If I may, installing any version of the MPX-65 -- or its variants -- and having the unit end up operating properly in a manner as would be expected of a Fisher product is quite an undertaking. Some time ago I took on that task with a 202-T, which is the higher tier version of the basic tuner/preamp series that includes the 100T/Coronet offerings in the lower tier. As is so often the case, the particular 202-T I serviced had been through the tech mill trying to get an MPX-65 to work properly in the 202-T chassis. It seems so easy right? Physically, it's a drop in fit, and the connections should be easy enough -- except that they aren't. However, with some thoughtful re-engineering, these units can be made to operate exactly as Fisher would have intended.
At the heart of the matter is that these units were designed to operate with the MPX-20 stereo decoder, the really never produced design that Fisher came up with to decode the Crosby FM stereo format that Fisher was betting was going to win FCC approval -- except it didn't, and the rest is history. That's why there were so few of these tuner/preamps produced, because they were ultimately an "oops". For those that were, Fisher simply recommended using one of their new outboard units based on the format selected (GE/Zenith), and let the issue quietly go away. But converting them is still so tempting. There are two main issues to deal with: Indication, and mono operation -- both of which are important.
Because the Crosby format utilized an FM sub-carrier for the stereo information channel, it was perfectly adequate to tune in an FM stereo station by the usual FM signal strength indicator (meter or eye tube), as the strength of the two signals (one within the other) would coincide with each other. The problem is, that the GE/Zenith system uses an AM sub-carrier, whose signal strength doesn't always coincide with the strength of the main FM carrier itself. This is why ALL of the Fisher FM MPX tuners and receivers of the GE/Zenith format base their automatic FM stereo switching or FM MPX Stereo eye tube indications on the strength of the AM sub-carrier of the MPX station being received, NOT the strength of the main FM composite signal itself. But the 100-T, 202-T units don't have any built in ability to handle switching of the eye tube from one type of indication to another. Therefore, even if you get the MPX-65 type decoder to "work", there is still no ability to tune it for optimum separation, let alone indicate that an FM MPX stereo station is being received.
The second issue is that if the MPX-65 type decoder is installed, you will find that you then no longer have any ability to listen to a distant FM MPX Stereo station -- or any station for that matter -- in true mono mode. Therefore, noisy stereo stations remain noisy, and mono talk radio/news stations must be listened to through the stereo decoder, which is not always a pleasant thing to do.
These things can all be overcome, but it does take some re-engineering of the unit -- which Fisher simply chose not to do. They just dropped the tuner/preamp line after these models, and went with separate tuners and the 400 CX-2 going forward. A proper install therefore involves installing a small DC switching relay to switch not only to FM Mono audio when selected, but also trigger the eye tube to function as an FM signal strength indicator as originally intended in mono mode, or as an AM sub-channel signal strength indicator when stereo mode is selected. The sensitivity of the eye tube also needs to be adjusted so that it closes part way in mono mode with full composite signal strength, but fully in stereo mode with full AM sub-channel signal strength. In this way, in stereo mode, stereo stations are properly tuned and indicated with full (or near full) closure of the tube, while mono stations only close the eye part way. This is exactly how Fisher chose to deal with these issues in the Fisher 400, allowing the single eye tube to act as an FM signal strength indicator, AM sub-channel signal strength indicator, and FM MPX Stereo Broadcast indicator all at the same time.
These issues may seem trivial compared to just getting the darn MPX unit to work, but without proper addressing, the beautiful Fishers that these units are just don't come off as working like a "Fisher". Other issues exist as well, but are much smaller and simpler to deal with by comparison. I offer this information to help guide any techs in their efforts to modify these models with an MPX-65 type decoder. Of course once installed, the decoder will also likely need alignment for best separation performance.
I hope this helps!
Dave