Minimoog Model D Pitch / Tuning Problems

[therealrx]

Finally for your filter cutoff sounding unpleasant when reaching 0, again I don't know exactly what you mean. Try this: open the filter cutoff to maximum, Emphasis to 0, Amount of Contour 0, all orange switches to Off, Sustain level to Max in the Loudness Contour, mixer volumes all to 5, waveform to sawtooth on all oscillators, and opening only one oscillator at a time in the mixer (blue switches), do you hear any unpleasant sound (or heavily distorted sound) from each oscillators then ?

Ok, Alain. So I tried the settings above - can't say that the sound bothers me in anyway with a sawtooth wave, but with the square wave, the problem remains. Where it used to distort in a more pleasant sort of way, now it has a discordant overtone to it.

Was looking at the circuit boards and notice that a few of the (I am guessing) transistors (items in the Q position) on the Power Supply board and a few of the same (as well as a small cap) on the Filter Board have a bunch of white gunk on the tops. Should they be replaced?

On the Filter Board, the relevant positions are Q8, Q9, Q10, Q11, Q13, Q14, and C10.
On the Power Supply, it's Q13, Q14, Q15 and Q16.

Any idea what the components are for these particular positions? Or any suggestions on where to find out / procure?

Again, thanks for your help.

[thealien666]

That "white gunk" is actually thermal compound applied to them at the Moog factory, in order to try and keep their operating temperature matched, as much as possible. Don't remove it. It's not something that has leaked out, don't worry.

And if your problem is only a distortion audible on the square waveform(s), you need to know that the amplitude of those waveforms is slightly higher than the others, and are more likely to saturate the filter input.
Are you plugged into the "high" or "low" output going to your mixer, or whatever you use to listen to your Mini ?

It might be a case of mismatched impedance maybe ?

Also, do you hear that same discordant overtone on the "phones" output, using headphones ? (you might already have answered that in a previous post, but I have a very short attention span... )

Alain.

[therealrx]

Leave the white gunk alone. Got it. Thanks, Alain.

Regarding the distortion, here is a sample running from the High output of the Moog to the Hi impedance input of the Apollo Twin.

https://soundcloud.com/therealrx2/disto ... quare-wave

The problem is the same no matter how the signal flows from the Moog - High or Low outputs 1/4 inch to DI to Soundcraft board or Mic or Line inputs on Apollo, or skip the DI and go straight to High impedance input of Apollo.

The tone can be heard pretty clearly at the onset of the sweeps in the sample above. However, that's not the case with the Headphone output of the Moog - there is no real noticeable overtone until the latter part of the sweep through the headphones. In fact, the headphones might sound exactly right (other than Oscillator 3 still sounding more like a triangle air sawtooth wave). Will have to listen again in the morning with fresh ears.

Also, I measured the voltage as per the power supply calibration instructions... got 9.96 for both measurements.

[therealrx]

Actually, disregard previous comment about the Headphone output sounding normal. It too has the same tone as the sample...

https://soundcloud.com/therealrx2/disto ... quare-wave

[thealien666]

I've compared your samples with my own Minimoog. And they sound pretty much the same.

But I think I know where the issue lies.

The Minimoog square waveform is NOT a pure square waveform. It is a pulse waveform with very close to 50-50% duty cycle ratio, but not quite. It can be easily observed on an oscilloscope. This is due to components tolerances. Also, you'll surely have noticed that the square waveform doesn't sound exactly the same between oscillators 1, 2 and 3. Again, a component tolerance issue. But for oscillator 3 it's a bit more, the design of the oscillator is somewhat slightly different than the other two, adding to the discrepancy.

So the less than perfect 50-50 duty cycle ratio (actually 48-52 in mine measured on OSC 1) creates a some additional harmonics to the square waveform, hence the impression of an overtone.
The ideal square wave contains only components of odd-integer harmonic frequencies. As soon as the ratio isn't 50-50, harmonics other than exclusively odd-integer appear.

Anyone with a Moog Voyager can experiment with that by slowly moving the waveform knob passed the point of perfect square wave, moving into the beginning of pulse width territory, and hear the results. It will sound like your samples, therealrx.

The fact that the "problem" seems to disappear when the filter cutoff is brought down to the zero mark, and below, is a confirmation of the presence of those additional "unwanted" harmonics on the square waveform, stemming from a less-than-perfect 50-50 duty cycle, filtered out by the LPF.

Alain.

[therealrx]

Here are some new sound samples from today's efforts...

https://soundcloud.com/therealrx2/osc-1 ... ted-square
https://soundcloud.com/therealrx2/osc-2 ... ted-square
https://soundcloud.com/therealrx2/osc-3 ... ted-square

I calibrated the power supple for (+) and (-) 10 volts - was previously 9.96 for both - although after replacing the Filter Board, when I checked again, the readout was no longer 10.00 but in fact, 9.98. Should I crank it up a bit so that when all put together it checks out at 10.00?

Anyway, that's how it sounds now. You are correct, Alain - Osc. 1 and 2 sound very similar and 3 is quite a bit different. Osc 2 sounds closest to how the Moog sounded before "repairs."

Any suggestions on:

A) how to get Oscillator 3 to sound like Osc 1 and 2 (again, before "repairs," all oscillators sounded pretty much exactly the same, and without the overtone.)

B) how to tune that ratio closer to 50-50 to get rid of the unwanted overtone?