I haven't owned or worked on a MAC1500, but from the schematic, it appears to be very similar to the MA230, which I have and have worked on. From the schematic, my guess is as follows: This has an FM pre-pre-amp section, (circuit surrounding Q5-8), which feed into the preamplifier section (circuit surrounding Q15-20). The phono section is a pre-pre-amp that also feeds into the preamplifier section. The phono section is the circuit surrounding Q9-14. There are no tubes involved in the phono section.
Thus, if the FM and Aux inputs work fine, then you need to look solely at the phono preamp section, which comes off the selector switches with a 1meg to ground resistor, followed by the .22uf input coupling cap, works through transistors Q9-14, and then out through some coupling caps and resistors.
On my MA230, there is a lot of signal leakage between inputs, so you could simply be hearing the working channel bleeding over to the nonworking channel. Thus, your problem may not be in the phono section at all. The first step I'd take is to ensure continuity between the input socket (clean the phono socket inside connectors well with some polish, and perhaps emery cloth if they're nasty, and give them a little squeeze with a small pliers to tighten up the connection with the RCA plug) and the selector switch. Dexoit the selector switch contacts, ensure continuity to the phono board, and then from the phono board to the preamplifier section (also check to make sure any caps on the selector switch are still good). If you've done those and are satisfied that the signal is able to make it from RCAs to the phono board, and then from the phono board to the preamplifier section, then you can turn your attention to the phono board itself.
First check that each channel is getting the proper dc voltage from the power supply. If they are, then turn to transistors. I believe the transistors are in sockets, which will make it easy to check them--swap Q9 with Q10, 11 with 12, and 13 with 14. If the problem switches channels, you know you have a dead transistor in the one channel. If not, then it would appear to be an open resistor, a broken/cold solder connection, or a bad cap. Measure all the resistors to make sure none have gone open, check the connections on the board itself looking for bad solder connections, then turn your attention to the capacitors. If they haven't been done, for $10 you can recap the whole phono pre section, and be sure they are good.
In short, verify that the signal can get to the phono board and out from the phono board. If that doesn't make it work, you've isolated it to the phono board. Check power supply voltage; transistors; resistors; and caps.
Good luck and happy hunting.
