Mini-Modules : Minimoog DIY clone on its way.

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oiseaux
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Post by oiseaux » Tue Apr 16, 2019 12:27 pm

Finished the build a few days ago- thanks Julien for the fun project!

Getting (mostly) good sound after powerup, but working on debugging a persistent issue:
I'm getting an additional high frequency signal showing up in the output mix, even when all the oscillator and noise pots are turned all the way down. Looks like a rough sawtooth on the scope, floating somewhere around 1500Hz; VCA shapes this signal in an expected fashion when gate is triggered (also can't hear the signal if gate is not triggered); VCF adjustment does seem to have some effect, but closing the filter down does not cut out this high freq. signal. Unwanted signal pitch is pretty consistent regardless of the input pitch CV (maybe about a semitone difference from the bottom to the top of the scale).
Tapping the oscillator output JPs gives clean signals across the board, so I've (rightly or wrongly) ruled those areas out as contributors to this issue.

Based on what I'm seeing / hearing, I'm thinking maybe a bad tranny somewhere in the VCF- any thoughts / tips on how to suss this out?
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Kevin Mitchell

Post by Kevin Mitchell » Tue Apr 16, 2019 2:03 pm

oiseaux wrote: Based on what I'm seeing / hearing, I'm thinking maybe a bad tranny somewhere in the VCF- any thoughts / tips on how to suss this out?
That's an odd one. Does it change when you adjust the filter controls?

Did you check the VCA as well? I would have guessed there's a short in the VCA somewhere if everything else already checked out as okay.

-KM

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oiseaux
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Post by oiseaux » Tue Apr 16, 2019 2:48 pm

Kevin Mitchell wrote: That's an odd one. Does it change when you adjust the filter controls?

Did you check the VCA as well? I would have guessed there's a short in the VCA somewhere if everything else already checked out as okay.
Yes, the character of the signal does change a little when twiddling the VCF knobs. But also as previously stated, closing the filter doesn't fully affect this mystery signal in the expected manner.
I'm a bit on the noob side- was applying some basic logic and thinking that when the VCA appears to behave correctly otherwise that the problem might lie elsewhere...
Will continue mystery-signal hunting with the scope tonight...
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Kevin Mitchell

Post by Kevin Mitchell » Tue Apr 16, 2019 3:50 pm

At least you're close to the finish line :hihi:

Check your micromatch solder joints for shorts if you hadn't already (JP14 for the VCF I believe) Though that could be an optimistic thought. If it's not coming from the VCOs and you find it at the start of the VCF (C68) it might be coming from the mixer.

Sorry if I'm not so helpful. Just shooting out ideas. I was stumped on this thing for a while not knowing that I had put fake JFETs in the glide circuit.

-KM

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oiseaux
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Post by oiseaux » Wed Apr 17, 2019 9:33 am

No- all ideas absolutely welcome- I'm really lost on this one now...

After sniffing around for a while I found the rogue signal lurking at the Preamp Output- feeling confident I disconnected the external signal preamp circuit (pulled C58), powered back up... and no change. So this weird high frequency signal in the audio path seems to be coming from multiple locations. As I write this, I'm now wondering if there isn't something wrong with one of he voltage regulators, even though they all seemed to check out on the DMM during board calibration. I did check a number of ground points around the board while looking for the rogue signal last night- they all seemed noise free... but I didn't think to scope the power rails...
Anyway, here's a portrait of the offender, scope misidentified the signal before and locked on a subharmonic or something- freq. is more in the neighborhood of 4500Hz (that's 200μS on the time scale of the scope).

Image
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jdelgoulet
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Post by jdelgoulet » Tue Apr 23, 2019 6:04 am

@oiseaux : If you change VCO-3 waveform, does it change on your scope too ?
I'm asking 'cos one possibility is that you've got some bleed thru coming from the mod-mix section ...

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oiseaux
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Post by oiseaux » Wed Apr 24, 2019 3:01 pm

@jdelgoulet: Sorry, been away from the bench (spring break and such...), haven't had a chance to circle back on this yet. But I can answer your query- VCO-3 did not seem to be tied to this issue. The rogue waveform is pretty consistent regardless of the front panel settings.

I did do some more research online trying to find a similar documented case- found this:

https://www.eevblog.com/forum/beginners ... cillation/

The OP over there seems to have seen a veeeerry familiar unwanted waveform on his scope, caused by the LDO regulator he was using needing more ESR on the output capacitor, if I understood the post correctly.
While I do have basic hobby electronics working knowledge, the subject of regulator / capacitor ESR requirements (limitations?) is a bit out of my range- needless to say I'm going to be looking at the voltage regulators and scoping the rails as I was previously thinking about once I get the chance.
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oiseaux
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Post by oiseaux » Thu Apr 25, 2019 9:25 am

So it's one of the LM2991 regulators (IC16 to be exact)- found the bad wave at TP1. I toyed around with C19, trying different components, even added a 1 ohm resistor in series with the 47uF cap to see if that would have any effect- no change. At this point I'm thinking the regulator is bad, and I'm going to try and replace it. Not looking forward to the upcoming challenge of desoldering the thermal pad from the PCB...

...and then things went from bad to worse. When shifting things around on the bench something conductive brushed across the back of the PCB, a crackle was heard and weirdness ensued. No noticeable magic blue smoke, but now VCO-2 and VCO-3 are misbehaving. VCO-2 now only outputs a clipped ramp for the TRI, nothing for SAW, VCO-3 output waves all now look "folded". Not sure which parts are fried- assuming one or more of the transistors within the VCO cores (and I'm reeeeally hoping not the LS3954s...); tracking and octave switching seems to be OK for all 3 VCOs, just waveforms 2 and 3 are messed up.
Will upload scope shots of this new madness tonight...
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Quadram
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Post by Quadram » Sun Apr 28, 2019 5:51 am

oiseaux wrote:found the bad wave at TP1
I had the same problem with both builds that I finished this week.
I realized that C19 was the problem, the capacitor was not a model with low ESR, I had ordered EEUFR1H470 Panasonic caps instead of EEUFC1H470.
I changed C19 for a test by a 47uf Audio Grade Electrolytic Capacitors that i had and the bad wave is gone.
I ordered some EEUFC1H470 caps for test and to make the good replacement.
I hope this help you

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kinrat
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Filter setup

Post by kinrat » Sun Apr 28, 2019 9:22 am

Having a bit of a problem with the Filter adjustment of the Emphasis control.
All other adjustments are good, and the filter itself works fine.
When adjusting the regen trimpot I cannot get the filter to self-oscillate.
I checked out all the resistors I thought could influence this, R311, R312, R389, R313,R296, R300 and even shorted out the emphasis pot on the front panel to see if that caused it.

The emphasis does work, I get the expected 'chirping' noise above about 8 on the scale. I compared it with my Boog D and it gets a similar sound through it but just tops out before it oscillates.

Any clue as to where I might look next?

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ganders
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Post by ganders » Mon Apr 29, 2019 6:07 pm

@jdelgoulet Love this project, it looks amazing!

Will there be any more PCB & Panel kits around anytime soon?

Cheers, George :)

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oiseaux
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Post by oiseaux » Wed May 01, 2019 10:14 am

Quadram wrote: I had the same problem with both builds that I finished this week.
I realized that C19 was the problem,
I had already tried swapping out C19 a few different ways with parts I had around the bench, but with no positive result.
Finally performed the painful surgery last night (removed IC16 from the board and replaced with a new regulator)- problem solved.

Now on to figure out what components are misbehaving in the VCO-2 and VCO-3 circuits...
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jdelgoulet
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Post by jdelgoulet » Sat May 04, 2019 12:12 pm

@ganders : they are back in stock :party:

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oiseaux
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Post by oiseaux » Mon May 06, 2019 9:12 am

So after extended reflowing, component replacement, etc. I've got good waveforms out of all three VCOs (via the Molex JPs)... but only when the sync switches are not on.
Turning on the VCO sync for either 2 or 3 causes glitches in the output waves. VCO-2 and 3 frequency pots affect the glitching- almost looks like the two wave frequencies (VCO-1 base freq. and variable VCO-2 or 3 freq.) are getting added and folded together- anyone else experienced anything like this?
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gslug
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Post by gslug » Fri May 17, 2019 9:21 pm

After my first build in about two years (Jasper, still need a case), the addiction seems to be back. I've just ordered the PCBs for this. I have a couple of other things planned too... :banana:
One day I'll get an avatar!

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gslug
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Minimoog metalwork

Post by gslug » Wed May 29, 2019 3:00 pm

Stupid question time! I've acquired a Minimoog front panel overlay, but does anyone have accurate drawings of the metalwork for the electronics enclosure. I've seen a few posts where people have built clones, so assume there must be some information out there somewhere.

Thanks,
Kevin
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Kevin Mitchell

Re: Minimoog metalwork

Post by Kevin Mitchell » Fri Jun 07, 2019 9:25 am

gslug wrote:Stupid question time! I've acquired a Minimoog front panel overlay, but does anyone have accurate drawings of the metalwork for the electronics enclosure. I've seen a few posts where people have built clones, so assume there must be some information out there somewhere.

Thanks,
Kevin
You'll have to be sure they used the same overly spec as the one you've acquired. I have one that should be an exact fit for the original (ebay reproduction part)

I've been slowly working up a CAD file so I can have one built for me. That's the only part holding back my clone!

Someone was awesome enough to dismantle their old MM and take measurements for me;
https://analogbros.com/pictures/MiniMoog/

I'll send you my files when I allot time to complete them!

-KM

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gslug
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Re: Minimoog metalwork

Post by gslug » Sat Jun 08, 2019 2:30 pm

You'll have to be sure they used the same overly spec as the one you've acquired. I have one that should be an exact fit for the original (ebay reproduction part)

I've been slowly working up a CAD file so I can have one built for me. That's the only part holding back my clone!

Someone was awesome enough to dismantle their old MM and take measurements for me;
https://analogbros.com/pictures/MiniMoog/

I'll send you my files when I allot time to complete them!

-KM[/quote]

My overlay is from the Argentine eBay seller and is meant as a replacement for an original Mini, so that should be OK.

I've also been having a go at designing the metalwork, mainly based on guesswork and what will fit in the wooden case. The linked pictures will come in very handy, but I'll probably wait for your files too as I'm bound to mess something up!

Thanks,
Kevin
One day I'll get an avatar!

Kevin Mitchell

Post by Kevin Mitchell » Fri Sep 27, 2019 3:36 pm

Overload mod anyone?
[video][/video]
Image

I'll update the video after I receive the new lamp. Trying to use a T1 lamp and 3mm LED bezel to mount a miniature version onto the panel.

-KM
Last edited by Kevin Mitchell on Wed Oct 02, 2019 7:53 am, edited 1 time in total.

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nosotokarjazzy
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Post by nosotokarjazzy » Sun Sep 29, 2019 5:21 am

Missing two components to finish my built : 5k & 1M-Log potentiometers from tayda shop (ref resp. A-1870 & A-1181).
Anyone has those two parts to sell me in Europe (or France) ? Don't want to pay enormous shipping fees for 1.70$ components ... :bang:
Cheers

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gslug
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Post by gslug » Sun Sep 29, 2019 2:59 pm

Kevin Mitchell wrote:Overload mod anyone?
-KM
Nice. :sb:
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Dominox
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Re: Mini-Modules : Minimoog DIY clone on its way.

Post by Dominox » Thu Jan 02, 2020 10:08 am

Hey,

Have my mme ready and it looks like working all...
But I have noise and hum in the vca...
Any idea's?

Greetings Dominic

Kevin Mitchell

Re: Mini-Modules : Minimoog DIY clone on its way.

Post by Kevin Mitchell » Fri Jan 03, 2020 12:55 pm

Dominox wrote:
Thu Jan 02, 2020 10:08 am
Hey,

Have my mme ready and it looks like working all...
But I have noise and hum in the vca...
Any idea's?

Greetings Dominic
Are you sure it's not the noise generator?
Does the modulation mix effect the hum or noise at all?

-KM

Dominox
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Re: Mini-Modules : Minimoog DIY clone on its way.

Post by Dominox » Wed Jan 08, 2020 8:55 am

No that's not the problem....

Noise volume is down all osc's too the modulator is of. I get no difrend sound wen I switch from pink to white, have no reaction with the filter on the noise. But I can modulade it with the env! I'm sure the noise comes from the vca!

The hum I don't know... must search again

Thanks for replay

Dom

Kevin Mitchell

Re: Mini-Modules : Minimoog DIY clone on its way.

Post by Kevin Mitchell » Wed Jan 08, 2020 11:52 am

At least you know the contour generators are working.

Record voltages from Q58 through Q66. These are all transistors in the VCA circuit. You may find a short or bad part somewhere.

When I was debugging the VCA for my unit I had made an audio probe to pinpoint where the signal was failing. That may help you as well. You can probe the VCA transistors and hear where the noise begins.

If there is noise at the base of Q63 (pin 2) then it's a problem before the VCA. If it's clean, keep following the transistors until you find the noise.

Here's an original schematic. The designations and some values are different but you should be able to use it to follow the signal.
VCA schematic here

-KM

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