fisher MPX decoder troubleshoot

Discussion in 'Fisher' started by vintelectra, Apr 21, 2013.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    To me, that points to an issue with the 38 kHz oscillator, as in the outputs are not equally balanced to ground on each half cycle. You might try triggering the scope by the pilot signal and see what that shows.
     

     

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  2. vintelectra

    vintelectra Active Member

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    455
    driving mpx with composite in 19K from gen, tapping trigger from tsp 101 is pic #1.

    Also I have replaced the right ring with one made with your diodes 1N459A. No change.
     

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  3. vintelectra

    vintelectra Active Member

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    455
    Is that showing the imbalance?
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    It would seem to be showing something. The bottom indices are distinctly different that the top ones. You might check it against another unit known to be working properly to see if the waveform is the same.
     
  5. vintelectra

    vintelectra Active Member

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    455
    A working unit should not have this high output on left ch without drive, but I will do that test anyway.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    You're checking for the waveform produced with your scope connected as in your post 442
     

     

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  7. vintelectra

    vintelectra Active Member

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    455
    Working perfect restored F500C connected as in post 442: pic #1, 35mV, 60hz noise, left mpx output. Pic#2 right ch out: 30 mV 60 hz noise.
    Without any drive = 0.3mV noise
    Nothing gets thru.. Martin
     

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  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Understood, but what I'm asking is for a scope shot of the output from each side of the push-pull 38 kHz transformer, with the scope triggered by the 19 kHz signal that triggers the trigger stage (pin 2 V101).
     
  9. vintelectra

    vintelectra Active Member

    Messages:
    455
    Sampling legs of Z101 in good unit: Pic#1: 36.7k ( in two separate freq meters) 6.5V
    Pic #2, the other leg 37k 5.5 V
    Sampling Z101. ( the only difference in connections is that the drive network is applied in the good unit without disconnecting natural input of the MPX)

    Pic#3 36k 15V
    pic#4 36k 11V ?????????
     

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  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Your good unit (left two pics) is producing a normal waveform if it was produced under the following conditions:

    1. 19 kHz +/- 2 Hz 1V signal is applied to the decoder input from the generator through the standard phase shift network. The receiver's MPX connection can remain in tact as long as the last limiter tube is removed.

    2. The vertical input is connected to either the output of C216 or C217. Frequency should be 38,000 Hz +/- 1 Hz.

    3. The External Sync is connected to pin 2 of V101.

    Is that how the last two pics were generated? If not, they need to be.

    Have you checked that the 38 kHz trap coils are both good (L102 and L103)?
     
  11. vintelectra

    vintelectra Active Member

    Messages:
    455
    yes, input as stated.
    yes, L 102 &103 good, 142ohm &133ohm
     

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    So if all four pics were taken under the exact same conditions as described, but on different decoders, then clearly something is causing the 38 kHz waveform to fill in on your defective decoder that should not be. Seems to me then I'd concentrate on the 38 kHz oscillator. You need to find out why its waveforms are different.
     
  13. vintelectra

    vintelectra Active Member

    Messages:
    455
    Agree. I will sleep on it and see what comes up, and let you know. Thank you Dave. Martin
     
  14. vintelectra

    vintelectra Active Member

    Messages:
    455
    SOLVED!!! a long story short: The new "1uf" coupling cap was off ,reading 7 uf & 1.2meg ! went thru the whole batch ,all bad, never had use them before. ( cant believe)
    Now I have 28 db separation. pic 1 sine at rt ch clean. Pic2 sine at left ch. Note residual 38K contamination on left only . any suggestion?

    I have replaced both rings twice, Z101 and caps , ceramics and audio path caps Thanks Martin
     

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  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Wow! Bad caps for sure. Need better ones! The resistance of the cap would allow DC to pass, tending to bias one ring on relative to the 38 kHz signal, and the other off relative to that signal, which would explain the different 38 kHz residual readings at the grids of V102. Usually, excessive 38 kHz in the audio output points to the trap and de-emphasis networks. Have you changed any of the values in those networks? Checked the cap values? One sure fire way to test those networks is to swap the ring outputs at the grids of V102. It will switch the channels, but not the components. If the problem remains at the same output point, then de-emphasis is the problem. If the problem stays with the channel signal, then re-examine what the residual is at the two grids of V102.

    Dave
     
  16. vintelectra

    vintelectra Active Member

    Messages:
    455
    You are exactly right! The process that led me to the fault, for some intuitive reason ,I checked D.C. Values at ring outputs and found high (70vdc)' while minimal vdc( mV) in working unit. Since the abnormally high Vdc
    Was equal in both rings outputs, the voltage had to come from the coupling cap, although I had replaced the new cap twice!!
    I will try the swap and report, but how significant is a minor 38k contamination which we cannot hear anyway?
    Not that I don't want to tinker further in this hypertinkkered mpx
     

     

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  17. vintelectra

    vintelectra Active Member

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    455
    Difficult to check network caps, better replace them if I must.
    Before swapping,: you suggested swap at grids of V102, but is much easier to swap at the ring output and should be the same effect yes?
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    That is maddening. Replace the parts only to find the new parts are also NFG. I've done it myself and it really does tend to throw a wrench in the troubleshooting process just because you think "I already replaced that and it didn't fix it".
     
  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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  20. vintelectra

    vintelectra Active Member

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    455
    Replaced the cap twice!! What are the chances that the whole Bach is in error?
     

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