Stereo/Mono Switch for Sys
I studied the service manuals for Marantz receivers from the 70s to figure out what they did, and I basically copied that design. The solution is very simple, a 4-pole double throw (4PDT) switch, when thrown one way, pass through of stereo, and the other, the stereo signals go through 2.2KOhm resistors and combine for mono. Even though it's a simple solution, it's kinda expensive because the switch I used is ~$18.
I used an infuriating online tool to draw the circuit diagram, so it's the best I could do quickly. They didn't have a 4PDT widget, so pretend the two DPDT are a single 4PDT.
For the wiring, I removed the output RCA jacks and bent the center pins up. Then I soldered wire to the contacts on the board for the input and then directly to the center pins for the output. In the picture, you can see the resistors soldered to the switch and the wire connecting all four top terminals of the switch.
As for mounting, I just drilled a hole in the Sys and used a finish nut outside, and a lockwasher and nut on the inside. Ended up looking ok, wish I would have been able to center it with the volume knob, but I think that was too low and the switch would have hit the circuit board. It was a bit tricky getting it mounted and back together because this mod ties the front/top panel to the bottom of the unit.
The way it's mounted, up is stereo, down is mono.
Parts List:
2x 2.2KOhm resistor RES 2.2K OHM 1/4W 0.1% AXIAL
https://www.digikey.com/short/p1vqhd
1x 4PDT switch C&K 7401SYZBE
https://www.digikey.com/short/p1vq95
1x dress nut C&K 776001201
https://www.digikey.com/short/p1vq9w
1x lockwasher C&K 700303202
https://www.digikey.com/short/p1vqbc
1x nut C&K 700201201
https://www.digikey.com/short/p1vqbj
Wire (I used some 22AWG solid I had on hand)
Imgur picture link:
https://imgur.com/a/oqSrbyp