Minimoog Osc. stability?

[cloudhop]

I'm considering another attempt to get my homemade Mini Oscillators working. The fault being the absence of the LS3954 which is unobtainable from anywhere. However I may have a solution .
One thing I would like to know is how are the tracking and stability of the early (pre 10175) models. Do they drift etc? I never owned an early Mini so cannot test. Whats the octave ranges like for accuracy?

Another question........the early 901b's sound superb but terrible drift, the cause being the 901a. Has no one tried a modified version of the 901a?
Its looks tantalizing simple to incorporate more stable IC's in there with a temp etc but I'm afraid actual circuit design is a bit beyond my talents.
But I would have thought not too difficult for anyone with experience in design?

[Just Me]

Tracking of the early ones was good, stability was non existant. (At least on mine back in the 70's) You HAD to tune between songs, or get an interesting fat drifty chorus effect. Keeping drafts off the mini helped and you could not just haul it into a stage setting from the car and expect it to stay in tune. My MemoryMoog was the same way, but it would auto tune in a few seconds. The wooden case in the ARP 2600 helped it stay stable and the ARP design was also much better than the Moog at the time.
Just my experiance.

[MC]

The pre-10175 oscillators can be stabilized if modified. I'm an experienced EE and have corrected not only oscillator design errors (thermal coupling of thermisters to 3046s, replace (almost) all 741s with TL071, etc), but also errors in the power distribution in the minimoog chassis (daisy chained grounding replaced with star grounding, tuning pots referenced off VCO card power rails). I took my modified minimoog to clubs and the only time I tuned it was at the beginning of the set. It was rock solid.

[Kevin Lightner]

The early boards with three tempcos and two 3046 tranny arrays (the most common VCO board) does drift.
But it can be minimized in many ways.
Experienced users usually come to understand one must allow warmup time, keep drafts away and not to expect absolute accuracy.
Five of the opamps on the board are ripe for replacement with lower drift types, there are lower drift resistors available, trimmer ranges can be limited, etc.

The range switching can be improved also.
The buffer board can have better opamps installed and the range switch contacts can be doubled-up for more reliability.
Very important is to make sure all summing resistors are closely matched too.
If they're not, scaling the vcos can be more difficult and other inputs (the pitchwheel, for example) will throw the tuning off when used.
It's come to an age where the original, closely-matched resistors may have drifted and so I stock complete sets of hand-matched precision resistors for my own restorations and for sale.

I have an IMF3954 here, but just one. (same part as 2N)
But there are other dual FETs that will work fine.
In the Odyssey and other Arps, they use 2N3958's.
I had a bad 3958 once and no replacement, but did have a bunch of 2N5524's (still do) and I found they worked better.
They are used in the keyboard circuit.
So you may find other dual FETs out there that might work fine, but just be slightly off in specs like pinch-off voltage or whatever.
Something minor that might not make much difference or even improve on the design in some way.

I should also mention that if you're building any of these older VCO designs, be especially careful in your choice of integrator capacitors.
While it's well known that polystyrenes are probably best, the most important spec to look for is a low dissipation factor.
There are some really nice caps, but lousy in this regard and if you use them, your reward will be lousy high frequency tracking.
If your DIY boards are like originals and you have long traces to these caps, you may also consider "flying" them off the board to remove the effects of board capacitance.

As to 901As and B's, I've seen (and also built) my own version of the 901A.
In my opinion, some of the drift does come from the A, but most of the criticism I've heard is towards the B's poor range switching.
Since it uses capacitor substitution, it's almost impossible... even with matched caps... to get a pair or triplet of 901Bs to range switch together well.
One can employ basic R/C design conventions and add resistors to trim the octave settings.
These would usually be very high values in the megaohms if you've already matched your caps as close as possible.
The larger problem is that 901As and Bs aren't especially linear.
They can track well over certain ranges, but their response isn't usually a very straight line.
More like a slightly bumpy line.
So one can take two 901s, scale them the normal way by adjusting them back and forth at 1,2 or 3 octave intervals, but then find individual notes within those octaves imperfect.
It can be very frustrating.
On a "normal" VCO, if your widest notes are in tune, so are all the notes in between. Not so with 901s.

Both A's and B's can be temperature sensitive. The effect is especially pronounced though because not all modules will share the same drift characteristics and not drift the same amount or in the same period.
So one ends up with oscs that drive them mad because they all seem to have a mind of their own.

One funny thing: take a look at the Paia 4720 VCO schematic and one for the Moog 901B.
Both use a single unijunction transistor as their core and in the same basic way.
It's amusing to see one of the most expensive oscillators sharing the same core design as one of the cheapest.

[cloudhop]

Thanks Kev for a very detailed and highly informative reply which I can assure you is very much appreciated.

I need to assimilate the information there but it has immediatly made me re-think the projects somewhat.

I'm not doing the Mini's Osc's board but have designed my own workings which is one board per osc.
I am about to test the J201 as a sub for the 3954, I will need two, very well matched.
Once (if) I get them working I will experiment with various caps and 741 replacements for stability then probably redesign a
better board maybe to hold 3 osc's. Its going to be a lot of trial and many errors (but good stuff) I'll keep the forum posted.

I have 6 x homemade 901b's in my home made 3C, all gorgeous in sound but lacking in discipline so I am (hopefully)
replacing them with 3 Mini's if the projects works. Ive got an Oakley VCO in there right now which is pretty close
to the 901b sound but not quite. And its that "not quite" that makes the difference. Apart from that I would like to retain all Moog
circuits in the system.

Change subject a little. I had in the system a 914 Fixed Filter clone using IC's etc but never really happy with it, it wasnt genuine Moog
so I decided to build the early 907. I just finished it this week and the sound is sweet and beautiful. I had some high value Toko inductors about
but had to wind my own whopping 5H's + others down to 800mH.
But it worked first time!!!!!! thats a first for any of my modules although the buffers are quite simple.
Worried a little about the tuning and the padders but luckily I have some photos of the components from a 907 and I am sticking to the values they have in there as far as I can make out. My other concern is my accuracy in the value of my wound inductors. But from the sound of the module
I must have done good enough!

I hope to post photos on the forum of the 907 and the Mini's..

[cloudhop]

also..many thanks for the other replies..again very helpful..much appreciated