My Mini D has started to exhibit intermittent and very faint pitch modulation bleed through from OSC 3, on other 2 osc, even when “Osc Mod” switch is in “Off” position.
I’ve read in the service manual that this might be caused by dirty or poor contacts from board connector lugs, and proximity of wires going to these lugs. But it doesn’t mention what board connector ? The Oscillator board I guess ?
Also, could it be due to something else ? I’ve made sure that the contact traces on the OSC board are very clean, and that the spring action of each lugs is strong (it’s actually quite hard to pull de board out and insert it back again). So I doubt that there could be poor contact there…
As I said, it is very very faint and not really noticeable under normal play, at the moment. But any insight on how to correct it, will be very welcome.
Thanks.
EDIT: it’s always more pronounced (although very faint still) during the first 15-30 minutes of warming up. And completely disappear after an hour or so of being powered-on. Could it simply be moisture on traces ? (the room it’s in is a constant 40-55 % humidity range, and it does that at 40% as well as 55%).
Offhand, I don’t know what’s wrong.
But here’s a few ideas…
Is there a jumper next to the +10 v trimmer on the PSU and not a 10ohm resistor?
Most have the jumper, but some early ones don’t.
Caps replaced?
Anything drawing excessive current? (mods?)
Everything clean? (boards, contacts, etc)
Any external connections hooked up?
Any parts previously substituted, especially transistors?
Is it grounded?
Does your Minimoog have any of these resistors below, especially in the power supply?
These particular resistors (brown body, colored stripes) often drift way off from their intended values.
It’s normal to find 5% tolerance types that actually are 10-70% off value.
I’ve had that problem. Cause was open connection on one of the connector flags carrying the supply rails. The supply rails each have a closed loop kelvin system with force and sense lines which keeps the rails stable at the oscillator card, if either of these go wonky at the connector then you hear a faint modulation on the oscillators.
Easy way to confirm this is to apply pressure on the oscillator board right above the connector, while listening to the minimoog. If the modulation goes away with pressure applied, then one of the flags has spread open.
The manual claims that the fix is to short the K/S lines - this is WRONG. Correct fix is to disassemble and repair the connector flags. The flags will have spread apart - carefully bend them so the gap is not so big.
The board is supposed to have a tight fit into the connector, that’s normal.
I’ve had to remove the osc board lately to install the sync mod, so maybe it is a loose flag. I will perform the test suggested by MC later on today, and I suspect (hope) this might be it, because it started not long after I did the mod…
If not, then I’ll have a look at those resistors on the PSU board. Electrolytic caps have already all been verified with a proper ESR meter and are all well below maximum tolerable values, indicating that they’re all still in great condition.
When I get home tonight, I’ll check all that, and will keep you posted. Thanks again.
I’ve never found that following Moog’s advice and shorting out the sensing is a bad idea.
The Minimoog is a fairly constant current instrument and sensing isn’t needed any more than in say an Ody or 2600 (which doesn’t use sensing either.)
For me (and apparently Moog), the additional lines that the sensing loop uses are not only unnecessary, but also provide more connections that could fail and cause problems.
To each their own I guess.
BTW guys, this wasn’t meant as a contest on who’s got the best advice. I’m taking all suggestions in, from anyone who has ever possibly encountered this little problem with a Minimoog.
There will inevitably be different school of thoughts on “tricks” or mods to get a better performance out of an analog synth, based on different experiences. And as long as these mods don’t risk damaging anything, they’re all valid to me. It’s just that some will be more effective than others at resolving the issue.
It’s up to me to try your valuable suggestions, and I thank everyone who has and will make them, and see wich one works best for me, regardless of who suggested it. We’re all here to help each other out, and what can work for one, might not so much for another. That’s normal, since not all Minis were created equal.
Like I said, I will start by checking for loose flags. And that, I think, we’ll all agree on the right first thing to do ?
There’s no pissing contest between MC and myself.
I have much respect for MC.
I’m sorry if I made it seem this way or you interpreted it as so.
It’s not a big thing anyway because I doubt your Mini even has sensing enabled.
But for those that don’t understand the issue, sensing is a method whereby a power supply provides additional wiring that goes to the destination of the power and “checks” if the power being delivered is correct.
It’s a loop to correct power problems and theoretically is… on paper.. very wise.
The problem is that the sensing lines require additional connections.
These connections in a Minimoog are all frictional, not hard-soldered.
So if any of these connections have problems, power regulation can become markedly worse.
Moog published a bulletin saying that problems with modulation bleed-thru could be due to sensing and they disabled it on all further production Minis.
This occurred very early and probably less than a thousand Minis were affected.
The way to disable the sensing is to replace a certain resistor with a jumper wire.
All later Minis had the jumper wire installed.
Sensing is a great idea, but can’t always be implemented reliably in the real world.
In the case of the Mini, the additional physical contacts create more chance of errors due to wear, lack of contact tension, corrosion and probability.
The resistor that was previously used (and later eliminated) could also affect drift.
So it’s my personal belief that Moog didn’t make their earliest Minis “right” and then proceeded to make another 11 or 12 thousand “wrong.”
In your case Alien, I’d guess some contact might have become a problem when you were doing the sync mod.
Not that you did anything wrong.
Normal manipulation of the boards can cause this.
It’s happened to me and apparently happened to MC according to his post.
Also, Moog also suggested soldering the wires to the power connection pins (NOT directly to the edge fingers!) in order to eliminate any problems from the physical crimps used on these connectors.
Wasn’t feeling contested at all. The respect for Kevin is mutual.
I can’t think of another electronic music gear that implements power sensing.
I come from a career of designing automated measurement equipment and know the value of power sensing. As Kevin said, it requires quality components along the path of power. In the price-competitive industry of musical instruments, quality is not always priority. It would had worked in the Minimoog with better board connectors. Many analog synths have their achilles heel because the bean-counters substituted cheaper inferior components.
Back when the Minimoog was first designed in 1969, Moog was suffering from the tuning foibles of the 901 VCOs. Part of their solution was power sensing. And it would’ve worked if they paid attention to grounding schemes and power distribution. Using my background in design, I corrected the power distribution in my minimoog. That was twenty years ago and my minimoog is still stable tuning.
ARP was just around the corner with better linear to expo converters that contributed to rock solid tuning stability in their VCOs. Technology was still evolving back then. The ARP solution was the one that everyone eventually adopted. And while ARP didn’t need power sensing, they sure needed better power supplies!
And what MC said about those board connectors, I can relate to. I remember vividly my Sinclair ZX81 micro computer (back in 1982) crashing ever so often due to the poor connector from the external 16K ram expansion box. Resulting in loss of hours of work, copying/typing lines of machine language code from a computer magazine for a new game…
I had finally resorted to soldering 40 wires, effectively bypassing the connector, and making it permanently connected ! But I can’t very well do that in my Mini, with soooo many contacts. Besides, it is practical to be able to remove a board for repairs and such.
As for other synth manufacturers sometimes having better technology/design, I tend to agree there also. Even in today’s Voyager, the trimpots for the scaling are single turn ultra-sensitive ones. Why not have used multi-turns on all of them ? Especially for the tuning ? I’ll never understand that.
But we see that only too often. Cost cutting measures that will eventually bite a company in the ass, by having higher than normal returns under warranty for recalibration, in the case of the modern Moog Music.
As for other synth manufacturers sometimes having better technology/design, I tend to agree there also. Even in today’s Voyager, the trimpots for the scaling are single turn ultra-sensitive ones. Why not have used multi-turns on all of them ? Especially for the tuning ? I’ll never understand that.
I understand this issue, but it’s hard to say.
It isn’t always the trimmer itself, it’s that the engineer may have allowed the trimmer to span too much range.
Arps were like that.
There would be a trimmer for the VCO tuning that could span several octaves.
I understand how parts tolerance could affect such designs and that a manufacturer would want to be safe.
They certainly wouldn’t want trimmers that didn’t reach their trimpoints.
But I feel they usually put way too much control on many adjustments.
In later synths with higher parts tolerances, it’s even less forgivable because all the accuracy and tolerance they gain in components, they might lose at the trimmer due to noise or movement.
As for multiturns, it’s likely a cost issue.
A good single-turn trimmer simply costs less than a good multi-turn one, especially in earlier made synths.
Yesterday, I came home and tried to replicate the problem to investigate what the cause could be. And wouldn’t you know it ? As hard as I tried (well I couldn’t really try anything yet, I had to wait for it to manifest itself) it never appeared !
I even powered down everything and waited another 3 hours, and then powered back on, and still, no faint modulation from osc 3… Darn it (in a good way !)
So, when I get back home today, I’ll try again to get that issue to appear… I didn’t even have to open the back of the Mini yet ! I strongly suspect a flaky flag on one of the connectors, but intermittent. So tonight I’ll try to “provoke” the issue by “testing” each flag (as suggested by MC) by gently pushing on the osc board near each one, and see, I mean hear, if that creates a change…
I finally managed to replicate the problem yesterday by moving/pushing on the oscillator board near the left side of the left connector on that board. Turned out to be, as MC suspected, a couple of “weakened” flags.
I carefully gave them a little more spring by slightly closing them, and re-inserted the board (which was even more difficult than before), and proceeded to try to provoke the osc 3 bleeding into osc 1 or 2 pitch with the oscillator modulation switch to off, and nothing happened.
So I guess you could say, case closed !
Thanks you guys for all your help !
Alain.
P.S. The sense resistor is absent on my PSU board from the factory.
