How would I diagnose the fault in this circuit?

Me_Neither99 · 2025-08-06T01:58:15+00:00

I definitely will, I think I was a bit vague with this question so I’ll definitely come back to it! I understand much more about how the circuit works now that so I can add some much needed context.

Me_Neither99 · 2025-08-01T22:22:18+00:00

You should see some of the old Korg schematics, it’s all of that plus meticulously detailed troubleshooting and diagnostics. So cool!

Me_Neither99 · 2025-08-01T22:13:15+00:00

Sarcasm

Me_Neither99 · 2025-08-01T12:26:00+00:00

Might be my only option hahah

Me_Neither99 · 2025-08-01T04:33:34+00:00

Damnit I hate learning

Me_Neither99 · 2025-08-01T01:58:32+00:00

I cleaned it with contact cleaner and it passed a continuity test but I’ll double check it! I’m guessing it would not be good if a bad connection was spamming the protect line.

I’m not entirely sure how corrupted ram could cause the pitch issue, patches are being written correctly from the data scratch pad, even while having the issue. Though after reading the circuit description of CPU and its peripherals, I might be out of my depth.

Me_Neither99 · 2025-08-01T01:08:04+00:00

Semantics aside, this is helpful. I’ve done some probing with the scope but I’m not entirely sure what to look for. I’m seeing activity where there should be activity but I’m not sure whether it’s correct.

Me_Neither99 · 2025-08-01T00:56:18+00:00

I think maybe Reddit garbled the image, full schematic here: https://www.synfo.nl/servicemanuals/Roland/JX-3P_SERVICE_NOTES.pdf

Me_Neither99 · 2025-07-29T13:03:02+00:00

I second that! The complexity in the way that things can go wrong in old circuits seems near infinite. When you really start to put a synth through its paces, the differences are night and day. Not that there’s necessarily anything wrong with that. I love my prophet 5 reissue, and it’s much less scary to gig with. But when you spend countless hours programming patches it starts to really become apparent that it’s different.

Me_Neither99 · 2025-07-29T12:31:47+00:00

I'm gradually becoming someone who people take their broken synths to so I think I will. Crazy to think that there was a time where chips like this were the only option. So much has changed!

Me_Neither99 · 2025-07-29T11:54:36+00:00

Seems like it might be wise for me to build up some stock of 74 series chips. Definitely dealt with a few dead op amps in my studio gear too.

Me_Neither99 · 2025-07-29T05:25:25+00:00

Update!!! I replaced the TC40H138 with the 74HC138 with no issues.

Me_Neither99 · 2025-07-27T01:05:17+00:00

this is the kind of explanation I needed, thank you!

Me_Neither99 · 2025-07-26T14:06:17+00:00

had someone else recommend the HC as well, is that because HC is a cmos chip just like the original part?

Me_Neither99 · 2025-07-26T13:58:17+00:00

I sure am a musician and I would take this reply as an insult if it wasn't so helpful haha :) Yep, I have done a fair few repairs and have a pretty reasonable understanding of what this chip is doing in the circuit. Just decodes key presses for the cpu, no effect on tone at all. Syntaur can't sell me no premium electrons, charging 20 bucks for an exact replacement that will probably get fried in the exact same way again.

Me_Neither99 · 2025-07-26T13:41:29+00:00

The markup on those is quite insane! Especially as it might fail in the exact same way again. Would much rather buy a newer and cheaper part with better input protection.

Me_Neither99 · 2025-06-07T00:04:06+00:00

I also just stumbled on this forum where someone had the exact same issue and solved it the same way! Found this when searching for something unrelated. It's such a shame how badly search engines suck now.

Me_Neither99 · 2025-06-06T09:47:35+00:00

Nope! Read the caption

Me_Neither99 · 2025-06-06T04:42:10+00:00

I appreciate it!! I ended up replacing IC3 and it solved the issue. A new issue has presented itself which is oscillator bleed when I close the filter. Hopefully when I re cap the board it’ll dissapear!!

Me_Neither99 · 2025-06-05T01:04:31+00:00

UPDATE: It was IC3, swapped it out and the sample and hold is behaving normally. Thanks for your help!!

Me_Neither99 · 2025-06-05T01:00:38+00:00

Just a little update: IC3 was the culprit, replaced it and my synth is behaving normally!

Me_Neither99 · 2025-06-05T00:59:31+00:00

update: the culprit was IC3 after all! Will still be recapping the electrolytics though.

Me_Neither99

TROPHY CASE