Mini-Modules : Minimoog DIY clone on its way.

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Kevin Mitchell

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

Post by Kevin Mitchell » Tue Sep 29, 2020 7:40 am

Hi, Brian. I'll have a crack at it.

You need to know if the trouble is in the mixer, VCF or VCA. Let's start at the VCA and work backwards.

Check for signal at R271 - the VCA input resistor. Also might be worth while to probe the base of Q66 to see if there's any envelope activity going to the VCA.

If nothing at R271 - your signal is getting lost in the mixer or VCF. If there's signal at R271 but no activity at Q66, there's an error in the VCA contour generator. Make sure the VCA controls are not at 0 while you're testing.

If both of those seem fine - there's an error in the VCA circuit.

-KM

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

Post by frozenkore » Tue Sep 29, 2020 1:49 pm

Hi Kevin,

Thanks for the help! I see an envelop activity on Q66, but I do not see anything signal on R271 whether I press a key or not.

*Edit: I just checked all the 741s. IC1 and IC2 read 0.00v on VCC+, while the rest read +10.00. My guess is I have an issue somewhere around there.

Cheers,
Brian

Kevin Mitchell

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

Post by Kevin Mitchell » Tue Sep 29, 2020 2:50 pm

No.

Those ICs along with some others have -10v for GND and 0v for the VCC rail. Others have -10v and +10v. So that's why you see 0v on some and +10v on others.

You said you can observe the waves of each VCO. You'll want to look at the mixer since it's next in the path. But first let's do the easy part and check the VCF. With the resonance (or emphasis?) all the way up do you have anything at the output? If not - well there's your clue.

Edit: Here's some places to look.
VCF input capacitor (from mixer output) is at C68. If there's no signal there, the problem must be in the mixer. If it's the mixer, I'd say it is actually a micro match issue since it's all passive with resistors and pots. Those connectors are RSW2,4 & 6.

Think about it this way, the gate controls the respected envelope generators for each the VCF and VCA. So either the envelopes aren't working or the signal is getting lost in the filter or amp circuit. So far we have suggestions that it isn't the VCA - or at last the signal isn't making it that far.
-KM

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

Post by frozenkore » Tue Sep 29, 2020 6:01 pm

Kevin Mitchell wrote:
Tue Sep 29, 2020 2:50 pm
Edit: Here's some places to look.
VCF input capacitor (from mixer output) is at C68. If there's no signal there, the problem must be in the mixer. If it's the mixer, I'd say it is actually a micro match issue since it's all passive with resistors and pots. Those connectors are RSW2,4 & 6.
I checked C68. I cranked the resonance and have the cutoff around 50% (no knobs so it's a guess). I've got nothing on C68. I'll check around those switches. Thanks!

*Edit: Are you suggesting I verify that all of the micro match connectors are correct? Just to confirm, bottom row should match bottom row and top should match top right?

*Edit: For the micro match connectors, I'm guessing there should be no connection between pins right? I'm looking at my board scans, and where I'm getting continuity does not look like a place that should be connected... On the jack board. I have continuity between two pins on each connector...

Kevin Mitchell

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

Post by Kevin Mitchell » Wed Sep 30, 2020 8:13 am

Many of the micromatch pads are joined. That's fine unless you suspect an error.

Let's trace the signal right before the mixer. There are 3 molex connectors on the back for each VCO wave. When I was working on my build I had measured and captured the waveshapes. Have you verified that the waves are present at the molex connector pins? They should look similar to my observations;
Image
Image

If you don't see those, your oscillators aren't working. Next down the line are the wave select rotaries and then the level pots. Probe the middle lug of each of the oscillator level pots. Each one of those gets mixed through a 33k resistor and then into the VCF though C68.

-KM

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

Post by frozenkore » Wed Sep 30, 2020 11:15 am

Thanks again! I have waveforms on all pins of the molex, at the the switch, and at the 33k resistors. However, I have nothing on the other side of the 33ks. On that note, I also noticed that there is nothing coming from the noise circuit. Should I be able to see noise on the scope? I have turned noise level all the way, but still have nothing. So I'm wondering if there is an issue there. Like you said, that trace goes right to the negative lead of C68. Unless there is an issue with C68 pulling it to ground or something.

Thanks,
Brian

Kevin Mitchell

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

Post by Kevin Mitchell » Wed Sep 30, 2020 12:02 pm

The noise enters the mix through an 11k resistor(R426). I can't describe what you'd see there - but there should be something.
There's also the external input preamp going to the mix through a 33k resistor(R427).

The mixer output enters the main board through pin 9 of JP14 & JP31. I would check there for shorts as well - there could be a bridge to pin 10, being -10v. That connection also passes two signals for the VCF resonance pot and the main audio output.
What voltage do you see at the mixing node (where the signals vanish at C68)? If it's -10v then we can guess why :hihi:

To eliminate the noise and preamp from the mix, remove R427 and R426. See if the VCOs show up at C68.

I suspect a short or bad part somewhere. We just have to find it.

Also; sorry for all the sporadic updates of my posts. I keep adding to it to explain furthermore. It's certainly not a simple circuit to work on - especially when there's something wrong with a fresh build.

-KM

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

Post by frozenkore » Wed Sep 30, 2020 3:59 pm

Hey, no worries about the sporadic updates. I think I updated my post four times :D. I have good news and bad news. I'm not sure about the noise part, but I removed both resistors, and that didn't fix anything. Then I removed C68 itself. As soon as I did that, I got a mixed signal on the negative pad of C68. So the issues must be in the filter? I'll start checking for shorts.

*Edit: (see I do it too!). In the case there was something wrong with C68, I replaced it, but as soon as I put it back in, no signal again at the negative pad.
Cheers!
Brian

Kevin Mitchell

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

Post by Kevin Mitchell » Thu Oct 01, 2020 8:01 am

Okay! So far we've narrowed it down to the VCF - which we keep going back to. Check for shorts around the filter ladder and JP14/31. It seems the signal is getting squashed immediately in filter the path.

Have you tried adjusting the filter resonance, range and scale trim (R389, R390 & R391)? I doubt that's the issue but I'm wondering if it'll produce any effect. Monitor the output of the VCF which is pin 3 of Q75.

You probably have a bad transistor or short somewhere between Q85 and Q70 (the designators are backwards for how the signal travels through the filter). I would like to rule out a bad transistor in the ladder since you technically tested them when matching, right?

-KM

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

Post by frozenkore » Fri Oct 02, 2020 11:09 am

Okay. Thanks again :). So... No unexpected shorts on JP14/31 (or JP16/32).

I will try the resonance adjustments.

So I had a question on this.

Couple other things I've noticed. If I check continuity across R312, my meter beeps, but if I check the resistance, it's 167ohms (it's a 200ohm resistor). So I don't know if that's a short, or if the signal my meter is sending through overcomes the 167ohms of resistance. I removed it (that was fun), and when I checked continuity between the two pads, I had a brief chirp, but that's it. I also measured the resistor removed, and it was almost exactly 200ohms.

Okay,this isn't an edit because I never posted it... I'm looking at the Minimoog D service manual. I see in the filter section the audio goes through a 10uf cap (which is likely C68). In that circuit I see a 200ohm resistor in parallel with a 220uf cap. I believe that 200ohm resistor is R312. Like I said above, it looks like my meter beeps, but I'm measuring 167ohms across the capacitor; so it must be fine.

I made the mistake of pulling Q83 (since it's lowest on the ladder I believe), and now I'm matching transistors again :sob:. What is the connection between Q85 and Q70. They seem physically distanced? I will try to follow that schematic a bit and do those other tests you suggested.

Thanks,
Brian
Cheers!
Brian

Kevin Mitchell

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

Post by Kevin Mitchell » Fri Oct 02, 2020 12:47 pm

Q85 to Q70 are all in the VCF circuit. That's why I wanted to draw your attention around those 16 transistors.
Can't you just solder Q83 back? It only needs to match Q84.
I'm measuring 167ohms across the capacitor; so it must be fine.
A charging capacitor should climb towards infinite resistance until it discharges. Also measuring a capacitor that is installed in a circuit doesn't really give you precise observations. But if it's a solid reading, I would suspect it to be bad. Try removing it and see if it's resistance climbs and discharges back to 0.

-KM
Last edited by Kevin Mitchell on Fri Oct 02, 2020 2:25 pm, edited 1 time in total.

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

Post by frozenkore » Fri Oct 02, 2020 1:01 pm

Kevin Mitchell wrote:
Fri Oct 02, 2020 12:47 pm
Can't you just solder Q83 back? It only needs to match Q84.
I broke it :sob:
A charging capacitor should climb towards infinite resistance until it discharges. Also measuring a capacitor that is installed in a circuit doesn't really give you precise observations. But if it's a solid reading, I would suspect it to be bad. Try removing it and see if it's resistance climbs and discharges back to 0.

-KM
Let me try that. Yeah, that makes sense.

*Edit: Also, when you said Q85 to Q70, I thought you meant those should have been connected, not that you wanted me to test the transistors within that range. Sorry about that.
Cheers!
Brian

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

Post by yan6 » Fri Oct 02, 2020 5:16 pm

any leads on the lm3046. Are these a possibility:

http://smallbear-electronics.mybigcommerce.com/alfa-lv-as3046d/

Kevin Mitchell

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

Post by Kevin Mitchell » Fri Oct 02, 2020 6:07 pm

yan6 wrote:
Fri Oct 02, 2020 5:16 pm
any leads on the lm3046. Are these a possibility:

http://smallbear-electronics.mybigcommerce.com/alfa-lv-as3046d/
Yes those should work. Likely the only smd variant currently in production along with the CoolAudio version.

Julien, feel free to say hello lol

-KM

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

Post by frozenkore » Mon Oct 05, 2020 10:23 am

Hey Kevin,

So I have a small update. I have my father's analogue meter. I started going through the ladder looking for shorts. I found that (I'm using http://www.synthfool.com/docs/Moog/mini ... Manual.pdf) that between the bases and collectors of Q70 and Q71 (the ones attached to R294 and +10v), the meter goes to 0 ohms. I measured across R294, and I'm getting aproximately187 ohms. Now, unlike the rest of the transistors in the ladder, these two are the only two that connect to a rail (well, primary voltage source). Is it possible these two are my culprits? I can pull them and see if I see a signal coming in to C68, but again, I'm not sure if in this case the meter reading is actually showing me a short, or expected behavior.

Thanks,
Brian

PS, I had actually PM'd Julien, and according to his profile page, he hasn't been on since mid-June.

*Edit: So this is odd. I pulled Q70 and Q71. I still got nothing on C68... However, I just happened to check the emphasis pot and checked the CW pin. I put the emphasis all the way down and cutoff all the way up, and I saw the the mixed waveform. I tested the transistors and they were good. When I put them back in, I no longer saw the mixed signal on the emphasis pot. Looking at the Moog schematic, I removed Q2 and Q3. If there is an issue with the collectors shorting to +10v, I can see why without them in there, I might see the signal on the emphasis pot...
Last edited by frozenkore on Mon Oct 05, 2020 7:58 pm, edited 1 time in total.

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

Post by Sim0n » Mon Oct 05, 2020 12:44 pm

Does anybody have any patches for the mme so that I can use one to test the frequency knob on osc2.

Kevin Mitchell

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

Post by Kevin Mitchell » Tue Oct 06, 2020 8:49 am

Brian, how are you matching the transistors? Can you prove that they are are within ~2mv vbe for each pair?

Simon, what?

-KM

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

Post by frozenkore » Tue Oct 06, 2020 9:34 am

Kevin Mitchell wrote:
Tue Oct 06, 2020 8:49 am
Brian, how are you matching the transistors? Can you prove that they are are within ~2mv vbe for each pair?
I'm using one of the Ian Fritz transistor matching boards. I'm powering it up with a Synthrotek USB Power. Then I connect my meter to the positive and negative posts, plugin two transistors, let them sit for a few minutes (voltage seems to fluctuate for a while), and if I find one that's under 1mv, I flip the switch and reverse the polarity to test it that way. I was trying to match another pair yesterday, and I kept getting strange results. I would get around -1.2mv and +0.3mv... Then decided to power it up with my row power 40, and then the same pair was around 0.3mv.

I never took the power supply into consideration. I wonder if I should have used the row power 40 to test those.

Does that sound right? I'm slightly concerned now. I got the kit from SynthCube. Of the 22 MPS8097, I was able to match 10 pairs... (since two don't need to be matched).

Should I go back through and retest them?

Thanks,
Brian
Cheers!
Brian

Kevin Mitchell

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

Post by Kevin Mitchell » Tue Oct 06, 2020 11:06 am

Contact synthcube and see if they supplied matched transistors and resistors. They do not elaborate this on their parts kit page. Finding 10 appropriate matches in an assorted lot of 22 is impossible. Best to be sure. If they matched them for you, you'd think they be packed in pairs.

I think you should also re-review Ian's matching procedure. Make sure the current resistors are matched as closely as possible - per the build instructions.

-KM

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

Post by Sim0n » Tue Oct 06, 2020 12:06 pm

Does anybody have any patches for this mme minimoog. By patches I mean the positions for every switch and potentiometer knob. I want to test the frequency knob for osc2 as its currently unresponsive. Everything works but that one potentiometer does nothing. All voltage test points are correct and I have checked everything. This is the final thing that I can think of before I start again on a fresh mme pcb. I just need the positions of all knobs and switches for any setting where frequency osc2 pot is responsive and changes the audio.

Kevin Mitchell

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

Post by Kevin Mitchell » Tue Oct 06, 2020 12:36 pm

Oh. Yeah. Patches... :lol:
The guide tells you how to tune each oscillator. That's how you isolate them.

-KM

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

Post by EOTS » Sun Dec 06, 2020 2:59 pm

Hey there,

in case this is of use to someone else,
I've created a version of the soldering guide where it is possible to switch the silkscreen between references and values.
Try it out at https://martinjankoehler.github.io/MME/ ... rd-sg.html

Image

Best regards,
Martin

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

Post by KSS » Mon Dec 07, 2020 1:30 am

Very cool, EOTS/Martin! :tu:

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

Post by EOTS » Mon Dec 07, 2020 4:10 am

KSS wrote:
Mon Dec 07, 2020 1:30 am
Very cool, EOTS/Martin! :tu:
Thanks KSS!

In my build, the VCF calibration gives me some trouble.

In the calibration guide, steps 1-3 work perfectly.
In step 4.1, my emphasis starts at 9 (not 7,5) with the trimmer in mid position,
and with the trimmer at max I get to 8 at best.

Also in step 4.2, 440 Hz can‘t be trimmed, mid setting gives around 140Hz.

Any idea what could cause this?

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

Post by KSS » Mon Dec 07, 2020 5:35 am

Sorry, I didn't build this great project. But I can still appreciate your build guide!

At one point I expected to get one of the bad switch PCB sets and put it in a full size cabinet with KBD, but went another direction instead. I'm sure one of the many builders will be along soon to answer beyond the usual troubleshooting advice to check soldering, component placement and value.

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