Tact switches + rotary switches + multiplexer logic failure help

[cincin]

I'm working on a MiroKORG digital synthesizer, and the only problem left is that the TECHNO rotary switch selection, the Program 2 and Bank A/B tact switches do not work. Pressing the tact switch, or rotating the switch to TECHNO, has no effect.

Looking at the schematics (page 9), you can see that the 2nd pole (labeled #3) of the BANK SELECT rotary switch (SW2) is on the same line as the Bank Select A/B tact switch (SW5), and Program 2 (SW12). I suspect the second pole on SW3 and SW8 (labeled #3) also don't work, I haven't tested them specifically.

I looked at the traces and there's strong continuity from pole 2 of SW8 all the way to pin 69 of IC18 (on page 6 of the service manual, following T1).
D14 and D20 test fine (page 9), C186 and L30 (page 6) also seem fine.
I've replaced the tact switches and cleaned the rotary switches contacts before realizing that wasn't the problem. All switches test fine.

And that's where I'm lost.

If it was a problem with the multiplexer (IC22, page 9) I would expect more than those buttons to give unexpected results, but I admittedly don't know what I'm talking about (that's why I'm in Beginner)

But what else could it be? a logic failure in IC18 (CPU)?

What should be my next troubleshooting step, swap the multiplexer?

Thanks!

[Kim Christensen]

You could have another stuck button (Shorted/on) which is confusing the key-switch polling. Also, check D15, D17 & D18.
If you have a scope, check the outputs on IC22. You should see some short logic low pulses on pins 7,9-12 as it scans the switches. If you see that, then IC22 is probably OK.

[cincin]

Good point!

The other tact switches work fine, and I just tested that they are not shorted on.
What I've been calling encoders are actually just mechanical rotary switches, and they all check out too.
I checked D15, D17 and D18 and they all test fine.

I will put IC22 on the scope in the morning. Maybe between the scope and touching up on basic diode scanning and multiplexer fundamentals I'll figure it out.

Will report back.

Thanks!

[cincin]

Well.... turns out I didn't have continuity all the way through after all... I think I got mislead by the fact that the components are not physically connected in the same order as they are in the schematics. That and following the trace back and forth on the double-layer PCB.

In any case, there was a small break by an oxidized via. Glad I double checked.
It has now been repaired and the synth lives again!

A nice reminder that schematics represent logical connections, not physical connections.

Thanks for the help!