202-R Arrived

Discussion in 'Fisher' started by audmod01, Jun 27, 2017.

  1. audmod01

    audmod01 Well-Known Member

    Messages:
    748
    Location:
    Tioga, TX
    Over the last several days I have been dealing with a strange AM problem with the 202-R. The FM sounded fine on all stations or when fed to both channels simultaneously (FM only function). When AM only was selected the AM audio coming out of both channels was strong and full bodied. However if I selected one of the AM-FM functions, either phase shifted or normal the audio was thin and reduced in volume. I measured DC voltages on the AM 12AX7 tube and everything was normal. I could see some slight differences in the AM audio at the output of the normal and phase shifted output coupling capacitors. So I changed the two output coupling capacitors. I also changed the one on the output of the FM 12AX7 too. Still the audio when operating in the AM-FM mode was thin (lacking in bass) and reduced in volume on the AM dedicated output. This issue developed while I was swapping the AM and FM Level meters around. I kept wondering if something I had done during that exercise had caused it.

    Finally I began to suspect the audio cables I used to the interface of the preamp to the audio system I was listening through. I swapped the left and right channel audio cables to the preamp and the problem went away! One of these two cables either has an open ground or a poor signal lead connection inside. I realized that I had moved the tuner about when I was looking at the two meters and evidently a poor connection developed inside one of the audio cables. Fortunately these two cables are the type that can be repaired. Both ends of each cable has a screw-on shield cover and can be removed to gain access to both ground and signal conductors to rework them.

    This is the second time I have been bit by defective audio cables recently. The first instance was on the Electra 440/490-T combination with a bad dual audio cable that was brand new. Now this issue with older cables that have seen frequent use over the years.

    So now the 202-R is fully functional in all modes and works normally with great audio! I can button it back up and concentrate of building up the MPX-65 that I plan to convert to WX functionality and use two 12AT7 tubes and one 12AX7 tube in it. I have two small power transformers I ordered from AES that will provide both HV and filament voltages for the MPX circuitry separate from the power transformer in the 202-R. The MPX sub-chassis will be built into a Bud aluminum box that can either be attached to the back of the 202-R or lie alongside it in a custom cabinet. I will work to provide a way to auto-switch when the MPX circuitry is functional. There is a lot of planning ahead.

    Joe
     
  2. audmod01

    audmod01 Well-Known Member

    Messages:
    748
    Location:
    Tioga, TX
    One item I noticed about the 202-R is that on the left side of the chassis behind the area with the FM tuning knob, there is a thin plated steel plate attached by two screws to the side of the chassis. There is no critical circuit in that area that I can see. The FM tuning knob does have the contact behind it that senses the touch of a person's hand to turn off the Microtune AFC circuit. I am wondering if the steel plate was added to help avoid sensing nearby objects that might mess with the operation of the Microtune circuit. Dave mentioned that it operates by picking up stray 60Hz energy in the air that is intercepted by the human body, amplifies that and uses the result to trigger the AFC defeat relay.

    Joe
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    8,668
    Location:
    Ball Ground, GA
    That's exactly what it's for Joe.

    Dave
     

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