SM-80: Connecting an MPX unit

[tutor_turtle]

I am putting the final touches on my SM-80 restoration.
I'm very pleased with the sound. I'm guessing it's putting out a solid 28 watts/per.
As anyone who is familiar with this tube-based stereo receiver, it was setup for Stereo AM and Mono FM!
But it does have two RCA's tabled MPX and an MPX button.
When MPX button is activated, the FM goes silent.
One looks to be an output, the other, an input.
By my reasoning, there should be one FM coded output and two inputs for the decoded left/right. (the system calls all I/O, channels 1 & 2)
I have an opportunity to get a (non-functional) Fisher MPX decoder for free.
I'm curious as to how this lash-up is supposed to work.
Has anyone played around with an MPX decoder? Better yet, hooked one up to a SM-80?

As a side question: I don't have all the original knobs, I have the two original tuning knobs, but the bottom row are substitutes.
They are close, but I'm short one.
Attached is what the originals should look like.

 

[Hyperion]

This might be of interest.

M

Thread 'Sansui MP-2 Multiplex Adapter'

Oct 19, 2014

Acquired one of these and could not find out which Sansui tube models this was
made for. Anybody know? Thanks.
MW

• mwwcam
• Replies: 6
• Forum: Exclusively Sansui

 

[Sansuiman]

I was always of the understanding that the stereo line level output was on the MPX adapter itself, thus the hook up was MPX OUT from tuner/receiver to IN of MPX adapter, then L & R OUT of MPX adapter to AUX or TUNER IN L & R of integrated amp or receiver's function select switch to directly

For receivers, this does means chewing up the AUX input, or a tape loop, as the adapter doesn't magically convert output from the existing FM mono function on the selector switch to stereo, but rather creates a separate stereo source from the "raw" FM signal output, which then has to be fed back into the receiver as left and right line level audio from the outboard adapter.

That said, I wonder if it would be feasible to connect the L & R audio outputs of the MPX adapter to internal points of the receiver, to "inject" the stereo signal for output from the receiver without needing to use the AUX or tape loop inputs. Obviously, this would entail adding two additional RCA jacks and associated shielded wiring form l said jacks, to wherever one could find an appropriate point within the receiver to inject the FM stereo signal to be routed directly through the function selector.