Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

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tempustry
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Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by tempustry » Sat Feb 26, 2011 12:00 am

Had a very intensive time (multiple hours) tuning this mini I am restoring for a friend. The on board scale & octave trims are insanely touchy, using a frequency counter, even turning minutely, it was near impossible to lock on to the right HZ values. The scaling is close but with a 3-4 octave spread between 2 oscillators, the beating is obvious. were they perfect 20 years ago?

Are these trims the culprit, can they be cleaned, or replaced with a new equivalent part? Below is a link (if I linked it right) to a few photos of the project. The oscillator board closeup shows the metal trim pots I'm talking about. 100 Ohm 28-206 and 1K Ohm 28-208.

http://www.facebook.com/album.php?aid=3 ... 1220656397

thanks!

-Allen
http://www.facebook.com/allenbrunelle
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Kevin Lightner
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Kevin Lightner » Sat Feb 26, 2011 1:43 am

There are many solutions....

You can buy replacements (I sell them at http://www.minimoogs.com.)

You can put in trims for the range that are 800 ohm instead of 1K and obtain less range for the same travel.

You can drill holes or add sub-boards to accommodate 25 turn cermet multi-turns.
But be forewarned that you must mount them to align with rear cover trim holes.

You can change or add resistors to allow less range for both trims types.
You can even go so far as putting in selected resistors and pots with much less range, thus allowing for only a few semitones of tuning span.

Generally I'll first wash a vco board and flush out the trims, drying them off using compressed air.
If they're still iffy after that, I'll usually replace them.
I do have artwork for PCB sub-boards that hold multiturns, but out of finished boards at the moment.
That solution is the most difficult, but arguably the most stable both in terms of vibration and temperature sensitivity.
It's also not stock, which some people take objection to and I can understand such concerns.

I also hugely recommend replacing all of the VCO summer resistors with new precision matched triplet sets.
These are all the resistors along the right side of the board (and one at the bottom.)
They are what's responsible for feeding in all CVs to the VCOs and must be matched perfectly as sets.
Over time they can drift in value and make it much more difficult to calibrate everything perfectly.
I sell those too, but you're welcome to make your own sets or check what the values are of the ones already installed.
You must have an accurate DMM for this if you do it yourself.
4.5 digits minimum.

Finally, remember it's a voltage controlled synth and all voltages and tunings are dependent on the power supply being stable.
Since the power supply uses the same type of wirewound trims as the VCOs and are likely the same age, those trims should be also be addressed.
If they wander, so does the whole instrument.
Again, it's possible to range-limit those trims so they span very little change from full CW to full CCW.
They're considerably easier to swap with good multiturns than the VCOs too.
Cermet multiturns tend to have less temperature drift than wirewounds too and being so close to the heatsinks, it's important they don't drift from temperature.
And suffice to say, tuning an instrument with trims that provide 10-25 turns is a whole lot easier than adjusting trims that provide less than one single turn of travel.

Hope something above helps.
Good luck!
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

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tempustry
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by tempustry » Sun Feb 27, 2011 9:39 pm

Thanks for the verbosity Kevin!! many thanks.

-Allen

http://www.facebook.com/allenbrunelle
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http://www.facebook.com/allenbrunelle
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Technician Larry
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Technician Larry » Wed Mar 02, 2011 11:50 pm

Those wire wound trim pots have always cleaned up nicely for me. I give them a few rotations back and forth and maybe a drop of Deoxit before attempting to adjust the scale. The 10V supply trimmers need the same treatment. 10 volts can be within tolerance but still have enough noise modulation caused by contact oxidation to cause pitch stability issues on the VCOs - especially on high frequencies. I see that Kevin advocates replacing with different trimmers, though I have never had enough trouble to consider doing that.

What i almost always have trouble with (on the original VCO board) is the absence of a separate range trimmer for each VCO. This is especially true if the unit does not have the octave buffer mod installed. Set the range switches to 8' and calibrate the VCOs from keyboard control only. Then compare to the response from the range switches. Without the buffer mod, the range switch settings cause a slight loading of the divider that sets the octave voltages and results in slight interaction between VCO octave settings. With the buffer mod, the accuracy of the octave switch setting is determined by the tolerance of R 43, R52 and R179 (15K 1%). It should be noted that the keyboard CV is routed through 1% resistors also, (R27, R80 and R129) meaning that the allowable tolerance of the CV control is plus or minus 2%. This is just not good enough. A surprising oversight that was corrected on the upgraded VCO board.

A Mini with the original VCOs could be improved by hand matching those 15K and 51.1K 1% resistors. Buffer the octave range divider and add the pitch bend dead band (return to zero) mod to avoid additional pitch problems.

LaxSlash1993
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by LaxSlash1993 » Thu Mar 03, 2011 4:01 pm

Technician Larry wrote:I give them a few rotations back and forth and maybe a drop of Deoxit before attempting to adjust the scale.
Isn't deoxit bad for synths?

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Kevin Lightner
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Kevin Lightner » Thu Mar 03, 2011 7:01 pm

Isn't deoxit bad for synths?
Salesman and tech shops will say no.
I personally say yes.

The stuff works great at first and everyone thinks all is well.
Then it corrodes metal, attracts dust and causes more problems than it had before.
I'm still looking for one maker of pots or switches that add this stuff to their products at the time of manufacturer.
One would think if it was so good, many would be using it.
But I can't find one.

Moog's early service manuals sometimes advocated it.
Later manuals discouraged against it.
They too learned what the long-term problems are.

Your mileage is up to you, but I don't use and haven't for decades and I'm very glad I don't.
It's snake oil designed to take your money and fix things for a short time.
(fwiw, I've been working on synths for 36 years... I think that's long enough to know.)

K
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

Technician Larry
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Technician Larry » Fri Mar 04, 2011 3:30 pm

From an email to Rosser Douglass - Moog tech support manager:

"Hi Rosser: I'm inquiring about the accuracy of some information posted on line regarding servicing and the use of Deoxit. There is a claim that early Moog service manuals advocated the use of - then later manuals discouraged it. I don't recall it ever being mentioned in the early manuals, and I have never seen any of the current or post Big Briar service info. The claim suggests it collects dust, which I would not argue against since it is sometimes errantly used as a wash, but also that it will cause corrosion. I've seen lots of corrosion in old gear, but it never occurred to me that any of it may have been promoted by the presence of residual Caig product. Soft drink spills (I always recommend drinking beer instead) , salt air from living at the beach, excess humidity from long term storage in unheated environments, rodent excretion, CAT PEE for gosh sakes, possibly air pollution, but deoxit? I can't say I have ever observed evidence of this. Have I missed something?

My 40 year old Heath kit integrated amp got twitchy and glitchy about 20 years ago and I swabbed it out with the old formula then called Cramolin - been working fine ever since. I chopped a Mini into a rack mount module for Jeff Alviani (Strong Wrong) probably 30 years ago - still have the remains of the keyboard in my graveyard pile for parts - just checked - tarnish but no corrosion. I know I serviced those key contacts with deoxit back when this Mini lived in smoke filled bars. The air in those smoke filled bars caused serious tarnish on gold resulting in stubborn key contact issues for which I found deoxit to be the only reliable solution. Should I be scolded?"

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Kevin Lightner
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Kevin Lightner » Sat Mar 05, 2011 1:34 am

"Hi Rosser: I'm inquiring about the accuracy of some information posted on line regarding servicing and the use of Deoxit. There is a claim that early Moog service manuals advocated the use of - then later manuals discouraged it. I don't recall it ever being mentioned in the early manuals, and I have never seen any of the current or post Big Briar service info.
Technician Larry... I made the claim.
Why not simply ask me for scans of these Moog service manuals?
I have them.

Btw, I would like to point out that Caig did not make DeOxit in the old days.
Then their product was known as Cramolin.
The same claims were made for both products by Caig (Cramolin and DeOxit)
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

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Kevin Lightner
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Kevin Lightner » Sat Mar 05, 2011 2:08 am

From the 1973 Moog Sonic Six service manual:

Image



From the 1976 Moog Micromoog service manual:

Image


There.
Now you won't have to bug busy Moog employees who may not know the differences between 35 and 38 year old service manuals. :wink:
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

Technician Larry
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Re: Mini-Moog Osc Board Scale/Octave Trimpot cleaning?

Post by Technician Larry » Mon Mar 07, 2011 8:50 pm

It sounds like an attorney wrote that statement from the Micromoog manual. It doesn't really say anything. Kind of like saying "if you drink coffee, you will die". Oh? Doesn't everybody die anyway? If you use Cramolin you will have to clean your contacts again. Really? Doesn't everybody that plays an old Moog seriously have to clean their contacts again anyway? If you use Cramolin, dust will collect. Really? Doesn't dust collect everywhere anyway? Extra dust may collect, I suppose, if you let the Cramolin puddle up somewhere. (Caig's motto: "The less you use the better it works") Moog obviously learned that you should not "spray a light coating" as in the Satellite manual recommendation. If dust was the problem it would be easy to deal with. Dust is not the problem. Tarnish is the problem and it is not so easy to deal with. Tarnish reduces the conductivity of the electrical connection made when the contact is closed, as will dust. Dust will blow away, or wipe away with little or no effort. Tarnish will not. I don't know if Cramolin will prevent tarnish but I do know it sure helps to remove it, and does so without abrasion to the gold plating. (Though I was seriously disappointed with the new formula sold at Radio Shack)

Kevin, the dominant question I intended in that email was not so much "what do the old manuals say?" as it was "what is your current policy?" or "is there evidence Deoxit causes corrosion?" Here is the answer.

Hi Larry-
I cannot say that I possess a ton of information on this subject. I know
that we are using Deoxit on our products now (and have been for years),
without seeing any obvious damage caused by it. I also use Isopropyl
alcohol on our boards to clean them after repair. I will follow up with
you on Monday after speaking with one of our engineers about this. Have
a great weekend.
All the Best,
Rosser

Tech Support Manager

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