(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.
