Technical question about Minimoog D

[thealien666]

This question is addressed to Minimoog D connaisseurs out there.

My 1975 Minimoog exhibits some odd behavior of the envelopes: the maximum attack and decay times are WAY off in regards to whats written around the knobs.
For example: maximum attack time for filter envelope is about 10 seconds, which is a little shorter than it should, but the maximum attack time of the VCA envelope is more like 16 seconds which seems normal. More importantly, the decay times of both envelopes is around 32 seconds which is way more than expected!

But more annoying is that, when all these knobs are positioned on an indicated number, like 1 second for example, the actual resulting times are way off making those markings unreliable as an indication of the settings.

Are my Minimoog's envelopes defective, due to aging components, or is this behavior typical of all Minis ?
There aren't any trimpots to adjust or offset these times on board number 2. I also realize that due to component tolerances there will always be some deviation from the norm, but this seems excessive.

Thanks for any input.

[cumulus]

Mine are not off, at least not by much.

Have you had it serviced recently? My guess is no.

It should be easy for a tech to straighten out.

[Kevin Lightner]

It does sounds like yours is out of spec.
Unless a previous tech replaced some of those pots, I would agree that aging and even dirt can affect these circuits.
However it IS normal to encounter Minis with considerably longer times available than what the panel markings dictate.
30 seconds of decay or release times are not uncommon for Minis or Moog modulars.

If per chance you open your Mini to inspect these pots, please note that if they are original pots, the part number should end with an "AA" marking.
If however you find them with a UA or BA suffix, then I would suspect someone replaced them at some point with the incorrect type pot. All attack and decay pots on a Mini should be 1 megaohm audio taper types.

[thealien666]

It does sounds like yours is out of spec.
Unless a previous tech replaced some of those pots, I would agree that aging and even dirt can affect these circuits.
However it IS normal to encounter Minis with considerably longer times available than what the panel markings dictate.
30 seconds of decay or release times are not uncommon for Minis or Moog modulars.

If per chance you open your Mini to inspect these pots, please note that if they are original pots, the part number should end with an "AA" marking.
If however you find them with a UA or BA suffix, then I would suspect someone replaced them at some point with the incorrect type pot. All attack and decay pots on a Mini should be 1 megaohm audio taper types.

Ahhh, thanks very much for that info, Kevin. From the photos I took of the inside when I first got my Mini, they are indeed 1 meg with AA suffix.
Could the electrolytic caps (C2 and C5 on board number 2) be causing such a difference in times maybe due to their higher ESR over 37 years ? (I don't have an ESR meter yet)

Anyway, it wouldn't hurt to replace those and maybe it might correct the problem... In fact, I should recap the whole instrument (it's been done only in the PSU so far). It isn't such a big job, but only to find axial type casings to fit in place of the originals.

Thanks again guys.

[Kevin Lightner]

Considering there are only two small caps on the env board costing less than a dollar total, I'd agree it wouldn't hurt to replace them.
I do to every Moog I work on.

But also take a look and see if the transistors on that board are different numbers than in the normally available schematics.
Several Minis used substitute transistors, especially on the env board.
While this lends to so many Minis having varying characters, if this particular problem is annoying take a moment to verify it wasn't a unit made with substitutes.

Also, while I've mentioned it before, the top horizontal row of components are the filter env, the lower row are the VCA's env.
In the top row there should be a resistor of 4.7Kohms labeled R13.
This resistor, at least in my opinion, was a bad value choice.
A better choice would have been around 3.6K to 3.8K.

The reason I feel this particular resistor was a mistake is that the attack env can often overshoot even the highest sustain knob setting.
One can argue whether this is right, wrong, part of the "Moog sound" or whatever, but it's contrary to how envelopes are generally accepted to work.
Normally an attack portion should meet the sustain level set exactly. Not under or overshoot.

To hear this effect, simply crank your filter emphasis so the filter self-oscillates, turn up the filter env's sustain fully and dial in the attack and contour depths for your basic siren sound. (one that doesn't go out of hearing range.)
If like many Minis you will hear a rise in pitch, then a sudden drop to the already maxed sustain setting.
To me, I find this annoying.
Other owners have also remarked they're bothered by it.

You can replace R13 with a 10K trimmer and adjust the attack to perfectly meet the maximum sustain level available.
I often do this mod to Minis, but never do it to the VCA's env as I consider this overshoot or "bump" to be part of the classic Mini sound.
I feel this is one of the secrets to the Mini's "punch" and anyone that understands synthesis will easily understand why.

The Moog 911 modular envelopes (at least later ones) had a trimmer to adjust this attack overshoot, but the Minis don't.
A trimmer that allows either top or side adjustment is your best bet if you decide to replace R13 with a trimmer.

Best of luck.

[thealien666]

...You can replace R13 with a 10K trimmer and adjust the attack to perfectly meet the maximum sustain level available.
I often do this mod to Minis, but never do it to the VCA's env as I consider this overshoot or "bump" to be part of the classic Mini sound.
I feel this is one of the secrets to the Mini's "punch" and anyone that understands synthesis will easily understand why...

Of course.
After checking that, it sure overshoots quite a bit on the filter envelope. But curiously not on the VCA envelope ? (at least not on my particular Mini as I've watched the waveform amplitude on an oscilloscope).
While I have that board on the bench, I'll probably replace that resistor with a trimpot, and check those pesky possible substitute transistors.

As always, your valuable personal insight and vast experience in fixing and restoring classic synths shows off.

I'm not expecting absolute precision on those envelope times, after all this is an analog instrument and not digital precision, but at least I'd like to get close to 1 second when I put a knob to that position, not 4 seconds as it is right now on the decay !

Again, thanks so much for all that precious info Kevin. (The least I could do was to mention your OptoKey optical keyboard contact product in another thread, in return.)