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 Forum index » DIY Hardware and Software » The Repair Shop
Rene Schmidt MS20 filter: when I crank CV output goes silent
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cslammy



Joined: Apr 27, 2018
Posts: 133
Location: USA

PostPosted: Sun Feb 03, 2019 9:49 pm    Post subject: Rene Schmidt MS20 filter: when I crank CV output goes silent
Subject description: OTA VCF "lockup"?
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Hey there, I've now built 2 rene MS20 filters: https://www.schmitzbits.de/ms20.html

First one works fine, second one built w/ same parts and PCB has an issue.

when I really push the cutoff CV (crank up the cutoff frequency pot, +/- 15V or that combined with CV modulation) the audio output goes dead.

When I crank the CV inputs back down the VCF output comes to life again after about a second, I can hear audio at its output again.

I can't see anything wrong w/ either circuit; the only difference is that I used smaller green LEDs in the one with the "lockup" problem to get more nasty "MS20 peak distortion".

Beyond that when I look with a scope etc. they look about the same until one's audio output dies.

The resonance settings doesn't affect this issue--the small LED one will get silent and conk out even with resonance set to 0.

I read about zeners used to protect against "lockup", is that what I am seeing here? I mean this: http://www.syntiac.com/syntiac_obvcf.html. What does this mean?

I thought maybe I was killing the 3080s by over currenting them but I have also found when you do this they die really fast, and if that's what's happening here, if the 3080's are getting knocked silly, these 3080s are coming back to life.

Any assistance would be greatly appreciated, this issue is driving me crazy!

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Grumble



Joined: Nov 23, 2015
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PostPosted: Mon Feb 04, 2019 1:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Could you post a picture with the not functioning module? (both sides when possible)
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cslammy



Joined: Apr 27, 2018
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Location: USA

PostPosted: Mon Feb 04, 2019 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Grumble:

I have not added bypass caps yet. I usually do that once I am pretty sure the build was OK. I assume that's not the issue here, I use a stable power supply while testing (siglent 3303X)

As I said, I have another identical board that works without this issue (Same parts, bigger LEDs) which is the same layout, same PCB etc.


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wackelpeter



Joined: May 05, 2013
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PostPosted: Mon Feb 04, 2019 8:52 am    Post subject: Reply with quote  Mark this post and the followings unread

does this appear also when you replace the 10k resistors at the OTA bias Inputs with larger ones say somethin between 15k or 27k?

The Filters behaviour then should be more tame and it's not working as in the original but perhaps to make sure the fault you're describing is connected to the OTA's or the bias Control Input.

I have this one on stripboard too, twice and both switchable between HPF and LPF... but i wasn't so much satisfied with my results, so i ended up replicating the whole original circuit with most additional parts until i was happy enough with my results...

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cslammy



Joined: Apr 27, 2018
Posts: 133
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PostPosted: Mon Feb 04, 2019 9:25 am    Post subject: Reply with quote  Mark this post and the followings unread

wackelpeter wrote:
does this appear also when you replace the 10k resistors at the OTA bias Inputs with larger ones say somethin between 15k or 27k?



Hi Wackel, thanks for the suggestion, no, I haven't tried that.

I haven't tried any troubleshooting yet....

If I was going to shotgun this I think the first thing I'd try is removing the LED's to see if that's the issue, since I have the same design already that works with larger LED's.

My hypothesis is that the small LED's cause the resonance op am via the 1nF cap to sink all the current coming from the first OTA to ground, so no audio makes it to the rest of the circuit.

If the problem goes away with the LED's out, I know more about the problem I figure?

But really, I want to understand why this doesn't work, that's how I learn things.... Smile

So here's a more general OTA question I know some of the readers here will probably just know: I thought (I could be wrong) that if the OTA +/- inputs see too much voltage (over 35mV? something like that?), or the control current goes beyond something like 2mA, you fry the 3080 immediately. No passing go, no collecting $200, poof, the 3080 is toast.

I have seen the too much control current quick-fry 3080s several times, but I am not sure about the inverting and non-inverting inputs on the OTA, does anyone know if overvoltage there causes the OTA to get stupid for a bit then, if the offense isn't too great, the OTA comes back to life when the voltage drops again?

If so then I agree i need to drop the voltage into the OTA stages (probably by decreasing the 220 ohm resistors' size).

Ah! And I just thought of something. On both units I built this weekend, one has this issue, one doesn't, I didn't have 220 1% so I used 270 ohm 1%.

But that's so close I doubt that's it....and it's not a problem on the same module I built with larger LEDs that also uses 4x 270 ohm 1%.

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Grumble



Joined: Nov 23, 2015
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PostPosted: Mon Feb 04, 2019 2:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

It looks like you have a cold solder joint somewhere, reflow all your solder joints, maybe that will cure your module,
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cslammy



Joined: Apr 27, 2018
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PostPosted: Mon Feb 04, 2019 2:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Grumble wrote:
It looks like you have a cold solder joint somewhere, reflow all your solder joints, maybe that will cure your module,


Thanks Grumble, i will try that today.

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cslammy



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PostPosted: Mon Feb 04, 2019 8:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

[quote="cslammy"]
Grumble wrote:
It looks like you have a cold solder joint somewhere, reflow all your solder joints, maybe that will cure your module,


Grumble nailed it. Using a scope I could see that High Pass in wasn't working and I traced it to a Cold solder joint on the wire that connects the normal of the High Pass input jack ultimately to ground. See my diagram.

Interesting that a ground problem with this cap manifests this way, not what I would have expected.

Thanks all!


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cslammy



Joined: Apr 27, 2018
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PostPosted: Tue Feb 26, 2019 7:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK thanks grumble as always, spot on.

I have a blog post about the whole MS20 adventure. Sound clip too.

Good news is that the rene MS20 filter, once you build it in a non-sloppy manner, works, is a simple VCF and (I think) sounds good. Requires 2x 3080's.

http://audiodiwhy.blogspot.com/2019/02/korg-ms20-filter-why-in-audiodiwhy.html

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