Multimoog double oscillation/harmonic problem

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richardm
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Multimoog double oscillation/harmonic problem

Post by richardm » Wed Feb 10, 2010 7:13 pm

Switched on my beloved multi today and thought it was simply out of tune. However it seems that when I select either osc a or b separately, there's some sort of harmonic being generated. E.g. at F the oscilator sounds OK, but f# or b and beats begin and get more horrible the further from F I get. SO it seems like ui's producing a fixed harmonic around F somehow. As I say this happens for both oscilators, all waveshapes, with everything else flat..anyone any ideas?

At higher pitches this just sounds like a distortion. MAkes it impossible to play, even when the rest of the tuning (pitch, scale etc) is correct.

Thanks -- Richard.

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Piktor
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Post by Piktor » Thu Feb 11, 2010 2:28 am

A fellow Multimoogist!

I think that I know what you mean, but I don't have a sure fire solution. Is the sound raspy, sort of like the oscillators are being modulated by another oscillator that is running in the audio range? I've got two Multimoogs and both had developed that problem. My tech friend and I went through one Multi, removed the bus bar and thoroughly cleaned all of the key contact points (including the J wires), using a very clean pencil eraser and alcohol. That did the trick. That Multimoog works great now.

I had a go at the second Multimoog, following the same procedure, but the distortion returned within a couple of days, so that seems to be a mystery at the moment. Apparently the old capacitors do have a tendency to fail over time, so, I'm going to have my tech friend have a look at that next, when I can find some time in his schedule.

richardm
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that's the sound

Post by richardm » Thu Feb 11, 2010 1:54 pm

that sounds like exactly the problem. I'm sure the bus bar does need a clean, but since this sound happens in 'drone' mode I assumed not related to dirty contacts. I'll clean it up though and see.

I'm finding it impossible to find anyone in Seattle to look at this thing, so if you find any likely culprits among the capacitors pls let me know, and I'll take a soldering iron to it myself.

Thanks -- Richard.

richardm
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Doesn;t happen when absolutely cold!

Post by richardm » Thu Feb 11, 2010 4:21 pm

New information! When I fired it up I notice that for a few minutes it sounds fine -- this happens after a minute or two as it warms up. Must be a clue in there for what's wrong. Atfirst, I though a quick clean of the busbar mght indeed have imprved it, but if I hold a note when it's cold, it gradually get more and more distorted til I hear this clear double oscillation thing. Almost like a sax player growling...

Hmph.

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Piktor
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Post by Piktor » Thu Feb 11, 2010 7:50 pm

Yes, exactly the same symptoms here with the second Multimoog Richard. As the machine warms up, the distortion fades in. I'm not too knowledgeable about these things, but my friend figured that this symptom indicated a capacitor problem.

I would be happy to let you know if I we discover the root of the problem.

richardm
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Post by richardm » Thu Feb 11, 2010 8:57 pm

Great. I have a mate who knows general electronics, and I have the ciurcuit diagram, so I'll ask him how to check the capacitors and post any joy here. I bet it's a simple fix once you know how.

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MC
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Post by MC » Thu Feb 11, 2010 11:35 pm

Jitter is a common ailment on oscillator A which is what you're probably hearing. The cause seems to be either the connectors to the extension board or the power supply. I helped a tech with that years ago but I don't remember the specific cause.
Gear list: '04 Saturn Ion, John Deere X300 tractor, ganged set of seven reel mowers for 3 acres of lawn, herd of sheep for backup lawn mowers, two tiger cats for mouse population control Oh you meant MUSIC gear Oops I hit the 255 character limi

richardm
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Post by richardm » Fri Feb 12, 2010 12:00 am

This is the same though on A and B, does that suggest something further down the line?

richardm
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fixed

Post by richardm » Sat Feb 20, 2010 10:37 pm

Bought volatage/capacitance meter. Downloaded service manual. Tested all voltages -- surprisingly, correct. Removed, cleaned, lubricated and reconnected all connectors (there are at least 14). Blew out all the accumulated dust.

Seems to have fixed it!

So thankfully no need to search for a dead capacitor this time round - -it seems lke it was an intermittent connection (which I understand can cause its own capacitance problems). Got lucky -- hope this works for yours -- let me know!

MC -- I think you were right -- many thanks!

electrojoe
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Post by electrojoe » Sun Feb 21, 2010 11:30 am

MC is correct. I have 2 of these as well and the power connector has always been the culprit for this issue. First you think its out of tune, and then the odd sound comes in. Clean or replace the power connectors to resolves this.

Just my 2 cents.

GD

richardm
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Post by richardm » Sun Feb 21, 2010 2:38 pm

BTW what is it with all these people with TWO multis? I thought ONE was pretty cool. Do I need to find another one?

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Piktor
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Post by Piktor » Sun Feb 21, 2010 6:54 pm

Thank you for the repairs information. That is much appreciated!

Two Multis? I bought a new one when they first came out (about $2000 at the time). When MIDI and the DX7 came along, I picked up a second one from someone who didn't think that it was as valuable as I did. Paid $200 for the second one in 1985. I was anxious to work with all the interconnections between the machines, so I got some Cinch-Jones plugs and some 3/16" jacks and went to town. It still wasn't a Minimoog, but the four-oscillator hybrid is a quirky, but cool instrument.

Along the way, I have discovered a couple of pretty cool (I think) tricks that you can do even with a single Multimoog. As soon as I get through the next really busy bit in my job, I'll start a new thread to share those ideas with anyone who is interested.

P

sunny pedaal
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Post by sunny pedaal » Mon Feb 22, 2010 8:49 am

Piktor wrote:, I'll start a new thread to share those ideas with anyone who is interested.

P
count me in ,
sunny ( owner of 1 multimoog )

richardm
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Post by richardm » Mon Feb 22, 2010 1:19 pm

Hmm -- pretty soon we'll have accounted for all the multis still in existence in this thread.

Proffessor Undressor
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Post by Proffessor Undressor » Thu Mar 04, 2010 8:21 pm

Well, you can check one more off the list. I just bought one at a local flea market for $200. It came in its original flight case and is in perfect working and aesthetic condition. Couldn't believe my luck!

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