400C - Still fighting hum issues

Discussion in 'Fisher' started by cbox83, Oct 5, 2017.

  1. cbox83

    cbox83 Active Member

    St Pete, FL
    I've been cracking at my 400C for a while trying to get rid of the hum, and it always creeps back in somewhere, somehow.

    I followed an earlier post about rerouting and cleaning things up, lifting the heaters and isolating them from the chassis. There is always a hum that creeps through and I think I finally found where its coming from. However, I'm dreading what I suspect it is.

    Connecting my scope from chassis to the heater, there is 280v AC floating between the two. Tried putting a 10uF capacitor between the heaters and the chassis, verified through the multimeter that there was no current going through it, and grounded it. Hum got quieter, but it's still really annoyingly there. Removed the heater lift circuit since the heaters had no DC return path to ground ground from the cathode (which should overcome H-K differential) and its still there, just ever so faintly quieter but still ever present. Am I seeing a failing power transformer?


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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Ball Ground, GA
    83 -- A little more information if you will. May I assume you referring to the DC heater supply? If so, the heater bias circuit in fact references the heaters powered by that circuit to ground both with regards to DC and AC components. DC elements are referenced to ground by the fact that the 270K/68K heater bias voltage divider is placed across the 320 volt B+ source, with the B+ supply itself grounded. Were the 68K resistor of the divider not grounded, then the divider would not divide, allowing a full 320 vdc to bias the DC heater circuit.

    AC components are grounded by way of the 10-11 uF cap placed between the output of the DC Heater bias supply, and ground.

    If I may, where exactly are you measuring 280 vac on this heater circuit -- or are you referring to the AC heater circuit powering the rectifier tube?


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