Suzuki Omnichord OM-27 with a buzz

[cincin]

Novice here. I'm trying to repair a Suzuki Omnichord OM-27 that has a buzz. I'm operating it with a new replacement 12V 1A center-negative AC/DC power supply. It wants 600mA, I believe.

The only schematics available for the OM-27 are here, and they are handwritten and pretty hard to understand:

https://drive.google.com/file/d/0B8-MOD-DNXkHNzY0ZWE4YjAtMDNlZi00YTY0LWI5YzYtNzNjMDhiYTI4MzJm/view?sort=name&layout=list&num=50&resourcekey=0-EqLlWTBcP2OV2e0ZDq-aXg

The buzz is present in both the audio out and the onboard speaker. It gets much worse when I put my hand near a specific 1uF 50V electrolytic capacitor near the volume pot, and it becomes deafening if I touch the top of it. (I know about the dangers of capacitors. This was a calculated move) Tapping it with a chopstick didn't do anything special. It seems to be acting as an antenna.

- I've replaced that capacitor, but that wasn't it. The new one behaves just like the old one. I also replaced a couple capacitors around this one: a 0.01uF and another 100uF electrolytic cap.

- I've tried a second, identical, brand new 12V 1A center-negative power supply, same noise.

- When I tried probing around with an audio probe plugged into a guitar amp, I noticed that the buzz disappears completely when I connect the ground trace to the amp cord's TS sleeve (ground). The antenna effect disappears, I can touch the cap and it doesn't make a noise. The amp and the unit are connected to the same power outlet. The same is not true if I ground the trace from the same point, or any other point, right to the unit's power connector's ground. So, I'm assuming it's not a broken ground trace in the unit.

I don't understand what's happening. I have theories and questions:

- Why would me touching or even just hovering above a known good cap (the first one I replaced) emphasize the buzz? What's happening here? I'm I acting like an antenna? Induction from my static?

- Why would connecting the ground trace to the amp's input ground remove the buzz? The current wants to go back to the power adapter, so it's not like it's draining out through the amp's ground into the wall.

- Should I concentrate my search near that antenna cap, or could it come from anywhere?

My current theory is that a component has failed and is shorting to ground, which causes a potential on some part of the ground trace, and/or a lack of potential on the circuit, giving a ground loop type problem. Does that reasoning make sense? It doesn't explain the whole antenna cap effect.

- What should be my next trouble shooting step?

Thanks! I learn every time I post here.

[Audiorepair]

Try removing the volume pot and hard wiring it on full (bypassed completely).
Maybe the pot body isn't grounded/shielded, or the track is somehow touching the body/shield, or its ground lug isn't actually grounded.

You could try grounding the pot body first to see if that makes any difference, or shorts stuff out etc.

[cincin]

Thanks, good call. I tried grounding it to the ground trace and it doesn't make any difference. I think it's properly grounded.

[NOON Design]

Is the new power supply a switchmode supply? Many can create horrible buzzes in audio equipment, especially if it was designed for a linear supply and has filtering for line frequencies, not switching frequencies. See if you can try it with a linear supply or even a different brand of switchmode, some are quieter than others.

[cincin]

Good to know, thanks. I tried powering it with my bench power supply and it sounds the same.

The buzz is STRONGLY influenced by my hand's proximity to that 1uF cap I just replaced. It's 100x worse if come within an inch of it. The buzz is actually almost acceptable when no one's around the unit, but it gets bad when you try to play it since you invariably need to place your hand over that cap... Could that weird effect really stem from the power supply?

Cheers!

[NOON Design]

Getting the same buzz from your bench supply should rule out the psu. Is there something particularly electrically noisy in the room that it's picking up via you? Have you tried it in a different room?

[cincin]

Nothing specifically noisy in the room, and it's the same everywhere in the house.

I'm not experienced enough to say that this is in fact strange, but it sure it to me.

Any random troubleshooting tests I could try that might shed more light on this? At this point it feels like I'm randomly replacing parts...

Thanks for the help!

[SMdude]

When you plug it into the guitar amp, the unit is getting grounded through the amp.
Currently I think your ground is just floating.
If you connect your ground trace to another ground source does the buzz disappear?
Theoretically it is probably better for it to ground through the guitar amp if you intend to use an amp with it, otherwise I'd be looking at tying the secondary centre tap to ground(research this and do at your own risk, it is something that I would do)(assuming you are using a transformer with centre tap).

[cincin]

My power supply only has 2 prongs, so that wouldn't work. But why is the ground floating to start with? surely it's not by design?
Cheers!

[Audiorepair]

Does it still buzz when powered by batteries?

[cincin]

Well well well, it doesn't buzz when powered by battery! That's a good clue. I didn't want to buy 6 C batteries just for the test, so I first tried plugging the replacement power adapter into a good battery pack / inverter I have. The buzz was still there despite that pack not being grounded to earth. Then I found an old 12V CCTV battery in my stash and hooked that up and TADA! Buzz is gone, the cap is no longer sensitive. Amazing.

What does this mean? An issue with the power supply being the switched type? and somehow my (cheap AliExpress kit) bench power supply also is? If so, a proper adapter (and a new bench power supply!) would solve all my problems. Well, with the OmniChord anyway...

Cheers!

[cincin]

Well... I got an official QChord power supply, linear, same specs as the original OmniChord power supply, and the buzz is gone...

I've leaned something today. About power supplies.

Thanks gang!