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 Forum index » DIY Hardware and Software » The Repair Shop
Diagnosing MG-1 hum
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Vincent Brick



Joined: May 29, 2014
Posts: 5
Location: SoCal

PostPosted: Fri May 30, 2014 11:01 pm    Post subject: Diagnosing MG-1 hum
Subject description: not sure where to start
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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.
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varice



Joined: Dec 29, 2004
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PostPosted: Sat May 31, 2014 12:22 am    Post subject: Reply with quote  Mark this post and the followings unread

welcome 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

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DES



Joined: Feb 28, 2003
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PostPosted: Sat May 31, 2014 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

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...

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Vincent Brick



Joined: May 29, 2014
Posts: 5
Location: SoCal

PostPosted: Sun Jun 01, 2014 12:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.


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


lol I have no clue what I am doing. Hoping to start electronics classes ASAP.
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Vincent Brick



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PostPosted: Mon Jun 02, 2014 9:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.
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Uncle Krunkus
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PostPosted: Tue Jun 03, 2014 1:58 am    Post subject: Reply with quote  Mark this post and the followings unread

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

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varice



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PostPosted: Tue Jun 03, 2014 3:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Uncle Krunkus wrote:
...Remember that an electrolytic cap which LOOKS fine, might still be dry.

I totally agree.

At this point I would recommend that this synth should be looked over by a qualified electronic tech. A good tech should be able to find the fault and correct it.

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Vincent Brick



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PostPosted: Wed Jun 04, 2014 12:23 am    Post subject: Reply with quote  Mark this post and the followings unread

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.
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varice



Joined: Dec 29, 2004
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Location: Northeastern shore of Toledo Bend
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PostPosted: Wed Jun 04, 2014 6:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Vincent Brick wrote:
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?...

Yes, I would suspect that the wire connected to the wiper of the Master Volume pot is picking up interference, possibly from the power transformer.

According to the MG-1 schematics, the wiper wire is connected between the Control PCB (behind the front panel) and the Keyboard PCB (at the bottom of the chassis) through a shielded cable with the shield apparently connected only at the Control PCB. Find that cable and make sure that the shield is connected properly. Also check the position of the cable and rout it as far away as possible from the power transformer.

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Vincent Brick



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PostPosted: Thu Jun 05, 2014 5:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

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



Joined: Aug 24, 2010
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PostPosted: Sun Sep 28, 2014 5:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

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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