Fisher 400 stereo beam bounce

Discussion in 'Fisher' started by ducati2, Aug 4, 2018.

  1. ducati2

    ducati2 AK Subscriber Subscriber

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    It can be a number of things, but there is also one aspect of the MPX-65 decoder that tends to promote a bouncing beam as well. Check it out in my thread on getting the most out of Fisher FM MPX tuners and receivers. It's a simple fix, and may solve your problem if all else is well.

    I hope this helps!

    Dave

    Edit: Does it act that way on commercial FM Stereo MPX stations?
     
  3. ducati2

    ducati2 AK Subscriber Subscriber

    Messages:
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    Location:
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    Yes it does same thing on any station strong enough to get the el84 to move.

    Been reading your post you mentioned, guess I have the ‘dancing beam syndrome’. Most of it so far is above my head, will start w some rechecking of tubes, then maybe some tube rolling. I also have three orphaned mpx 65 units I could swap out if needed —as last resort.
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    OK. Next question: Does the eye tube (an EM84A) dance when set to mono mode? Or is it only in stereo mode? That will help get to the bottom of things pretty quick. After I made my post, I went back and looked at your video. The eye tube's action does not look like one attributable to the typical "dancing" eye tube scenario when the unit is receiving an FM Stereo MPX station. If that video was made with the unit in stereo mode, then it is indicative of an MPX-65 sub-chassis that is either well out of alignment, or has had a failure somewhere in the pilot signal path. The huge clue, is that with the normal dancing eye scenario, the eye tube bar is there, but dances with modulation. What I saw in your video is that the bar is NOT there, but appears with modulation. Would this be a correct assessment?

    Dave
     
  5. ducati2

    ducati2 AK Subscriber Subscriber

    Messages:
    566
    Location:
    Oklahoma
    The bar is there with a stronger station and dances. When switching to mono, the beam is spread wide open and is not moving.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,558
    Location:
    Ball Ground, GA
    OK -- For reference, in a properly operating receiver, a typical station of normal signal strength should make the bars -- when set to mono mode -- each come in about 1/2 way from a fully open position, and there will be no movement of the bar. The bars should react to tuning input, opening with no station, and closing to the point indicated with a typical good signal level. A very strong signal will not have them close much more than indicated. In mono mode, the eye tube operates as a conventional signal strength indicator.

    In stereo mode, the eye tube indicates the presence of an FM Stereo MPX signal. When tuned to a mono station while in stereo mode, the beams will go to basically a full open position (you still see the beams, but they are notably wide apart). On a strong stereo station, the beams should approach the center, but not necessarily touch, and should always be present, but will dance on some stations if the fix I published is not installed. On weaker stereo stations, the bars will not close as much, and will likely dance, but should always be present.

    What I observed is that the bars of your eye tube went away when no modulation was present. I would suggest that your best move might be to install one of the other (hopefully good) MPX-65 sub-chassis, to either resolve the problem, or prove that the sub-chassis is not the problem. If the tuner operates completely normally in mono mode, with good tuning characteristics (not touchy, easy to tune, no distortion), then the sub-chassis is likely where the problem resides.

    Dave
     

     

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