since I only have a MK1 I can't tell you about the headphone jack for sure but:
- the slider switch should be set to MIDI
- doesn't matter if MIDI 1 or 2 is used, I would stick to MIDI 1 just to be on the safe side
- the RCA output is line-out level, it needs amplification

Yes the headphone port should always output something. I take it the volume dial on the front is not set to mute?

If you are using the 5-pin MIDI port, then the switch is supposed to be set to MIDI. If you are using the 8-pin mini-DIN port with a serial cable, then the switch is set to RS232C-1 for 31.5kbps and RS232C-2 for 38.4kbps. RS422C is for the Macintosh. If you connect the SC-55mkII with a gameport/MIDI cable, you should have the switch set to MIDI. The MIDI cable should be connected to the MIDI IN on the back of the unit.

Thanks for the info, looking at it again I'm pretty sure the headphone port is broken (the volume seems fine as it does affect the slight volume out of the RCA output) but it also seems to have other issues.
While testing every so often when playing the built-in midi songs the unit would power cycle, now when using a USB to Midi adapter (cheap one off DealExtreme) and in Dosbox outputting via Midi it just restarts the unit over and over but you can see it accepting the signal for a short second before doing so.

I'll just have to track down an amplifier as that has always been in the picture and hope it stays solid through the gameport to midi cable, at the same time keep an eye out for another unit and look to sell this one as is (sad though as it is in mint condition with original adapter and manual)

Now I just have to find a way to determine if my Roland SMPU/AT ISA Card works and if it's worth tracking down a breakout box for it. Thanks Guys

So after testing with an amp the output is still extremely low, after opening it up and inspecting the board I found the large 2200uf 16V capacitor has leaked. I will purchase a new one and replace it and continue testing.

Not sure if anyone see's any value in me documenting the teardown process or the repair.

that's glue from the assembly line, not leakage.
I would check the audio ports for fractures on the solder joints

Hmmm, I could be wrong I guess... though there is no other glue on the main board. I've replaced quite a few caps from older video consoles and that looks pretty consistent with what I've seen. This issue with this unit are quite numerous

Headphone jack doesn't work
RCA Output sound is extremely low even when connected to amp or not
Device randomly and occasionally power cycles over and over

I'll replace it anyways and see if that makes effects anything, I may just replace all the caps as I've seen that do miracles for older electronics. If that isn't it this thing will be nice for parts if I can find a working SC-55 MKII that may be banged up.

That's glue 101% , if that amount was acid leakage the sc-55 board would be a mess ... look here: Have you opened your SC-55 to check the battery inside?

If you are unlucky and the unit is indeed damaged (could be the onboard ICs , you never know...) please don't throw it away , as you can see on my sig I am looking for a mk2 housing/display/ports 😉

If it is glue then the 2200uf cap is the only surface mounted capacitor on the board, perhaps someone can take a pic and we can see if this is normal. It's also possible with the flash the picture I took just doesn't give the correct detail, but the brown's and mustard colours as what I'm used to seeing in blown caps (though normally from the top)

I will agree that with a flat top and no bulging you are almost winning me over. Don't worry I won't toss it, I'd just give it to anyone who wants it. Once I'm done build a couple of my machines I will take dozens of extra video and sounds cards, plus processors and P1 mobo's that I'll just dump off to retro enthusiasts.

I've uploaded another pic of some other caps (and various wiring) to show the rest of the board looks clean of glue

basically this is the only cap on the mobo that has that glue... my mobo also has it, your has it and even Stefan_L's one has it:

http://www.ym2149.com/roland/roland_sc55_pcb_partside.jpg

it's there on this cap because it's the biggest one on the mobo, can't really tell you how that affected assembly so they used glue 😀 but it's normal

kudos on your decision to offer stuff to people 😊

edit:

here's a pic of one of my MK1s , you can see the glue as well

or a CM-32L:

maybe the roland people liked the smell of glue back then 🤣 🤣 🤣

The evidence just keeps mounting, the second pic does show some of the brown and dis-colouring that I assumed by electrolyte. I'll look around for any issues with the PCB and swap out the capacitor if need be. Thanks for all the info keropi, I appreciate it.

don't mention it, glad I could "help" ... my bet is that the ports just have fractured solder joints 😀

So I got around to looking at this, Keropi you were correct that the capacitor looked good even after removing but appeared a little loose. I replaced it anyways and found 2 cracked solder connections which appeared to be grounds under the 3 midi ports. I fix that with some extra solder and tada, the sound is fine and the headphone jack works. Likely the last user put a lot of strain on the midi ports with his cables, a good thing to note if you have a headphone jack issue with this system.

great news then!!!! 😀