Problems with Fisher FM 100B

Discussion in 'Fisher' started by musicgeorge, Jan 4, 2019.

  1. musicgeorge

    musicgeorge New Member

    Messages:
    38
    Hi all. Here again with another issue. I'm restoring a Fisher FM 100B, did the usual replacements, selenium rectifier changed for new silicon unit. Some caps and resistors as well. Here are my main two questions for you and problems:
    a) When tuning in a stereo signal, I get a weird noise, like when the needle of your turntable accidentally crosses the vinyl, sort of a weird growl. This I had before going about the replacements.
    and
    b) Had 182 vdc out of the old selenium rectifier into the can capacitor (schematics calls for 195) but now I'm getting 230vdc after the silicon unit is in place.
    I'm guessing these are 2 separate issues, so your help is more than welcome to get my fisher tuner working properly. How to get the signal back without any interference and if I need to add a drop resistor (value?) and where to place it in case it is necessary
    Thanks in advance!!!!
     

     

    Please register to disable this ad.

  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,905
    Location:
    Ball Ground, GA
    Music -- Install a 100Ω 5W resistor between the output of the new silicon bridge rectifier, and the FIRST filter cap (no cap connected to the rectifier). This will mimic the normal internal resistance of the old selenium rectifier, and bring your voltages back in line. Remember too, to only measure voltages with no antenna connected, and the pointer turned to the left end of the dial (no station). There is a pretty big voltage swing between receiving and no reception conditions. The indicated voltages are based on the unit not receiving any station.

    Set the unit for mono reception. If the sound is normal in that mode, then the MPX sub-chassis is the likely culprit producing your noise. First up -- if not already done, change out the 1 uF 350 volt electrolytic cap in the MPX unit. When it goes bad, things sound really weird.

    I hope this helps!

    Dave
     
  3. musicgeorge

    musicgeorge New Member

    Messages:
    38
    Thanks Dave, really appreciate this. Will install the resistor.
    Regarding the 1uf electrolytic cap, I changed that one precisely. As you correctly mention: in mono, sound is coming out perfect, it is when receiving an fm station and the beacon is turned on, by the way, that 1uf electrolytic cap is the only cap I replaced in this section.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,905
    Location:
    Ball Ground, GA
    The output coupling caps on the MPX chassis are fairly hard to get to. They usually don't fail by leaking, but can go intermittent. Therefore, I doubt that they are the problem -- and they wouldn't fail with the symptoms you have anyway. If the stereo beacon is turning on and off appropriately, then you're likely into the ring diode de-modulators, or the alignment of the 38 kHz oscillator coil. There is a blue separation control on the top side of the MPX chassis. It's normal position is typically in the 5-7 o'clock position. If it is turned up near the 10-11 o'clock position, that will usually cause significant distortion, so you might check if it has been disturbed as well.

    If the MPX chassis needs to be serviced, it's best to get it to someone who has the proper equipment, and thoroughly understands the theory of FM MPX Stereo so that they can properly diagnose the problem and fixed it, without rifling through it replacing everything until it works......

    Good luck with it!

    Dave
     
  5. musicgeorge

    musicgeorge New Member

    Messages:
    38
    Many thanks, again Dave. I think it is time to take it to a professional, wiggled the blue separation control a bit and reception continues to be the same.
     
  6. musicgeorge

    musicgeorge New Member

    Messages:
    38
    Just want to report that thanks to Dave my Fisher FM100B tuner is up and running wonderfully, thanks Dave for taking care of my MPX sub chassis issues!!!!
     

     

    Please register to disable this ad.

  7. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,529
    Location:
    Melbourne, FL
    Great news and yet more good work by Dave!

    I'd be curious as to what Dave found in your FM100B. I'm in the process of refurbishing a KM-60 and I think the MPX section may be the same. I am not far enough along to know if I have any issues, but the information would be good to know.

    In my KM-60 I did replace the 1uF cap in the MPX section and I've located and tested some tubes for it, but that's all I've done to the MPX section so far. I'm right at the cusp of powering it up on the variac but work travel and family activities have paused my work on it for a couple of weeks.
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,905
    Location:
    Ball Ground, GA
    Tim -- There were three issues with MG's MPX sub-chassis, but only two of which caused it to be nonfunctional:

    1. As is so typical, at some point along the line, the MPX section started to give trouble, which somebody somewhere thought could be corrected by twisting and turning things (adjustments). Other than the blue separation control getting bumped (particularly on the 400 and C receivers), this is rarely if ever true. Casually turning the various adjustments will invariably make things worse, which was certainly the case with MG's unit.

    A first step then was to attempt alignment. This proceeded normally, with the adapter able to produce normal scope displays of and between the 19 kHz pilot and 38 kHz suppressed sub-carrier signals. But proceeding through the rest of the aligment process then only returned about 15 db of separation left to right, while near normal separation was achieved right to left. This usually indicates trouble with the ring de-modulator diodes.

    2. His being an older MPX unit, the diodes for each sub-channel phase were mounted on individual terminal tag boards (as opposed to shrink wrapped), making it easy to check the diodes -- except that each diode tested fine. Still, the scope waveforms don't lie, and clearly indicated something was going south with the left channel de-modulator. MG had previously replaced the 1 uF composite signal coupling cap, so any leakage from that source that could bias the diodes into a lop-sided condition of conduction was already eliminated as a possibility.

    The answer lies in the test conditions that the diode test function of most DVM employs. They give a good indication of a diode's Forward Drop for matching purposes, and surely indicate a shorted condition. But since most meters use the 2 KΩ scale for the test, they do a poor job of measuring the amount of Reverse Leakage for a DUT (device under test).

    Further testing of the left channel de-modulator diodes indicated that one diode actually had about 10-12 MΩ of reverse leakage, which was the original problem with the sub-chassis to begin with. Normally, the amount of reverse leakage of a good diode is too high for even a 20 MΩ scale to give an indication on. The bad diode was replaced with a good one of near identical forward voltage drop characteristic to that of the remaining three diodes making up the left channel de-modulator ring, and the alignment of the adapter then touched up. This allowed the unit to return over 35 db of separation L to R, and R and L, repairing the original problem and current complaint of the unit.

    3. Finally, one component in each of the de-emphasis networks had also been replaced with an alternate value -- which based on all my testing, is something I never recommend doing. Using the commonly recommended alternate value results in an increase in 38 kHz artifacts in the recovered audio, and has not been shown to be necessary to any practical degree based on frequency response measurements and listening tests. As a result, the components in question were returned to their original value, and the unit shipped back to MG.

    I hope this helps!

    Dave
     
  9. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,529
    Location:
    Melbourne, FL
    Dave,

    I can’t thank you enough for the explanation, effort and time it took to answer. Each time I learn a little more.

    When finishing up my project I’ll have these thoughts and concepts to consider as I’m powering up and checking out my vintage Fisher tuner.

    Thanks again Professor Dave!
     

Share This Page