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Fisher MPX-100 questions

Carbsandcaps

Carbsandcaps

Super Member
I picked up a fisher MPX-100 on the cheap and I’m trying to find some info on it. It seems to be an early model. I’ve seen some with stereo separation controls on the back and high and low inputs. This one has neither. It has a stereo balance control inside and a single input. My questions are: is the stereo balance channel seperation? It has channel levels on the back too. Hooking it up can I just hook any non MPX tuner to it then put the output directly to my amp?

Sorry if these are dumb questions but theres not much info on the different models of these. IMG_0775.jpegIMG_0774.jpegIMG_0776.jpegIMG_0777.jpegIMG_0778.jpeg
 
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The tuner will have a RCA jack either on the top of the chassis somewhere near the discriminator(or ratio detector) area marked MPX Output. Plug in a single RCA interconnect (Fisher says t keep it as short as possible, but in any event no longer than 3ft long). This goes into the single RCA in the lower right hand corner, marked "TO Tuner MPX OUTPUT".

A double RCA Interconnect goes from the OUTPUT (lower of the 2 pair) to your amps (or receiver) input. AUX, TAPE MONITOR, TUNER are all good. DO NOT USE Phono or Tape Head inputs. Need to know what tuner you plan on using so I can look up the connections and figure them out. It gets a bit complicated as I use the MPX-100 with Receivers and even then it's a headscratcher. Try the manuals below.


Owners manual here.................... http://www.fisherconsoles.com/non console manuals/fisher mpx100 Operating Instructions.pdf

Service Manual............................... http://www.fisherconsoles.com/non console manuals/Fisher MPX-100 Service Manual 10001-19999.pdf
 
The two input connections are if you want to use it in a tape loop, or don't want to give up an input. Looks like on this one you set it to "Tuner" and it simply passes whatever is fed to the input on to the output jacks.

1705846919226.png
 
Ok makes sense. What I’m getting at is i currently have a fisher fm-100 which I’m not too fond of only on looks because it’s in rough shape. If I was to get something like an 80 mono tuner would this put out a multiplex signal or is there extra circuitry in the front end of the fm-100 to “pick up” the multiplex signal.
 
In a sense NO. FM has an AM carrier that has the encoded signal in it that is decoded by the Multiplexer. All MONO units can't decode this, and it's passed to a connected X'er. The 80-R does have a MPX'er output jack(see attached pic) which is on the top of the chassis near the last tuner coil. Use that to the mpx input of the mpx'er.

ScreenHunter 33.jpg

You can check a unit's schematic and see if it has a MLPLX output jack on it. It'll be near the discriminator/ratio detector tuner coil or tube. Most late 50's FISHER tuners will have this RCA JACK.
 
larryderouin said:
In a sense NO. FM has an AM carrier that has the encoded signal in it that is decoded by the Multiplexer. All MONO units can't decode this, and it's passed to a connected X'er. The 80-R does have a MPX'er output jack(see attached pic) which is on the top of the chassis near the last tuner coil. Use that to the mpx input of the mpx'er.

View attachment 3095340

You can check a unit's schematic and see if it has a MLPLX output jack on it. It'll be near the discriminator/ratio detector tuner coil or tube. Most late 50's FISHER tuners will have this RCA JACK.
Click to expand...
Ok so no MPX output, no MPX capability.
 
I have a Harmon kardon tuner that I really love the look of. It has a spot on the chassis for a built in MPX unit which it does not have. However it looks like I can use an external adapter too if I’m reading this right under the sterophonic simulcast section IMG_0829.jpeg
 
Oh wait I just re-read that it looks like I would still need the hk adapter. Or maybe I’ll get into it and see how the MPX adapter hooks up and put in some rca jack in place of it
 
you can add an MPX out jack on most things, but how well it performs is debatable. There is a thread on here specifically about Fisher MPX stuff but it applies to everything. The gist is that not all tuners have a wide enough bandwidth to get really good FM separation. Fisher stereo stuff has a different detector transformer that makes the difference. I have one to install in my TA-600 for that reason. I've run MPX decoders on other tuners though, and sometimes the separation just is not very good. I tried it on a Stromberg-Carlson using a Fisher WX decoder from a 500B and I think I got maybe 10db at best separation. Its enough to give a left and right, but one designed for stereo will give double that much.
 
gadget73 said:
you can add an MPX out jack on most things, but how well it performs is debatable. There is a thread on here specifically about Fisher MPX stuff but it applies to everything. The gist is that not all tuners have a wide enough bandwidth to get really good FM separation. Fisher stereo stuff has a different detector transformer that makes the difference. I have one to install in my TA-600 for that reason. I've run MPX decoders on other tuners though, and sometimes the separation just is not very good. I tried it on a Stromberg-Carlson using a Fisher WX decoder from a 500B and I think I got maybe 10db at best separation. Its enough to give a left and right, but one designed for stereo will give double that much.
Click to expand...
That’s what worries me about this fisher unit not having a channel separation control. I’m assuming it just goes for maximum separation?
 
My mistake on location. It's actually on the back panel on an 80-R & 80-T. Here's my 80-R from the back.
80-R Rear.jpg

80-R close up.jpg

On your MPX-100, the stereo balance not sure if it's analogous to the pot on the top of the WX and MPX-65. Unless you have the equipment to do an alignment, just ensure it's at or as close to center as you can get it.

You have the 1st version. 2nd and 3rd versions had a separation control on the back panel. These are the 20001-39999 and 40001-49999 versions. Their Service manuals are just below the 10001-19999 version listed above on Fisherconsoles.com
 
Carbsandcaps said:
That’s what worries me about this fisher unit not having a channel separation control. I’m assuming it just goes for maximum separation?
Click to expand...
The control doesn't matter, its a case of the tuner itself not having the bandwidth available to produce a lot of separation. The control only lets you reduce it from maximum.

This explains it all much better than I can

Avery's "First" Maker: The TA-800

Talk about lousy Rhythm.
audiokarma.org
 
gadget73 said:
The control doesn't matter, its a case of the tuner itself not having the bandwidth available to produce a lot of separation. The control only lets you reduce it from maximum.

This explains it all much better than I can

Avery's "First" Maker: The TA-800

Talk about lousy Rhythm.
audiokarma.org
Click to expand...
That was a VERY interesting read even if some of it went over my head. So my fm-100-b that I’m using now which says “wide band” on it would actually be less “stereo” affect than an older fm-60 or fm-80 with my MPX 100 adapter. All because fisher was hedging its bets on the Crosby multiplex design.
 
You've got it backwards. Wideband tuner will produce a better Stereo than narrow band.
 
Carbs -- Any of the Fisher tuners and receivers that include the "Wide Band" label were designed from the ground up based on the GE/Zenith multiplexing format. Therefore, they have the wide band Ratio Detector transformer and WX or MPX-65 MPX sub-chassis installed and will deliver the FM Stereo MPX performance that Fisher was famous for. For receivers, this includes the 400, 500B, 500C, 800B, and 800C. For tuners, this includes any of the "B" versions or later (FM-50B, FM-100B, and FM-200B), the FM-1000, and the KM-60 tuner kit. The later consoles employing console specific tuner/preamp heads that came with built in MPX capability (as shown on the input selector switch) also included the wide band technology, although these units didn't always include the Wide Band terminology on the front panel. I have an Ambassador tuner/preamp head that has all the good stuff in it, but no labeling that it was wide band capable. That's likely because when the choice was initially made to go with the GE/Zenith format, everybody scrambled to modify their existing equipment to work optimally with that MPX method. That resulted in Fisher and a number of other manufacturers labeling their modified designs during that time period as "Wide Band" to distinguish the updated units from their previous non-wide band designs. After a period of time however, it was simply accepted that all current offerings were based on the GE/Zenith format, so the Wide Band label was no longer needed. For equipment built during and around the time when the FCC announced its decision however, the Wide Band label is the key to look for to get the best FM Stereo MPX performance. Of course, all of those type designs will already have MPX decoders installed, making the need for any external device in that case a moot point.

Dave
 
dcgillespie said:
Carbs -- Any of the Fisher tuners and receivers that include the "Wide Band" label were designed from the ground up based on the GE/Zenith multiplexing format. Therefore, they have the wide band Ratio Detector transformer and WX or MPX-65 MPX sub-chassis installed and will deliver the FM Stereo MPX performance that Fisher was famous for. For receivers, this includes the 400, 500B, 500C, 800B, and 800C. For tuners, this includes any of the "B" versions or later (FM-50B, FM-100B, and FM-200B), the FM-1000, and the KM-60 tuner kit. The later consoles employing console specific tuner/preamp heads that came with built in MPX capability (as shown on the input selector switch) also included the wide band technology, although these units didn't always include the Wide Band terminology on the front panel. I have an Ambassador tuner/preamp head that has all the good stuff in it, but no labeling that it was wide band capable. That's likely because when the choice was initially made to go with the GE/Zenith format, everybody scrambled to modify their existing equipment to work optimally with that MPX method. That resulted in Fisher and a number of other manufacturers labeling their modified designs during that time period as "Wide Band" to distinguish the updated units from their previous non-wide band designs. After a period of time however, it was simply accepted that all current offerings were based on the GE/Zenith format, so the Wide Band label was no longer needed. For equipment built during and around the time when the FCC announced its decision however, the Wide Band label is the key to look for to get the best FM Stereo MPX performance. Of course, all of those type designs will already have MPX decoders installed, making the need for any external device in that case a moot point.

Dave
Click to expand...
Ahhh ok that makes sense than I had it backwards.
 
Well after I finally got back to it I found out what’s wrong with the MPX unit. The stereo light relay windings are open. The coil is powered but 220v b+ then feeds the 19kc amplifier tube. There’s no voltage on the plate of that tube so not only can’t the relay close but the MPX can’t work correctly either. Now to find a replacement relay or bypass it with a 10k ohm resistor
 

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