Not too long ago I picked up a junker 500-C because Number One Son began expressing interest in lugging one of my darlings off to college with him. Well, not that I wouldn't do anything for him but the thought of him or one of his cute little drunken buddies splashing suds all over it with the resultant sparks and smoke gave me the shivers. So for what was a very good deal, I picked up the junker. The unit was sold "with a weak but working channel" so I thought nothing of it attributing the weakness to a cap or something dirty. I went about tackling the usual mods and exercising the usual gremlins along the way: new bridge, can caps, socket and switch cleaning, and misc. cap replacements. As I made changes I checked to see if the weak channel was corrected. Sadly, not. Now it can't be the volume pot since that was initially completely shot so I popped in a replacement from my stash. Still no LEFT channel. It's not the dastardly TAPE MON switch that has given my grief in the past since I removed it and thoroughly cleaned it. Still no good. (On deck is the MPX FILTER switch which has me wondering.....) To be clear about what's now going on: With the selector set to MONO, I get full sound from both channels. With AUX engaging a CD player-- both channels strong. With the selector set to forced STEREO, STEREO BEACON indicator comes on and left channel drops right out! With selector switched to FM AUTOMATIC, STEREO BEACON indicator remains lit regardless of signal strength. That is, when turning the tuning dial, the relay doesn't click and the BEACON light stays lit regardless of station signal strength. With the beacon lit as described, NO left channel (or extremely weak one.) As part of the recommended restoration protocol, I've replaced the 1uf cap off the MPX. Likewise, C 221 & 222 as well as C-214 within the MPX module. No impact. I've checked the diodes on the matrix board and all have one way conductivity. The associated resistors are good as well. I haven't yet checked voltages within the MPX but my thinking is IF the MPX is the problem, then neither channel would be working and, as I've mentioned, the right channel is fine. I'm bothered by the relay being always on but my thinking is that would not be channel related. Oh yeah, it's not the 6gk5 since it like all the other tubes that came with the unit have been changed. I've hit a wall! Anyway, any new thinking would be appreciated cuz my thinking module is exhausted!!