I have a Roland SC-55 I bought used a few years back. At the time, I tried the MIDI input on the back and it didn't work. I then tried the MIDI input on the front (MIDI IN 2) and it worked fine. So I kept using that input and didn't worry about it. I thought maybe it was just a faulty jack on the back of the unit or something.

Recently, I tried testing all MIDI jacks (IN/OUT/THRU) and discovered none of the rear jacks appear working. Only the MIDI IN 2 on the front works.

I put it in test mode and ran the MIDI test and it came back with errors. The manual suggest faults in either IC3, 4, or 30, and/or IC 3, 19, 21, or 30.

Now MIDI IN 2 takes a completely different path than MIDI IN 1, and runs through IC 19 and 21. So presumably those ICs are okay. IC 30 is the main CPU so if the fault is there, well, it's probably FUBAR. That leaves IC3 (SN74LS04NS Hex inverter) or IC4 (PC-410 photocupler). So I imagine if there is a repairable fault it's somewhere in and around those circuits.

I'm still novice at diagnosing and repairing electronics, so I'd appreciate some advice as to what and how to test things. I've already tested the jacks themselves and they are okay, plus some of the resistors. I've also only got a basic multimeter at my disposal; not sure if that will be enough to test everything.

Below is the circuit diagram for the rear MIDI jacks: