Fisher FM-100 & MPX-65

[junkjer]

Hi-
Could anyone offer any guidance on connecting an MPX-65 to an FM-100?

The label inside the tuner identifies it as chassis FM-100, serial #10106 A. There's a space for the MPX-65 and a 9-pin connector next to it.

My best guess so far:

FM-100 connector position #1: Black wire (MPX input) from MPX-65 (the one next to the orange wire on the MPX)
#2: Orange wire from MPX (FM-100 schematic indicates this connects to a 200v source)
#3: ?
#4: ?
#5: Brown wire from MPX
#6: Green wire from MPX
#7: Yellow wire from MPX (the yellow near the black wire on the MPX)
#8: Shielding from the FM-100 output wires
#9: Black wire from MPX (near the yellow on the MPX)

There is another yellow wire on the MPX, which appears to be for the stereo multiplex indicator on tuner models that have one -- I don't think the FM-100 does. So I imagine I leave this unconnected.

This would be much simpler if I knew what I was doing or I had a FM-100 schematic that includes the MPX-65 connections. The schematic I have is apparently for connecting an MPX-20 to the FM-100.

The tuner sounds fantastic in mono so if mating the MPX-65 is extraordinarily complicated I probably won't bother or will take it to a pro.

I could also use some guidance on how to clean the knobs (or where to find new ones). They're pretty ugly.

Thanks for any help you can offer!

 

[dcgillespie]

Junker -- There are no "instructions" for installing an MPX-65 into and FM-100, as that tuner was designed for use with a non-existent Crosby based MPX-20. That being said, it can be done -- but to do the job right and have it operate as a "Fisher" (i.e., really great), it takes more that just mounting and hooking up the MPX-65. That's because the basic operation of the Crosby decoder system is inherently different from that of the GE/Zenith based system that was ultimately approved. As a result, different circuit and user features are needed on the Crosby based tuner itself to get the best performance out of a GE/Zenith based decoder. For example:

1. The Crosby based Fisher FM tuners typically have no ability to indicate the reception of an FM MPX Stereo station when being received. As a result.....

2. They therefore offer no ability to tune the stereo station based on the strength of the all important AM sub-carrier within the composite signal, that contains the stereo information. And,

3. They typically (and importantly) offer no ability to listen to a stereo station in mono, once the MPX-65 decoder is "installed". As a result, noisy stereo stations just stay noisy.

I recently did the very thing you are looking to do with a Fisher 202T tuner/preamp. Same story. Wonderful tuner. Wonderful preamp. Designed for the Crosby decoder. It had been through a number of techs trying to make it work before I received it. The point is, you can do a simple install of a MPX-65, and it will "work". And while I haven't looked at an FM-100 schematic, I'm rather certain it won't just marry into the unit and become a FM-100B by hooking up a few wires. The 202T project required modifications to the eye tube circuitry, and the installation of a relay controlled manual stereo/mono switching system to have it really operate like a Fisher.

I'm hardly trying to discourage you from your project. It will work. But the devil is always in the details, which is also where the stuff that makes up a Fisher lies. I just want you to realize this.

As to your specific questions regarding the 9 pin connector (by pin number):

1. This pin is for manual stereo/mono triggering of the Crosby decoder. Do not use.

2. B+ for the pilot and sub-carrier circuits of the MPX-65. You will need to devise an additional B+ to power the audio based circuits of the MPX-65.

3. A tie point. Do not use.

4. Variable de-emphasized L+R audio for the Crosby decoder. Do not use.

5. Heater voltage.

6. Heater voltage. These should be balanced to ground.

7. Ch B input.

8. Circuit ground.

9. Ch A input.

The MPX input should be taken from the ratio detector signal, which is not available on the 9 pin connector.

I hope this helps, and good luck with your project!

Dave

 

[junkjer]

Thank you for so much detail. This is definitely beyond my limited skills to perform and your point 3 is the key. This thing sounds - to my starved ears raised on solid state stereo radios - fantastic beyond belief in mono. So there's no point in messing with it unless I can find a pro who can do what you outlined with your tuner. And how much time am I going to spend sitting right in the sweet spot between the speakers, anyway?

Thanks!

 

[dcgillespie]

Mono can certainly sound wonderful, but I surely didn't want to scare you off of your project. Maybe down the road after you've had some time with it. Just be sure that whomever you select to install your decoder (if you do) is well versed in the theory of how in fact FM Stereo operates. I say this primarily to prevent you're wasting time -- and potentially $$ as well -- by giving it to someone who thinks its as simple as connecting a few wires, but then leaves you disappointed in the end.

Besides these issues, it really is a wonderful tuner, so enjoy!

Dave