Fisher Coronet 100-T - FM Issues RESOLVED

Discussion in 'Fisher' started by mrphilco, Dec 31, 2017.

  1. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    445
    Location:
    Ferdinand, IN
    A few months ago I treated myself and bought a 100-T Coronet tuner/preamp.

    It was advertised as working on AM but not on FM. When I received it, I discovered that the seller was telling the truth. AM worked great. FM was dead.

    [​IMG]
    After I put it on the workbench, Viktor the cat had to inspect it to make sure it was to his liking. He approved, so I proceeded.

    [​IMG]
    Without flash

    I have replaced the few dog turd caps it had, rebuilt all of the electrolytic cans, and replaced the original selenium rectifier with 1N4007 diodes and a terminal strip. I found that a 39 ohm resistor across the two 500 uF caps, in place of the original 30 ohm resistor, was enough to keep 12AX7 filament voltages very close to their original levels.

    [​IMG]
    With flash

    I do not have the original Fisher service manual but I have the Sams Photofact for this set, so that is what I used to go by in servicing it.

    I discovered there was no plate voltage at the 2nd FM IF amp tube. The 3rd FM IF coil (single coil transformer) showed infinite resistance.

    I pulled a similar IF coil from an FM-50-B chassis. While I was at it, I replaced the ratio detector transformer with one from an FM-100-C chassis to get the "wide band" transformer as my intention was to eventually add a WX multiplex decoder.

    Once I did all that, FM was back, but was very weak. Alignment using a VTVM was not much help. I could not get my scope to display the waveforms which the Sams manual was showing, so I used the VTVM instead.

    I could not get anything out of the null adjustment on the ratio detector transformer. It was as if adjusting the slug had no effect.

    Upon inspecting the original 3rd IF coil, it looked clean - no indications of silver mica disease. So I opened it up and found that one of the wire leads was broken right at the solder terminal.

    I removed one turn of wire, resoldered the lead to the proper terminal, and then I removed the FM-50-B 3rd IF and reinstalled the original.

    After I did that, FM performance improved noticeably.

    However, four problems remain:

    1. While FM is now loud enough that I can hear the inter-station noise, it still seems to be lower than it should be.
    2. I still cannot adjust the "null" of the ratio detector.
    3. The FM magic ray indicator - instead of the shadow narrowing, it widens when I tune in a station.
    4. It seems to be "double-spotting" when tuning. In other words, when tuning in a strong station, it is as if it has two peaks, one better than the other. I realize this is likely an alignment issue, but until I can get FM to be a bit stronger/louder, I don't think a realignment is going to help at this point.

    Yes, I did replace the 8 uF electrolytic in the ratio detector circuit. I had read in another Fisher thread that this electrolytic should be installed backwards when replacing the ratio detector transformer with a "wide band" unit since the internal diodes are reversed, so that is what I did.

    Any help on this would be very much appreciated.

    Happy New Year to all!
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Location:
    Glen Burnie Md.
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,476
    Location:
    Ball Ground, GA
    Sounds like the + and - outputs of the new ratio detector transformer are connected into the circuit backwards. If I recall correctly, Fisher changed either the number location or terminal location on the wide band transformers -- Check it closely. As I recall, the transformer still wired in the same but maybe the cap and output leads needed to be reversed, or something like that. Bottom line: If the eye tube is working backwards, then the polarity has shifted due to the new ratio detector transformer install.

    Dave
     
  4. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    445
    Location:
    Ferdinand, IN
    Thank you Dave - I will check that closely. I was wondering if that might be the cause of at least part of its problems. I will study the FM-100-C schematic closely as well to see if that ratio detector transformer's pinouts differs from the original. I can reverse the 8 uF cap again as well, if I need to.

    I don't know if it makes any difference or not, but I had replaced a ratio detector in an FM-100 tuner, wired it in the same but installed the 8 uF cap with reverse polarity, and it works very well. Now you have me wondering if the pinouts of the FM-100-C transformer is different? I will look. I really appreciate the help.

    Larry, thank you also for the link to the Coronet service manual.
     
  5. audmod01

    audmod01 AK Subscriber Subscriber

    Messages:
    1,165
    Location:
    Tioga, TX
    Ron;

    Once you get the ratio detector working properly, if the reception is still weak that may be the time to check alignment. I would check the Mixer output 1st IF transformer alignment. Use a good 10.7mHz signal source and make sure that particular part is really peaked at 10.7mHz before moving on to the rest if the IF circuitry. There is no telling what someone before you may have adjusted in vain. If I remember correctly the KM-60 alignment information is helpful in aligning most Fisher FM tuners.

    Joe
     
  6. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    445
    Location:
    Ferdinand, IN
    Well, guys,

    I printed out copies of the ratio detector circuits of both the 100-T Coronet and the FM-100-C, from which the wide band ratio detctor transformer was taken.

    Here is the original 100-T circuit:

    [​IMG]

    and the FM-100-C:

    [​IMG]

    I had my ratio detector wired as per the original 100-T circuit above, save for having the polarity of C66 reversed to accommodate the reversed diodes inside the wide band FM-100-C ratio detector transformer.

    I looked over every connection...every resistor...every capacitor.

    All was wired correctly.

    So...

    I removed the wideband ratio detector transformer and reinstalled the original 1960 transformer, also replacing the 8 uF electrolytic with another, connected as per the first diagram above, and...

    I fired it up and it works great! :thumbsup:

    Reinstalling the original ratio detector transformer seems to have taken care of all of its issues. Volume seems normal now, the magic eye is responding properly, and the mysterious "double spot" tuning issue with two peaks is now gone. It tunes normally, sounds normally, and works normally. So the FM-100-C's ratio detector transformer obviously has issues, and it will find its way to the "round file" when I go back to the basement workbench shortly.

    I think that I will recheck alignment on FM and AM, replace the lamps on either side of the dial, and then call this one finished. Yes, it's mono FM, but listening to my Fisher 202-R tuner on a regular basis has made me become accustomed to Living Mono FM.

    Thanks again for the help!
     
    Last edited: Jan 1, 2018

     

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  7. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Did you change the output to match the later configuration, with the feed to the audio amp coming from the junction of the two 6.8K resistors, or did it remain as original with the output from the junction of R50 and R53 and the junction of the two 6.8K grounded? Not smart enough to know precisely what effect the difference would make but its the only obvious difference, besides the direction of the two internal detector diodes and the electrolytic cap. Those I don't see making any difference.
     
  8. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    445
    Location:
    Ferdinand, IN
    Gadget, no, I did not. I wondered about that after reinstalling the original, though.

    Perhaps if Dave sees this, he might chime in with some words of wisdom...
     
  9. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    445
    Location:
    Ferdinand, IN
    Hey Gadget,

    I believe that I have figured it out.

    In order to understand this, you have to compare the 100-T/Coronet C-808 tuner schematic with the 202-T schematic.

    In the 202-T, which Dave Gillespie successfully converted to FM Multiplex, the signal for the FM Magic Eye is taken from the 4th IF which has its own set of diodes internally, connected to a network consisting of a 15K resistor, 5000 pF capacitor, and 8 uF electrolytic. The signal from this is split between the EM84 Magic Eye grid and the FM muting circuitry. So adding a Wide-Band ratio detector transformer to the 202-T would not affect the operation of the FM EM84 Magic Eye tube.

    [​IMG]

    In the 100-T, the signal for the FM Magic Eye is taken from the junction of R39, R40 and the negative lead of C4 (as shown in the schematic below). Now with a Wide-Band ratio detector transformer installed and C4 reversed, the signal polarity going to the EM84 grid would be reversed! So, to make the Wide-Band transformer conversion work in the 100-T, I believe one would have to either reverse the polarity of the Wide-Band ratio detector transformer's internal diodes, or else modify the circuit to add a diode or diodes to the 3rd IF along with a network like that in the 202-T. And then one would need to move the grid of the EM84 to that output, fed through a 220K resistor.

    [​IMG]
     
    Last edited: Jan 14, 2018
  10. mrphilco

    mrphilco AK Subscriber Subscriber

    Messages:
    445
    Location:
    Ferdinand, IN
    Epilog: If anyone is still reading, I did redo the FM and AM alignment, set the tuner up in my home office and let it run for most of a day, continuous (about seven hours).

    It ran flawlessly, worked just fine. Of course, without AFC the tuning drifted a bit early on, but I reset its tuning once after full warmup and that was that.

    I had initially planned to install a WX MPX decoder, or an MPX-65 converted to the WX design, in this tuner but I may just leave it mono. It is ready for daily use again.

    Конец.
     
  11. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    35,843
    Location:
    Southern NJ
    Makes sense. I'll have to look close at my TA-600 if/when I convert that to see if it has the same design.
     

     

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