Minimoog Model D Pitch / Tuning Problems

Hello folks.

Wondering if anyone can help a novice diagnosis and correct some pitch issues with my Mini.

A couple of months ago, I brought it to a local repair shop to deal with some issues - pitch wheel would be out of tune when returned to detente, some scratchy pots, noise function didn’t work etc, and also wanted the Moog tuned and the power supply calibrated.

After a month and a half, I get the Moog back and it works properly for a couple of days, but then the pitch goes schizo. Will oscillate very quickly sharp and flat by a few cents, giving the pitch a jittery sound, and any or all of the oscillators may (or may not) drift 10 or 20 cents sharp or flat over the course of 10 minutes to an hour, so I now have to tune before every take. Not that big of a deal, but this wasn’t the case before visiting the repair shop. More problematic is the jittery pitch. Also, oscillator 3 sounds a little distorted even at low levels, and when all three oscillators are tuned to the same octave, the Moog will distort at low levels. Not horrendous distortion, but enough to make a square wave sound more like a sawtooth wave.

Here’s a sample of each oscillator, set to square wave, individual volumes and master volume set to 5, all orange buttons off, mod wheel is all of the way down.

https://soundcloud.com/therealrx2/moog-pitch-problem-oscillator-1
https://soundcloud.com/therealrx2/moog-pitch-problem-oscillator-2
https://soundcloud.com/therealrx2/moog-pitch-problem-oscillator-3

Am thinking that maybe something got jostled in transportation, or maybe the power supply calibration is off or possibly the oscillator board was not properly reattached? Again, am a total novice - have owned and played this Mini regularly for a dozen years but have never gotten into the guts other than to perform the basic oscillator tuning calibration via the rear trim pots. So any advice / suggestions / do’s and don’ts would be appreciated. Am thinking to check the power supply and oscillator board connection but am anxious not to make matters worse.

And can’t waste another month or two without it while it’s in the shop and don’t have the money anyway.

Hoping you folks can get me on track.

Thanks.

Rx

Wasn’t the work done by that “repair shop” guaranteed ?

Send it back to them and tell them to do it right this time !

Probably needs cleaning on edge connectors of the PCBs…

You are correct - the work was guaranteed. But they said they needed another month (and from prior experience, that means longer) to get it done and I simply don’t have that time to waste. They already had it for a month and a half and I’m trying to finish a very Moog heavy record by mid-summer. Am hoping that the repair is something I can handle. If not, I’ll bring it to another shop where I previously had some work done, as they did an excellent job. However, they’re expensive and also have a 2 - 3 week average job time and are a couple of hours drive time each way. At least the first shop refunded the money so even if I end up bringing it to the other, more expensive shop, it’s not the worst thing. But am also feeling like I should be learning to handle my own repairs - if for no other reason than to save myself the prolonged downtime.

Any suggestions on what to use for cleaning the PCB’s?

Thanks.

Unplug each board and carefully clean the edge traces with isopropyl alcohol.

You can use some Deoxit D5 if there is a lot of oxidation present (dull brown traces instead of shiny gold-like color) but it will only last for a few months, maybe a year.
Also with the D5 you can spray some into the connectors themselves inside the Mini. Let the product evaporate completely before re-inserting the boards.

There might also be other things that could affect the pitch, unstable power supply source, flaky connections of the Mod Wheel assembly (you could also clean those cinch-Jones connectors (black bakelite type underneath the Mod Wheel assembly), also the pitch bend potentiometer might be dirty.

Hope this helps a little.

Thanks for the suggestions.

I finally had the chance to get back to my Moog today. Popped it open and removed the Oscillator board and the board behind it as well. The gold contacts all look very clean. When you say “carefully clean the edge traces,” - again, I’m a novice - does that mean the actual traces running aver the board? Or the edges of the gold contacts that insert into the Oscillator connectors? And does it make a difference if I use Deoxit D5 or Deoxit Gold?

Also, have been looking at the tuning instructions on this site here - http://search.retrosynth.com/ah/search/lookit.cgi?-v9901.2166
Regarding the power supply calibration, am wondering about the following section of instructions:

Observe the Oscillator board connectors. There are two connectors, the A
connector is on the right, the other is B. The connector pins number 1A,
2A, 3A and so on starting from the right of the connector.

Have attached 2 photos here of my own Mini. The first is a picture of the Oscillator board showing what I believe to be both the A and B connectors (the white plastic pieces that the board is inserted into). Am I correct about this?

http://s1167.photobucket.com/user/therealrx1/media/IMG_0425_zpsk29kdhns.jpg.html

And if so, the following picture is a close up of the connector on the right, what I believe to be connector A (as mentioned in the instructions above). Again, am I identifying this correctly?

http://s1167.photobucket.com/user/therealrx1/media/IMG_0427_zps0pk4jmae.jpg.html


9) Connect the positive probe of the voltmeter to Oscillator board pin 1A

Does this mean remove the Oscillator board and connect positive probe? Or can I simply connect the probe to the first hole from right on the side of the connector?

10. Connect the negative probe of the voltmeter to Oscillator board pin 2A

Same question as above (except) can I insert probe into the second hole from right on the side of the connector?

11. On the Power Supply board, adjust the (+) 10 volt trimpot for +10.00
    volts.

12. Connect the positive probe of the voltmeter to Oscillator board pin 3A

Same question as above except can I insert probe into the third hole from right on the side of the connector? And do I keep the negative probe connected to Oscillator board pin 2 A at the same time?

Thanks for any and all info and suggestions. Am anxious to get this sounding better without destroying it and / or electrocuting myself in the process.

Right away from the first photo, the screws that hold the board in place have their insulating washer on the wrong side. Those washers should be between the printed circuit board and the frame of the Mini. Right now they’re between the head of the screws and the printed circuit board. Those insulating washers are there to prevent the printed circuit board (PCB) from touching the metal frame.

Second, I was talking about cleaning the edge traces (those that are used as connector).

And yes, the connector on the right is A and the one on the left is B. So they are C01B and C01A (for the oscillator PCB looking from the back of the Mini).

And yes, you need to keep the oscillator PCB inserted in order to properly calibrate the power supply (PSU), because you want to do it with a power consumption on that PSU.

And yes, you need to measure at the oscillator PCB to make sure it is +10 VDC.

And no, you won’t electrocute yourself, or damage anything, if you’re careful not to make any short circuit with the probes, and you don not touch the mains power switch exposed wires near the transformer on the far left.

Finally, if you’re not entirely comfortable poking around inside your precious Minimoog D, maybe it would be advisable to consider having a qualified and trusted tech check it over and calibrate it for you ?
Because sweaty hands and nervousness aren’t recommended when measuring things whilst doing some guess work too…

Hmm… there is a thin layer of electrical tape separating the circuit board from the frame - is that not sufficient buffer between the circuit board and frame?
The same thing applies to the board underneath the Oscillator board (don’t know what it’s called) - there are 2 screws with washers, then the board - and the screws go into posts that connect to control side of the frame. Should these washers also be on the other side of the circuit board? And regarding the board to the left of the Oscillator board, the washers are in the same position - and there is also a strip of electrical tape separating the board from the frame…

From what your saying, I should move the washers to the other side of the respective boards. Correct?

Thanks for the help. Will give it try (gotta learn sooner or later, no?), and if I can’t get it right, will drag it to the shop.

“Because sweaty hands and nervousness aren’t recommended when measuring things whilst doing some guess work too…” http://forum.moogmusic.com/posting.php?mode=reply&f=1&t=23359#

Those washers are insulating washers made of non-conductive material and are there specifically to keep a safe distance between the PCBs and the frame, on all of them.
There shouldn’t be any electrical tape anywhere inside the Minimoog. If there is electrical tape, it was put there by someone who didn’t know what they were doing when they tinkered with your Mini, and you should remove it and put the washers where they belong, so that they can fulfill their purpose.

And yes, we’ve all got to learn somehow (that’s mostly how I did it myself too) but, as you wisely said, when it becomes overwhelming and you feel too uncomfortable, there’s always the option of relying on a professional to carry on (or start over if we’ve messed-up something ).

Ok. Got those washers switched around so everything is in the right place. Too early to tell what effect the the deoxit / washer swap / has had, but it’s been about a half hour and all three oscillators are holding a steady 440hz, so that’s a massive improvement as far as the pitch drift is concerned. Hoping that continues to be the case.

As for the pitch jitteriness, that problem was intermittent, so will see how it goes over the next few days.

Didn’t do the power supply calibration yet because I have to get a multi-meter, but am wondering what effect power supply calibration will have on the overall tone of the moog. For instance, I love it’s general sound (aside from distortion issues which get progressively shittier as Cutoff Frequency is increased), and want to lose the unpleasantness while maintaining the overall great tone of this Mini. Will power supply calibration effect the overall tone or maybe / hopefully just get rid of brittle distortion and / or eliminate pitch problem>

Btw, I realized that the tech replaced what I believe are the power capacitors.

Here’s a picture of the original…
http://s1167.photobucket.com/user/therealrx1/media/IMG_0433_zpsxqrjao52.jpg.html

And here’s a photo of the new capacitors…
http://s1167.photobucket.com/user/therealrx1/media/IMG_0432_zpsj7ucy5ty.jpg.html

Am wondering if this could be a source of the distortion that is a little more evident in the sample of Oscillator 3 as opposed to the Oscillator 1 and 2 samples
https://soundcloud.com/therealrx2/moog- … cillator-1
https://soundcloud.com/therealrx2/moog- … cillator-2
https://soundcloud.com/therealrx2/moog- … cillator-3

And while all three Oscillators have at least some bit of unwanted distortion when outputting to the High and Low main outputs, I don’t hear the same distortion when listening through the headphones, and in fact, through the headphones, all 3 Oscillators sound great - just like they did before I took the Mini to the shop.
Any thoughts on if this could be related to either the power supply calibration or the new capacitors? Or what else might explain why the outputs and the headphones sound different?

(note… am listening to the Moog again this morning and now the headphones sound the same as the High and Low outputs - so either I was mistaken lat night, or the difference in the sound from headphone output is intermittent).

Also, it looks like there are some cracked solder joints on the new caps - hard to be certain because I’m looking through the crevice underneath the caps assembly, but fairly certain at least one joint is seriously cracked.

If so, any suggestions on how to get access to the under side of the plate that the caps are soldered to? Looks like there are a whole bunch of short cable runs soldered to the same board and not much room for maneuvering. I’m comfortable desoldering / desoldering the joints but not sure how to get at them - also, any suggestions about how not to electrocute myself in the process?

Will give the pitch issues a a few days to see if they are indeed resolved before getting into the power supply calibration and depending on what that gets me, will take a look at those solder joints if necessary - or bring to the shop if I must.

Thanks again for the advice.

These questions/comments alone suggest to me that you are better off not tackling this yourself..!

You say you’re OK with soldering but don’t know how to get to the parts that need the attention - also you don’t want to “electrocute yourself”…? Do yourself a big favour… take it to a reputable technician.

Cheers,
Tom

Well, the tuning issues seem to be resolved. Thanks, theailen666, for your suggestions. You’ve suddenly got me back into recording my record after literally months of downtime. IOU. Don’t know if it was the Deoxit or the washers - although since the pitch issues followed the “repair,” am thinking it was the washers. Either way, the Moog has gone from mostly useless to mostly useful.

Oscillator 3 continues to have the same distortion, and increasing cutoff frequency (on any and all Oscillators) continues to sound unpleasant as it reaches “0” - will post some samples tonight - but I can definitely plow through some tracks before tackling the busted solder on the power caps. Can anyone advise if this boosted solder would explain the aforementioned distortion issues - or if those issues are more likely caused by uncalibrated power supply? For that matter, if I do the power supply calibration, will that affect the overall tone of all Oscillators? I’m really happy with Osc. 1 and 2 so don’t want to make any changes that will effect their sound.

Tom, you’re advice, of course, is well taken. Still, depending on what I can learn here and elsewhere am probably inclined to take a whack at it myself and if I run into trouble, will take it to tech. As mentioned in previous posts, the problem with taking it to the tech is a couple of hours drive each direction, drop off and pick up, and about 2 - 3 weeks downtime while it’s repaired, not to mention the money that I don’t have. And am trying to wrap up a very Moog heavy album by the end of July so it’s horrible timing to lose it for that much time.

So if anyone has experience soldering the power caps and can offer some detailed instructions about how to properly and best access the underside of the assembly in order to desolder and re-solder, that would be greatly appreciated it.

Thanks.

You’re welcome Rx ! Glad to hear that things are improving !
I’ll send you my bill…

As for the “boosted” soldering on those caps, I really don’t know what you mean. A solder is either making proper electrical contact, or it’s not. It can be cracked (often due to time, vibrations, or heat, or a combination of the three), or look dull (what we call a cold solder joint caused by excessive heat during soldering and cooling too fast), or be bulging (caused by excessive application of solder by the connection might still be ok, it’s just doesn’t look good). Anyway, if one of those solder joints wasn’t good on those power filtering capacitors, I’m pretty sure it would affect all of the power going to the different boards, and not just Osc 3.

As for the distortion on the square wave of Osc 3, it sounds rather as if the waveform is not quite a square wave, but a slight pulse wave (pulse width ratio not quite 50-50%. Although it’s normal to have very slight differences between Osc 1, Osc2 and Osc3 on the square wave, due to the way Osc 3 circuit differs slightly from the others, what I’m hearing in your samples of Osc 3 in your first message seems a little too “pulse”.

This might be a question of possibly a power supply calibration, but I’m no tech so take this for what it’s worth.

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 ?

If so, are you plugged in the “high” output or the “low” output on the back ? And what are you recording the Mini with ?

Alain.

P.S. Thanks Tom for the pertinent advice. It’s always preferable to have a qualified professional do the work whenever possible. But there are always some things that we can try ourselves when in a bind, either for time or cash or both. But as soon as some things get worst with our tinkering instead of better, even slightly, it’s time to call a pro. Especially if we don’t have a clue as to what caused it.
Cheers !

Ha. Do you accept monopoly money?

Seriously, though, thanks again Alain, or getting me back on track.

As for the solder, it’s cracked - looks like cold solder joints. I had a Neotek II mixing console that had the same sort of cold solder joints and they would cause intermittent issues with panning / mutes / distorted audio / dead channels etc. When I desoldered and resoldered, those problems all went away. What you’re saying makes sense that all 3 Oscillators would be affected the same (or not at all) - so maybe this explains the Cutoff frequency issue?

Have tried both High and Low outputs, to mixing board, direct to Apollo Twin, with and without DI, and any and all variations, including the Headphone out.

Will try your test this evening or tomorrow and report back.

That rectifier board, where the three big electrolytic capacitors are situated, is removed by squeezing the top of the small nylon posts with a pair of long nose pliers, and pulling up on the board. When put back, the corners of that board will “click” back into place.

I don’t know exactly how you can see cracked solder joints from the top (component) side though ?

Well, I finally had a chance to get back to the Mini today… was going to try the above settings, but wanted to have another look at the rectifier board. As I mentioned in a previous post, I was able to see through a gap where the rectifier board is pinned to the frame, and it looked like the solder joint on one of the capacitors was cracked (as in a cold solder joint). However, looking this time, I couldn’t be sure - maybe it was the way the work lamp reflected off of the solder joint.

Anyway, while I was in there, I removed the Filter Board and looked over both, the Power Supply board and the Filter Board. Both had what I think were transistors (in various Q positions) with white gunk on top. Do you know if these are in fact transistors in the Q positions - little black plastic things mounted on silver legs? And there was a small blue cap (or resistor?) on the Filter Board with some white gunk as well, in fact, the cap (resistor?) looked cracked.

Will inspect those boards again tomorrow to see if I can get any markings from them, but maybe these bits need to be replaced?

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

Am way too exhausted to do any meaningful work tonight, but if you have any thoughts on all of the aforementioned, please let me know.

Thanks!

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.

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.

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/distorting-square-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.

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

https://soundcloud.com/therealrx2/distorting-square-wave

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.