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Fisher 80-T tuner problems, any suggestions?

Discussion in 'Fisher' started by gkargreen, Aug 2, 2018.

  1. gkargreen

    gkargreen Active Member

    Messages:
    459
    Location:
    Fairfax, VA
    Hi all, I actually have a pair of these, one I bought and one given to me to fix up. The first is working fine, nice AM and FM reception as well as great sounding! However, the second is super weak and I can only get anything out of the AM section right now, nothing from the FM section. Unit has been recapped and I have started with the back end first as most of the front end voltages seem to be fine or close (a bit high compared to the working unit with its original filter cap...). So right off the bat V12 is waaay funky, pins 6 & 7 having no voltage whereas on the working unit there is the proper 32/55 vdc on them, respectively (should note the working unit is slightly different as to have the MPX output on it but that does not affect the circuitry of the output @ V12). I have checked the resistance of all the resistors affected (R433, 46 & 47) all are good as well as cap C60, all measure in-circuit the same in both circuits. Any suggestions? Thanks.
     

     

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  2. gkargreen

    gkargreen Active Member

    Messages:
    459
    Location:
    Fairfax, VA
    Did a bit of poking around with the sig gen and scope, found that the signal is good into pin 6 of the CF (V12) stage, nothing on pin 7, get the same readings on the good 80-T except pin 7 has full output, so the AM tuner section is definitely good but the CF stage is not. Not sure what is going on here, will shotgun the stage and see what happens...
     
  3. audmod01

    audmod01 Super Member

    Messages:
    1,219
    Location:
    Tioga, TX
    Do you have B+ on pins 1 and 5? That is the plate circuit of V12. If you have no voltage there you will not have any conduction through the tube and consequently there will be no cathode voltage either. There could be a shorted C54, open R38 or no B+ coming from the switch S1-C.

    Joe
     
  4. gkargreen

    gkargreen Active Member

    Messages:
    459
    Location:
    Fairfax, VA
    Thanks Joe, I have been checking it out, I do have B+, what I didn't have was filament voltage! The service manual says to short pins 7 & 8 together to provide filament voltage on V12, which is odd since I didn't need to do that on the working unit. Shorting those two pins (this is on the 11 pin connector between the tuner and preamp section) jumped the current draw to 2 amps, so there is still something not right here, further digging in the works...
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,674
    Location:
    Ball Ground, GA
    G -- Looking at the schematic, the intention of the jumper is to light the heater of V12 when the unit operates as a tuner ONLY (no audio control section) model -- designated as the 80-R. In that case, the jumper should and needs to be in place for there to be any sound at the CF output on the tuner chassis. However --

    In the model 80-T, which includes the audio control section, the jumper MUST be omitted, so that when the 11 pin connector is then connected, the 6 volt heater for V12 (on the tuner chassis) is then connected in series with the 12 volt configured heater of V21 on the preamp chassis (tone control amplifier tube). This 18 volt heater requirement is then powered by the DC heater supply located on the audio control chassis. So the benefit of the 80-T over the 80-R in this case is that V12 has its heater powered by DC voltage instead of AC. If you had you meter set to AC, then you would have (theoretically) measured very little voltage on V12's heater terminals -- even if the DC as present -- as the meter would respond to just the ripple of the DC heater supply on the audio control chassis.

    If there is no DC power to the heater of V12, then I would check that V21 is good (if it's heater is open then no voltage will be supplied to V12). Also check that the tube socket is clean for V21 as well as the pins and receptors of the 11 pin interconnecting socket between the tuner and audio control section. If all is good with these things, then likely the DC Heater Supply in the audio control section needs to be rebuilt or serviced, so that it is supplying the necessary 17-18 volts or so to light the two series heater strings it powers. Besides the string of V12 and V21 discussed here, it also powers a second string consisting of V22 (CF output in the audio control section), and V23 (magnetic phono preamp) in the audio control section as well.

    I hope this helps!

    Dave
     
  6. gkargreen

    gkargreen Active Member

    Messages:
    459
    Location:
    Fairfax, VA
    Thanks Dave, found the problem for that. I have been use to reading pin connections from the bottom (CW), that I forgot I need to read CCW when looking down into the socket. Once I figured that out, the jumper worked fine and V12 came right up so that I now have the AM working great. However, the FM section appears to be completely dead, so that is the next part of this I need to fix. Thannks!
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,674
    Location:
    Ball Ground, GA
    So I take it that you have an 80-R with no audio control center -- not an 80-T (that has the ACC)???
     
  8. gkargreen

    gkargreen Active Member

    Messages:
    459
    Location:
    Fairfax, VA
    Actually, I have the control center part as well, I took it off to be able to read and diagnose the problems in the tuner section.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,674
    Location:
    Ball Ground, GA
    OK. But THAT is exactly why the heater of V12 was not lighting without the jumper in place. When the two units are packaged to operate together as the 80-T, then the jumper is not required since V12 would then receive its heater voltage from the audio control center DC Heater Supply. When you separated the two chassis (to make the tuner effectively an 80-R), then the heater of V12 was left dangling in the wind without the jumper. Installing the jumper then brought it to life on AC power. That is fine while servicing the tuner chassis as a stand alone piece that's disconnected from the audio control center, but before rejoining the two chassis, you must remove the temporary jumper you've installed. Otherwise you'll have AC trying to back-feed the DC Heater Supply in the audio control chassis -- not good.

    Dave
     
  10. gkargreen

    gkargreen Active Member

    Messages:
    459
    Location:
    Fairfax, VA
    Thanks Dave, yes that jumper is now removed and the two halves mated together and everything is working fine, not sure why it did not before I took it apart as I have really don nothing new to it other than check out voltages, etc. Electronics can be funny that way leading you on a wild goose trip!
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,674
    Location:
    Ball Ground, GA
    My guess would be pins in the 11 pin plug went wonky from time. Simply disconnecting them and reconnecting them may have done the trick.

    Good luck with it!

    Dave
     

     

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  12. audmod01

    audmod01 Super Member

    Messages:
    1,219
    Location:
    Tioga, TX
    Dave;

    I agree with your diagnosis. I have run into similar issues on older equipment where disconnecting and reconnecting plug-socket connections was enough to restore operation of a circuit.

    I noticed a strange thing about the Fisher service manual for this model - the size of the pages is smaller than 8.5 X 11 inches. I used Adobe Photoshop to resize the pages and joined the two pages that are the schematic for the tuner section into just one 11 X 17 inch page. The part of the schematic that shows the plug and jumpers plus the filament circuitry was the most difficult to read as the original scan was poor in that area. I had to look at the Sam's schematic in order to determine the V numbers used on the Fisher schematic. I finally worked it out and entered some new text to show that area of the Fisher schematic. I went back and entered all the component values where I could. Some were not given (certain inductors). I like having the component values on the schematic to avoid looking at multiple pages while tracing circuits in a unit under test.

    Joe
     
  13. audmod01

    audmod01 Super Member

    Messages:
    1,219
    Location:
    Tioga, TX
    Something I mentioned in a private message to gcargreen was in regard to the first two FM IF stages. Fisher had added high value resistors across the primaries of both FM IF transformers. This reduces the Q of the circuits and broadens the IF response. I suspect this was done in preparation for the advent of FM Multiplex stereo broadcasting. After the decision was made by the FCC for the GE-Zenith MPX broadcasting method, manufacturers then had to develop IF and detector circuits that could deliver the best overall results including best channel separation. To use one of these units with a Fisher WX or MPX-65 decoder might require adding another high value resistor across the primary of the FM discriminator transformer here. In their later receivers designed for the approved MPX broadcasting method the FCC chose, it turned out that a detector transformer with wider bandwidth was needed for best results. Also by that time Fisher had changed from FM Discriminators (Foster-Seeley design) to Ratio Detectors. The ratio detectors provided additional noise suppression which turned out to be a real plus for best reception on FM stereo signals.

    Joe
     

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