Fisher 100 T and MPX question

Discussion in 'Fisher' started by Leland47, Mar 14, 2017.

  1. Leland47

    Leland47 AK Subscriber Subscriber

    Hi all, newbie...and a non tech, here, with a technical question:

    I've read lots of the threads here about the 100 T and adding a multiplex. Seems a very technical problem.
    In the Fisher MPX sticky says that it can be done...but rather complex. The MPX-65 is not a drop in solution.

    Here is my question: I noticed that the Fisher MPX-125 has a relay on board.
    Dave Gillespie states that a relay has to be added to the MPX-65 to get the function of the stereo eye tube to switch over to do dual duty as a true stereo indicator....and more...

    So....would starting with an MPX-125 board be a better beginning than starting with a MPX-65 board? (due to the relay already there...?) ( I suppose the relay is in use for something else...but on another AK thread, poster states that he can hear the MPX-125 relay clicking when he switches from mono to stereo....)

    I don't have the knowledge to just look at the schematics to tell this or not. ( and don't have the schematics to look at.)

    There is not much information in the Fisher MPX sticky thread about the MPX-125.

    Plus...the Fisher 100T....has an MPX separation trim pot on the back of the chassis. What does this tie into there is no MPX on board the 100T? ( Yes...I need to look at the schematic! )

    Plus...I suppose it would be simpler all around to just use an out-board MPX....but I would like to put one on the chassis...

    So...generally speaking, is there any new, more current information about adding an MPX board to the Fisher 100T? ( I just recently acquired one.)

    Thanks so much!


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  2. Leland47

    Leland47 AK Subscriber Subscriber

    Could anyone recommend a Tech, qualified to do this type of work on a Fisher 100T?
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Ball Ground, GA
    Leland -- To answer your first question, the MPX 125 does in fact include a relay in part of its circuitry, with it doing the double duty of providing Stereo/Mono audio signal switching of the stereo outputs, as well as turning on and off a conventional #47 bulb that acts as a Stereo Indicator when lit. To that specific point then, the 125 makes for an easier install. And, it appears that the 125 may even be able to drive an eye tube as well -- although the one circuit example I have of its use (Custom Electra VIII) does not have it driving the eye tube included in that model). However,

    Against this are two factors:

    1. The relay I referred to as being required was specifically relating to the 202T, where if you want the eye tube to mimic the action of the eye tube in a 400 receiver (acts as an FM carrier signal strength indicator in FM Mono, and a Stereo Indicator in FM Stereo), then the relay not only supported allowing the MPX-65 to engage the eye tube when in stereo mode for accurate stereo tuning, but also allowed for receiving stereo stations in true mono mode if desired. Without the relay, these features are simply not possible when the 65 is installed into a 202. However, I've also installed a 65 into an FM-100, and in that case, the relay was not required for two reasons:

    A. The front panel controls allowed within their original scheme to provide for true mono reception of stereo stations if desired, and

    B. The client did not want to go to the added expense of tying the 65 into the eye tube when set for stereo reception, so that in that case, the eye tube always acts as an FM carrier signal strength indicator, and does not indicate the reception of a stereo station.

    As a result, I did not pursue the circuit changes required to cause the eye tube to act as a stereo station indicator. It may very well have required a relay as well, but this point was not of significant concern to the client, so it was not pursued. The point is, a proper installation can be different in each application, and is further governed by what features are deemed important to the user.

    2. The other point of consideration, is that the MPX-125 does not produce the same level of performance as the MPX-65, WX, and other classic Fisher MPX decoder chassis do. This is not just opinion, but supported by Fisher as well, where the published specifications for channel separation performance of the 125 are not as good as for those of the classic designs. At mid frequencies, the performance is pretty good, but at higher frequencies, the separation is notably reduced over that of an MPX-65.

    The MPX-125 was the last tube based stereo decoder that Fisher designed and produced. It was clearly a cost saving model to finish out production during the last couple of years of the slower moving products -- most typically found in the consoles. In the vacuum tube line of dedicated higher tier tuners (FM 50B, 100B, 200B) and receivers (400, 500C, 800C, 1800), the MPX-65 was the stereo decoder of choice to the end of production for all of these models.

    I hope this helps!

  4. Leland47

    Leland47 AK Subscriber Subscriber

    Thanks so much.
    Seems a complex (for me) topic.
    The info on the MPX-65 vs the MPX-125 is great, and thanks for that, too.
    I realize now...the MPX separator trim on the back of the Fisher 100T...was designed for the now defunct Crosby FM broadcast system, most likely.

    I do have an out-board, powered MPX, of a different brand. I think I should start with that....and see how it goes.
    Afterward, I can decide which way I want to go.

    The idea of having a Fisher stereo tube me hooked on this model, and restoring it to everyday use.


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