Peachtree original Decco, troubleshooting balance issue

I recently purchased an original Decco on FB Marketplace. It generally sounds great and seems to be in great condition despite one issue (and I honestly don’t think the seller on FB, who had bought the unit recently at an Estate Sale, realized this was happening): The balance seems to be off/not working. 

When initially testing it out, (with balance centered of course), I first realized the right channel seemed to be playing at a slightly lower volume. When I shift the balance to the right channel, the volume level gets much lower and the audio is coming from both speakers. When I shift the balance to the left, the volume level is much louder than when shifted to the right and, again, both speakers are playing audio. The result is so far is I’m able to play music through it and it even sounds ok when I have the balance set to just right of center. The problem of course is it’s not a stereo signal. I’ve definitely ruled out cables and source components and this happens no matter the input (SPDIF or RCA) or output (loudspeakers, pre-out, or headphones).

I have very little repair skills or knowledge and realize I’ll probably have to get this diagnosed and repaired by a professional but was curious to ask if there might be a relatively simple test or solution, or possibly known issue, I’ve not considered.