Hey there
So I am buying a used Mini Moog
Are there any major problems I should look out for that can’t be repaired
Peter
First of all, welcome Peter.
Second, a little more info might have been useful from you. Like how much is the seller asking for it, what is his description of the state of the machine, are you buying it on eBay or in person, what are your skills at fixing electronic instruments, etc…?
If that Mini is in good shape it might only need to be retuned (relatively easy to do).
If it’s in bad shape, and you don’t have any skills in electronics, stay away. Or be prepared to spend lots of $$$ trying to find a good tech and have him fix it.
If it’s in bad shape, and you have some skills in electronics, you might have to deal with cleaning and re-adjusting the keyboard contacts; buss bars and springs. Replacing the keyboard bushings (noisy “clackety” keys); available new replacement kits online. But be prepared to spend many hours doing these things. (spent about 4 hours on mine doing all this)
Some 741 op-amps on the Osc board might be faulty; replacement readily available in electronic parts stores. There are, however, some electronic parts that are discontinued in there. But they seldom fail (mostly discrete transistors), but then again sometimes they still do. But, fortunately some NOS can still be found online sometimes. Also, some of the Allen-Bradley potentiometers might be very difficult to find, but again they seldom fail as they are of very high quality. I’m sure I haven’t thought of everything…
Anyway, my best advice is to spend a fair amount of time testing it (if you’re buying it in person), or take a gamble on buying it online without the possibility of testing it for yourself. Not all eBayers are honest and fair. But I guess you could always get your money back…
Anyway, good luck to you, Peter.
My two cents on it. TIFWIW
Note: all the info provided is based on my personal experience of buying a Minimoog D in relatively bad shape due to having been stored for a long time in a very humid environment, resulting in corroded keyboard contacts, scratchy pots and switches, intermittent circuit board contacts inside, as well as some cosmetic damage. But I got it for cheap, fixed it, and now it’s in perfect shape electronically at least… 
almost everything can be repaired.
afcourse a mini with all osc tuning and sounding well and with the keyboard functioning good and all keys on the same hight is less work .
also some woodparts disappeared need special craftsman to get it restored.
other then that , always calculate an service for cleaning, tuning/ calibrating and ( if not done ) changing the elco’s. a good technician with experiences with minimoogs can do that on a regular base. most is workhours, but it might set you back anything from 300-800 extra. when keyboard is not good and needs b.e. new rubbers, or when extra woodwork has to be delivered you’re on the higher side, when only cleaning/calibrating on the lower.
electronically almost all can still be replaced.
one thing however: the pitchwheelpot and/ of modwheelpot, might be a pain in the ass when leaking thru. to find the correct one is a bit difficult.
also look for the serial: the newer osc boards are more stabile but the older ones especially 2000-3000 types i prefer for the sound and touch.
overall: congratulations already.
The Allen Bradleys can be replaced if they sound a bit scratchy, as tends to happen with pots over time. Best replacement for these are Clarostat pots made by Honeywell and are sold on Mouser.com.
Yes, of course. But some of them are not readily available, like the one for the filter resonance (emphasis) which is a 50k reverse audio… Of course, there is always the less expensive but reliable Alpha brand. (that’s what Bob Moog chose for the Voyager BTW)
Alphas are good. I use those for my own projects. However, they don’t feel the same as the Clarostats, which have a sturdier feel.
50K reverse audio taper pots (Emphasis/resonance) were often used in vintage amps.
The few I’ve found came from amp restoration companies.
50K audio taper pots are hard to find, but they’re still made.
The big problem is finding ones that truly turn off when fully rotated.
They need to read 1 ohm or less when turned fully. A dead short.
Few I’ve tested do.
(for those unaware, a Mini’s mod wheel shorts out already full modulation instead of the more normal method of providing it.)
There is a mod I conceived for eliminating or lowering modwheel bleedthru on Minis.
It consists of soldering a small glass encapsulated reed switch across the two mod pot wires, then gluing a small, but strong magnet at the proper location on the wheel.
When correctly aligned, the magnet will actuate the reed switch and short out the modulation pot, thus lowering it to zero ohms or close to it.
(I once told Bob Moog about the mod and he said it was “genius.”
I replied it was actually “desperation.”
When shopping for a Mini, check all pots and switches for noise.
Make sure the keyboard is in good condition and plays every note fine.
You can often judge how much wear a synth has seen by looking at the power cord where it enters the chassis.
If the insulation is broken or cracked, it’s likely seen some moving around.
That and the bottom of the Mini can offer clues to how much wear it’s seen.
A scratched up Mini bottom panel with worn rubber feet generally signifies the instrument has been used a lot.
I also suggest that one do some investigation into the seller to see if they’ve misrepresented synth sales in the past or is generally honest.
Shipping can often cause problems too, so don’t skimp on having it packed well even if it’s in a flight case.
Good luck! A good Mini is a wonderful instrument.