Diagnosing MG-1 hum

Vincent Brick · Joined: May 29, 2014 Posts: 5 Location: SoCal

I just pulled my MG-1 out of about a 2 year hibernation, I have owned it for about 18 years now. It has always sorta had a hum to it, its seemed to get worse with time, but now its gotten bad to the point of unusable. It sounds similar to a normal 60hz hum, constant when powered on regardless of any slider positions, even when all tone sources are turned down. How do I go about diagnosing and repairing something like this? When asking on another forum it was suggested I needed to replace the electrolytic caps in the unit, that they had perished from age, but then another person said that was a horrible place to start yet offered no alternative. Someone else suggested I come here, so here I am...

Right before I put it away I had installed a new cutoff slider and converted it to 1/4" input/output (I did the filter input mod ~15 years ago), I'm sort of a novice, how likely is it I screwed something up? I can solder alright and have a decent adjustable temp iron, but maybe I overlooked something? Is there anything in these modifications that could cause or exacerbate this hum if performed incorrectly? The synth and mixer are plugged into the same power strip so there should not be a ground loop issue.

Thanks so much for any info you guys can provide, this doesn't seem to be the most common of problem on these synths. Sorry to be long winded, just trying to be thorough.

varice · Posted: Sat May 31, 2014 12:22 am Post subject:

to the electro-music forum Vincent Brick Exclamation

In my experience, the first thing I would suspect is that a filter electrolytic cap in the power supply has failed. If you have a multi-meter or o’scope, check the DC power supply voltages for excessive AC ripple. If you don’t have a meter or scope, you might be able to see if the power supply filter caps are leaking and/or swelled.

Good luck with the troubleshooting Exclamation

DES · Posted: Sat May 31, 2014 8:47 am Post subject:

Go thru and check that all connectors are seated well..might want to unplug-plug them in several times to make sure the connections are making clean contact. Also, loosen then tighten all mounting screws..some might actually be providing a ground path...especially ones holding circuit boards down. WRT the 1/4" jack mod...was the original jack electrically isolated from the chassis and if so, is the new one? I've seen where some people will remove a plastic jack and replace it with a metal jack which is tying the audio ground to chassis ground which might cause hum issues.

As mentioned before, power supply caps can be suspect as well as power filter caps on the other boards...look to see if any look slightly swollen or are leaking.

As always, follow electrical safety precautions when doing these inspections...

Vincent Brick · Joined: May 29, 2014 Posts: 5 Location: SoCal

I'll try the connectors, inspect the caps and look at the old I/O board and make sure it was set to ground out through its own mounting. I guess I thought all equipment grounds out the negative on the signal cable.

Vincent Brick · Joined: May 29, 2014 Posts: 5 Location: SoCal

Update:

I see the caps near where the main power goes in and they all look fine, along with every cap in the unit that I can see. No expansion or leaks, the whole thing looks super clean inside. While in there I reseated all the connectors.

Upon inspecting the original I/O board I had previously removed, the board itself had no direct grounding point, but with the mounting and placement of the jacks within the chassis I can't see how there would be an expectation of ground isolation. To confirm, I isolated the 1/4" I/O I put together from ground and undid my VCF mod entirely, no change.

None of this had any effect, its still damn noisy.

I did previously perform a mod that boosts the output, it simply involved removing a resistor, I'm going to reinstall the resistor and see if that makes it any less noisy. If anyone can suggest anything else that would be great, otherwise I guess I'm just going to try to use it as-is, maybe look into some sort of noise gate to use with it.

Thanks guys.

Uncle Krunkus · Posted: Tue Jun 03, 2014 1:58 am Post subject:

A noise gate should be the last possible resort.
Remember that an electrolytic cap which LOOKS fine, might still be dry.

varice · Posted: Tue Jun 03, 2014 3:14 am Post subject:

Vincent Brick · Joined: May 29, 2014 Posts: 5 Location: SoCal

I think it is in the master volume control, the hum is worse at 3:00 than it is at 5:00 max. Does that indicate anything?

Unfortunately having someone else work on it is not an option right now, even if I could afford it I'm not sure where I could find anyone capable.

varice · Posted: Wed Jun 04, 2014 6:44 am Post subject:

Vincent Brick · Joined: May 29, 2014 Posts: 5 Location: SoCal

Sweet! Thank you so much, it means a lot, I'm really trying to learn about this stuff.

BrassTeacher · Joined: Aug 24, 2010 Posts: 27 Location: Kennesaw, GA

Not sure if the issue was resolved, 'cause the thread is a few months old, but in case anyone else has a similar issue:

There is a green/white wire that is run from the bottom board to the top board, from the + pin on C8 on the bottom board, to the ground pin of the Main Volume pot on the top board. If it is broken, fix it, if not there, add it!

See the pics below!

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