Fisher FM-100-B MPX Stereo Confusion

RadioOn

Active Member
I recently acquired a Fisher FM-100-B and I'm puzzled about the MPX Out, Stereo Beacon light and stereo/mono settings.

I guess what I don't understand is that it looks like there is both onboard stereo and a MPX Out jack (for going to a MPX-100 or MPX-200, let's say).

The Stereo Beacon light and Stereo-Mono Automatic setting on the FM-100-B makes me think this thing should output stereo standalone without a multiplexer. And if that is the case, what is the MPX Out for?

Blessings to anyone who can straighten me about this.

Asking because the stereo beacon light never lights and no stereo no matter what I try.

Nice sounding tuner other than that.
 
The best way to answer your question is to determine if the unit contains an on board multiplex unit, the way later FM stereo offerings did.

Generally, early units produced shortly before the time of FM stereo approval often either contained provisions to have a multiplexer board installed at a later date, or had provisions to control an external multiplex unit from the front panel, giving the desired look that it was in fact a stereo unit from the get-go.

If there is no on board multiplex unit installed, then it will require an external adapter to be used to decode the stereo information. Fisher multiplex adapters are always built on a separate chassis of their own, and when provided, mounted into a cutout provided on the main chassis for the adapter.

Dave
 
The FM-100-B has both. The MPX out may be a carry over from the original model or another model that has the same metal chassis. If you look at the schematic you'll see that the MPX out jack is seriesed between L23 and the #1 pin for the Multiplex board. On your's it's a 3 tube board right in the middle of the chassis 2-12at7 and 1-12ax7. You DO have an External Antenna connected, right? You'd not be surprised at how many people don't connect one, or think that the AM Ferrite antenna takes care of the FM also.



Manual here......... http://www.fisherconsoles.com/non console manuals/fisher fm100b 40001 49999 sm.pdf

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FM100B was introduced in 1962, the year that multiplex broadcasting began. At the time of design, the multiplex sub-chassis may have been planned as an option (the hole in the chassis would have been filled with a blank metal plate). If it was sold without, then an outboard multiplex might be added later. The RCA connector is cheap enough - and maybe it was used as a wiring tie point - so it was left in. Kinda like an appendix?
 
Thanks to all for the replies. Very helpful.

Yes, this 100-B has the onboard multiplex unit installed (with the 2-12at7 and 1-12ax7 tubes).

And yes, I have an external FM antenna hooked up (basic Radio Shack indoor dipole).

So armed with an understanding of the big picture, I went back and retested. The lack of stereo I talked about in my original post was most likely the MPX-100 I was using. I switched to a MPX-200 I also have and stereo improved greatly. And I tried going from the L and R outputs straight to the preamp and get stereo that way also.

Stereo beacon on the tuner never lights up, but I'm chalking that up to a burned out bulb.

Btw, fwiw at least with my units, the MPX out through the MPX-200 sounds better than the onboard multiplexer.

THANKS AGAIN!!! You guys made me smarter. I really appreciate the help.
 
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