Vintage Sanyo JCX 2300K receiver for repair

This is a lovely Sanyo JCX 2300K receiver that I enjoyed for several years. It has a wonderful sound and I paired it with set of bookshelf vintage speakers. The right speaker is no longer outputting much sound hence selling as is. I had the receiver repaired several years ago and the repairer pointed out that the volume knob was fixed by a prior owner who didn't use a proper spare part but functions well. I haven't it professionally looked at but here's a detailed description of the sound issue in case this is something you can fix! 1. The right speaker side of the stereo stopped working. I noticed that the right speaker wasn't producing any volume. If I adjusted the balance dial I could hear the speaker very faintly. I thought that maybe it was the speaker or the speaker cables, so I swapped the left and right speaker. But the problem persisted (the left speaker now on the right side was producing a very faint sound). 2. I thought maybe there is something up with the Speaker A set, so I tried Speaker B set. Same problem as above - the right side wasn't working of Speaker B set. 3. I tried Speaker A system and Speaker B system both on the left side and both speakers worked at the same time! 4. I tried taking the lid off and looking inside, but couldn't see anything visibly wrong