although it's overkill in the tuner. The new rectifier will bolt to the chassis in the place of the old one. Remove the old one with the wiring attached, mount the new one to the chassis wall, and transfer the leads with the ~ sign to the leads on the new one with the ~ sign. These are the AC input leads. Then you have the pos + and neg - leads. Connect them to the correct lead terminals on the new rectifier and you're done. No muss, no fuss! Check your voltages at the 1st and 2nd can caps to make sure the voltages are within 10% of nominal posted on the schematic. If so you're golden. If not you'll have to figure out value and wattage for a new resistor to drop the input voltage between the rectifier and the 1st cap.
     
  3. fred soop

    fred soop Super Member

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    The TFM-300 is the tuner portion of the 600-T receiver. The voltage WILL be higher with the new rectifier. Total draw of the front end and IF is approximately 26 mA. However, the TFM-300 also powers the multiplex and an audio stage with the same supply so the total current is closer to 42 mA and slightly higher if the stereo beacon is lit. That will give you a starting point to calculate the resistance. Be aware that the IF uses 19 V and the multiplex uses 22 V. These voltages are derived through a large resistor from the high voltage supply. There is no regulator (there is in my restoration because that voltage will vary depending on current draw of the front end tubes). Make sure the 19 V and 22 V are not high as the 2N2654 and 2N2925 transistors are rated at 25 V. Might be good to check those voltages with the tubes out of the front end as that will be a slightly higher voltage when powering up before the tubes draw current.
     
  4. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

    Messages:
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    Location:
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    Thanks FRED.
     

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