Moog model D osc bleed.

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Moog model D osc bleed.

Post by analogmadness » Wed Dec 30, 2009 3:56 pm

Hello everybody,
im new to this forum and what a great forum it is 8)
Im having a little problem with my first mini moog i just got for a few weeks.
Its a 3640 snr and the sound is everything i expected.
One problem tough
when all modulationbuttons are off i still geth bleed from osc 3 on both osc 1 and 2.
As im starting my electronic studie's im whilling to give it a try to fix that problem.
So i downloaded the minimoogs service manuell.
And yes in the troubleshooting page's i found my "trouble"
i need to replace a 10 ohm resistor with a straight wire in my power supply board next to the +10volt adj trimpot.
to my opinion a 10 volt resistor would be coulerd
brown black black gold.
The manuell also stats that its for snr below 2000
The only 10 ohm resistor i can find near the +10v trimpot
is standing vertical right between the + and - pots next to a couple of resistors standing horizontally between the 2 pots.
My bet is that thats not the one i want 8)
The manuell also says that you schould look out that the wire dosent touch
the surface of the trim pot.
Cant fint the resistor that should be near the +10v trimpot.
Is this different with later snr boards i guess.
Or might i have a outer problem.
So before i heat my solderiron i would like to hear another opinion.
Thanx in advance.
Sorry if my spelling is incorrect
M [/img]

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Post by analogmadness » Thu Jan 07, 2010 2:10 pm

Hey guyz,
diggin deeper in the service manuelli found what seems to be another possibility for the modulation bleed.
It could be the power supply board connections and flags witch are found in in the print socets.
Anyone ever had this problem?
any help would help!
Thanx in advance

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Post by dlearyus » Sat Jan 09, 2010 2:21 pm

i would be very careful about power supply mods without the correct mistake could fry the entire unit.

Cheers ;)

DL a resistor band identification chart also ;)

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Post by analogmadness » Thu Jan 14, 2010 4:05 pm

Thanx DL @ some point i have to try and solve the problem.
And indeed the power supply is verry dangerus to service without the right schematics.
Thanx for the info
Btw was i wrong with my colours of the resistor?
Ill check the chats 8)
Anyone els with some ideas
everything is welcome thanx in advance.
Ill try to post a picture of the prints soon.

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Post by acorkos » Thu Jan 14, 2010 6:16 pm

dlearyus a resistor band identification chart also ;) ... ge.en.html

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Post by analogmadness » Thu Jan 21, 2010 7:58 am

A friend of mine got hiz hands on cheap busted minimoog
and he came to my place to compare it with mine.
Its terribly wrong keys go up in pitch, choke, etc
So i askt him if he agreed to open him and chek out the cirquitboards.
Maybe cleaning up the cirquitboards connetions would help for a start.
To my surprise there where RA moog cirquitboards inside for the filter,
powersupply.. but not for the oscs DAMNNN.
So i tryed the RA filterboard in my mini and it works perfect!
Osc board works perfect to.
Now i want to try the power supply in my mini to check if the unstoppeble modulation from osc 3 is stopped.
Then im shure (if it works) theres no problem on the connection soccets and the problem will probably be something on the power suppy board.
Now the RA filter board has a slightly different layout but the connections are at the same place.
Will the connections on the powersupply board also be the same?
As i dont want to fry my mini as refferd earlyer in this thread.
Any help or advice will be much appriciated.
Thanx in advance

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Post by BrianK » Sun Jan 31, 2010 4:09 am

CLEANING up old Minimoogs is common and not hard. You need to clean each circuit board in the back - those connectors and edge-card traces clean easily and it makes a huge difference in most old machines.

Underneath (take off the wood bottom) you can clean the contacts under each key. You can also separate and clean the Cinch-Jones plugs under the Mod and Pitch wheel section - these connect those wheels and switches, plus the keyboard connection.

Most times, this will solve your problems with triggering and pitch instability. I did this for 3 Mini's just last week.

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Post by Portamental » Sun Jan 31, 2010 8:49 am

BrianK wrote:CLEANING up old Minimoogs.

I did this for 3 Mini's just last week.
Only three??? Last time around, i did 4 in an afternoon. I was just starting a fifth one, that's when I woke up all sweaty, rubbing the bedposts with the sheets. I turned around. Old Shool's still there. World ain't that bad afterall.

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Post by analogmadness » Sun Feb 07, 2010 2:41 pm

Thank you verry much BrianK for the info!!!!!
I cleaned the bars under the keys but is there any way to clean the springs?
Allready im getting much better results!
Still ive got the problem with the osc bleed.
Any suggestions on that one.
I swiched power boards from one to another and still ive got the bleed.
Theres a mod thats gives me an jack strig input and the original s trig input is replaced with something like an xlr cable input but just a little smaller.
Maybe thats where the trouble's at.
Thanx guys

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Kevin Lightner
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Post by Kevin Lightner » Mon Feb 08, 2010 4:18 pm

(my first post of the month.) :-)

If you have VCO3 bleed-thru, even with the osc modulation button off, there are several possible causes.
But first, you'll need a good tech.
Someone that knows Minis well or at least has mad skillz at work like this.
Below are some tips or cures to you and whoever that tech might be.
I take no responsibility if someone causes damage.

And yes, you could also send it to me, but it'll be a very slow return and a high price.
What you get back would be guaranteed a good Mini and with a better resale value, but I'm guessing you'd like a more immediate solution.

You're correct about the 10 ohm resistor mentioned in the service manual.
But I think yours already has that change implemented.
If not, do the mod as outlined.

Brian is correct that a clean Mini is a happy Mini.
Dirty contacts, especially on grounds, can cause bleed-thru, offsets and other undesirables.
The edge connectors, both of the board and connectors should be clean.

One major consideration is simply that the electrolytic capacitors are old.
These are on almost every board and as they degrade, they lower their ability to filter the machine as a whole.
The unit should be completely recapped.
But if you can just do a couple, I recommend replacement of the large ones located on the rectifier board.
You can use 85C rated caps, but 105C rated caps will last longer usually.
50 volt ratings are suggested, even if the originals were 35 volts.

The solid tantalum axial caps on the VCO board are usually fine and do not require replacement.
You can replace them, but probably won't see a big improvement and tantalum caps are expensive.

Replace any non electrolytic caps that appear to be cracking at the edges.
Do not replace the VCO polystyrene caps unless you have the same or better quality caps available.
They don't have anything to do with bleed-thru anyway.

Some revs or production runs of the Mini possess a certain type of resistor notorious for drifting in value over time.
They're dark brown, cylindrical and exactly 90 degrees at the edges- no paint coatings, just their value and tolerance stripes.
I've seen them as much as 45% off intended value.
These too can cause such problems, especially in the power supply and modulation mix amplifier.

Sometimes mod bleed-thru can be due to the osc mod switch rubbing the silver plating from the contacts onto the phenolic base material underneath.
This causes a low resistance bridge between contacts.
Rare, but can happen.
Again, cleaning is the answer.

The ground wire from the transformer should be firmly soldered to the rectifier board.
No frayed wires.

Eliminate any IC sockets on the VCO board (unless a later, uA726 based rev.)
Solder the ICs directly into the board.

Make certain none of the IC pins are tarnished.
They may appear black or even fuzzy.
Clean or replace these parts.
Same for any components with leads that have turned black, white, ashy or fuzzy.

Tighten down (or check) the edge card socket tightness on the bottom.
Make sure none of the edge card socket pins (flags) have been overly widened or stretched.
Also make certain there are no extremely deep gouges in the fingers of the PCB edge card fingers.
This for all boards.
It is better to try and re-seat such boards on unscathed gold-plated finger areas by moving the connectors and/or filing of the board itself to allow such placement, than it is to add solder.
(Solder is a poorer conductor than gold and once soldered, it's very difficult to remove.)

If the edge card flags are very corroded, you can use 600 or greater grit paper lightly.
I recommend new 1500 grit over new 600 grit too.
If already worn, 600 is ok though.
Do not over sand these pins.
Just two or three swipes at most.
Do not sand the gold-plated fingers of the board under any circumstances.
If absolutely necessary, polish LIGHTLY using aluminum oxide chrome polish or finer grit.
Do not remove the gold plating.

Inspect and re-solder as needed, any questionable connections on all boards, especially the VCO, PSU and VCF board.

Make sure the mod section connectors are in good condition.
They should not be corroded (white or ashy looking), bent or have enlarged or corroded female connections.
The wiring should not be frayed or have broken conductors.

It is doubtful that the S-trig jack is causing any of this, but it does indicate someone has been inside and may have caused problems.
Swapping boards might help locate problems, but the boards you used may have just the same problems as yours, even if the Mini they came from doesn't.

There is also this trick or cheat....

Solder a heavy gauge wire directly from the rectifier board ground (center term/white wires) and run it to a central ground point of the VCO board (solder it or use a good connector.)
This may lower the bleed-thru considerably.

There are several other tricks possible if your mod wheel itself is causing the bleed-thru, but you mentioned the mod button (switch) is off.

Hope this helps.

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Post by analogmadness » Thu Feb 18, 2010 3:17 pm

Thank you verry much Kevin!!!!
Not that i understand everything in there but the tech will!!!!!
Hope will find the problem now
Many many thanx

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