Mono / Stereo in Rockola 460

I have an issue with a Rockola 460 playing in mono only. The amp is perfectly good, proven by installing it in 474 with proper stereo performance. I have replaced the cartridge, too, but still no luck. If I switch the amp to stereo, one side drops out. Reversing the pickup connection at the amp reverses the dead side. Switch to mono and both channels play. The tone is good and there’s plenty of volume. I have not changed the pickup cable, but I did check for continuity from the cartridge connections to the pins that plug into the amp. I’d appreciate any direction on where to go with this.
Patrick.

Patrick,
It seems to me that you have proven the problem is between the cable plug that goes to the amp, and the cartridge ?
To test, either swap the connections at the cart. or jump the two "hots"--if you get both sides that way--it's GOTTABE the cart--- Ron Rich

Thanks, Ron. Dead side on cartridge. I'd replaced it from an unplayed 480 parts donor and, lucky me, apparently have two with dead right sides to throw out. Sounds even better now!
Patrick