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    FM-1 (with multiplex board) hum --filter caps replaced

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    boeingpilot

    Posts : 20
    Join date : 2015-06-20
    Location : Central PA

    FM-1 (with multiplex board) hum --filter caps replaced

    Post by boeingpilot on Tue Oct 16, 2018 6:04 pm

    Ok, among a pile of Dyanco stuff was a FM1 with the stereo board in it. Before I even bothered powering it up, I purchased the capacitor board and sonic update kits from FM3 zone.

    Did it in stages, first did the capacitor kit, made sure it powered up and then did the remainder (misc. caps, new PECs), and aligned it per the Dynaco instructions.

    Connected to a PAS3X and ST70, if I have the bass 'nulled' out, I get a low but noticeable hum. If I increase the base even a bit, it's awful. Mmmm - Looking at various threads, I saw the recommendation to run a ground lead from the RCA jacks to the multiplex board to ensure good signal ground. Still no good.

    So I've done the obvious, any thoughts people?

    Thanks for you help
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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: FM-1 (with multiplex board) hum --filter caps replaced

    Post by peterh on Wed Oct 17, 2018 2:21 am

    boeingpilot wrote:Ok,  among a pile of Dyanco stuff was a FM1 with the stereo board in it.  Before I even bothered powering it up, I purchased the capacitor board and sonic update kits from FM3 zone.  

    Did it in stages, first did the capacitor kit, made sure it powered up and then did the remainder (misc. caps, new PECs), and aligned it per the Dynaco instructions.

    Connected to a PAS3X and ST70, if I have the bass 'nulled' out, I get a low but noticeable hum.  If I increase the base even a bit, it's awful.  Mmmm - Looking at various threads, I saw the recommendation to run a ground lead from the RCA jacks to the multiplex board to ensure good signal ground.  Still no good.

    So I've done the obvious, any thoughts people?

    Thanks for you help
    As always, it should have been nice to have it tested before any rebuilds, now you cannot
    say if it is your soldering excersises or a previous fault.
    Said that, a coarse cable between the PAS and FM1 chassies will prevent ground loops through
    the signal cable. Easy to test, easy to remove if not working.
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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: FM-1 (with multiplex board) hum --filter caps replaced

    Post by Peter W. on Wed Oct 17, 2018 7:29 am

    boeingpilot wrote:Ok,  among a pile of Dyanco stuff was a FM1 with the stereo board in it.  Before I even bothered powering it up, I purchased the capacitor board and sonic update kits from FM3 zone.  

    Did it in stages, first did the capacitor kit, made sure it powered up and then did the remainder (misc. caps, new PECs), and aligned it per the Dynaco instructions.

    Connected to a PAS3X and ST70, if I have the bass 'nulled' out, I get a low but noticeable hum.  If I increase the base even a bit, it's awful.  Mmmm - Looking at various threads, I saw the recommendation to run a ground lead from the RCA jacks to the multiplex board to ensure good signal ground.  Still no good.

    So I've done the obvious, any thoughts people?

    Thanks for you help

    First, a few questions:

    a) Have you done the alignment - these beasts may be user-aligned. I will have "hummy" 3s that clean up after the first few steps of the alignment.
    b) Have you checked the chassis connections, especially from the main board? Age will sometimes crack the solder between the chassis and the board.
    c) Have you checked the RCA Jacks for good contact? If the shell is making poor contact, you will often get a hum - and these jacks were cheap even for back in the day.
    d) You are dead-sure you installed the new cap board correctly?
    e) Are you able to test if the tuner is drawing excess current?
    f) Lastly, do you have the means to test tubes? Try swapping identical tubes to see if the hum changes.

    Some suggestions:

    - Make sure all the internal grounds from the chassis and jacks are intact.
    - Clean the volume control thoroughly, and exercise it. While playing, it should be at max for best performance, but try varying it.
    - Make sure you have no incandescent dimmers, cheap CFL, and worse, cheap LED lamps nearby. Also older oil-burner igniter transformers.

    Let us know. These tuners should not have any inherent hum.


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    Re: FM-1 (with multiplex board) hum --filter caps replaced

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