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Need Help with Roland SC-88 Pro distorted sound!

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First post, by Dusko

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Hello All,

I know what I'm asking here is not easy to answer, kind of need an expert but, here I go anyway. A couple of years ago I got a Roland SC-88 Pro (120v version), it was like new, manual, original box, everything was great, not a scratch. Played with it for a couple of months and it worked as expected. A few days ago I wanted to use it again and now the sound it's completely distorted, it's like a big crackly sound (I'm attaching a sample using the sound preview button: one short press, one long and another short at the end) .

So, I opened it up and noticed that in the main board, some of the Alum/Poly (smd) caps leaked, not a lot but they did. I replaced them (8) with spare ones I got from Digikey, nicely done, no traces lifted whatsoever and used the same values of course but, that didn't fix the problem. I got the Service Manual and compared the voltages from the transformer, there are 5 output pins as you can see in the picture. I did two tests, one with the transformer on it's own, (ribbon) cable disconnected from the Main board, and the second with the cable connected.

My readings with a Fluke multimeter: (AC)
with the cable disconnected from the main board:
on the 12v ones I get 16.77v each
the 5v gives me 6.22 and the other 2.17

with the cable connected to the main board:
on the 12v ones I get 16v each
the 5v are aprox. 3.98v each

Either way, theses numbers are all wrong, correct? am I missing something? I don't see any signs of heat damage or leakage (except the caps I mentioned from the Main board), everything looks good, no funny smells, etc. Does anyone that has this unit can check those voltages and let me know? If mine are all wrong, that probably damaged some chips right? I guess replacing the transformer would be too late...

More finds:
I tested some voltages that go to the main board and got these values: (DC) 4.9v, 3.6v, 2.48v, 0.9v, some numbers look really odd but I don't know.
In the "analog" or secondary board, the (Japanese) electrolytic caps look in great shape, no swollen or leakage. Also, I did a test with the line-in and the sound is correct, no problems there.
Midi inputs get the signal but, that's when audio gets bad, same if I press the Preview button.
Display and buttons have no problems at all.

I did some of the tests from the Service Manual and got several errors with the "Memory Test", I'm attaching a screenshot of what the manual says it's wrong and it matches exactly with what is reported on the screen. I looked at all those chips but I can't find broken traces or burned marks.

If anyone has this problem before drop me a line please, either you fixed or not.

Thanks for reading!

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 1 of 7, by Dusko

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A little clarification, one of the ribbon cables that goes to the Main Board, attached near the transformer on the secondary board, reads 4.95v (DC) at two points, this may be a correct voltage. Another ribbon cable that is coming from the other end of the secondary board (passing thru the analog section, I guess), is where I get the other voltage readings (3.6v, 2.48v, 0.9v), this probably makes more sense... ? I'm starting to think that maybe the voltage is not the problem.

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 2 of 7, by Deksor

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Unearthing this topic, because I just bought a defective SC88 pro that has *exactly* the same issue.

Mine had bad caps, so I replaced them, but the issue remains (and the damage they caused was very minimal thankfully)

I ran the memory test, and I got ABCDE blinking, which means that the wave roms are dead (and that seems to make sense considering the issue I can hear).
Now they all blink together which should indicate they're all bad at once, so I'm a bit scared that it might not be the roms that are bad, but something wrong in the control logic ... which seems to go straight into the proprietary Roland RA01-05 chip so I hope it's not that ...

Now idk what test makes, maybe it checks the checksum of all the roms together ? Or maybe the (allegedly) bad rom causes a big problem on the whole bus ?

In case the wave roms are bad, is there any place where I could find a dump for them ? Please dm me if you have them.
My synth runs on Ver 1.02

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 3 of 7, by Deksor

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Well ...

When I said the damage they did was minimal I was wrong !!
turns out it was a corroded via that looked like it was still connecting, but it had a tiny amount of corrosion on the connection between the trace and the circle of the via, and it got cut there. After a bodge wire, the RAM test says a happy "ok" and the sound is fine again !! 🥳

@Op if you still have yours, make sure to check each traces going to the ROMs, especially those around the capacitor. Use a magnifiying glass or a microscope, it helps a ton 😀

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 4 of 7, by Dusko

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In my case it was a via that got corroded in the internal layers of the board due to a cap that leaked. I was able to fix it back then.
Now I'm having a similar problem again with a couple of instruments. I haven't open it yet, but I'm sure it's some leakage residue that I was not able to spot somewhere else.
I'll post my findings when I get to it.

Retro PC games channel: https://www.youtube.com/@myOldPC-75
Electronics, mods and tools channel: https://www.youtube.com/@RetroRust-75

Reply 5 of 7, by sl1pkn07

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Hello.

i have the same problem. but i have a request. seems my Sc-88pro have different main board than the service manual in internet. my main board is the model 70895190, not the 70895189 like i found in that service manual

any one have a second edition or something for this manual?

greetings

Reply 7 of 7, by sl1pkn07

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seems is the same manual. in the pcb picture in the manual show in the silkscreen part nomber like my unit

just repaired two traces of the 4 dots near the C50 (seems C54 leak to this part). the trace from pin 4 in IC5 to pin 2 in RA8 and the pin2 in Q2 to pin 4 in IC13 (mute line) the rest of the circuits seems intact. but yea, i need recap, at least the smd caps with ceramic ones

https://i.ibb.co/STyQh04/photo-2025-04-14-18-42-08.jpg

greetings

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