Tim Escobedo ms20 clone - no resonance with LP

[ChrisKelly]

I've built the ms20 clone based on the schematic from this forum
https://www.synthforum.nl/forum/synth-hardware/zelfbouw-reparatie-en-onderhoud/141915-diy-ms20-filter-voor-in-pendaal?t=138512&highlight=ms+20+filter

Its the one using a DPDT switch to use the filter as either LP or HP.

Both modes work great except in LP mode I get no resonance at all  in HP mode the resonance work really well.

Is there any particular part of the circuit that would cause this? I've tried different cap values instead of the normal 680pf but this hasn't worked.

Can any of you guys help?

[ChrisKelly]

BTW i should have mentioned:
- i'm using an LM13700
- i've tried swapping this for other 13700's in case of a faulty chip.
- no other changes to the schematic

[noisette]

Hi
Have you checked wether you used the right (+/-) ota inputs according to the schem? they change from ota 1 to ota2 iirc: Res only works with the right polarity (of feedback)
layout or photo would be helpful!

[ChrisKelly]

Hi Noisette
I think so - i followed the exact pinout. Do you mean where the 100k inputs terminate? (Pin 4 of OTA 1 and pin 13 of OTA 2)

[noisette]

Exactly only it´s pin 14 of ota2
which inverts the signal for resonance to work---

[ChrisKelly]

 
Oops i meant pin 14!!
Yep i've got the inputs to pins 4 and 14

[ponce]

I was planning to do that circuit as well, but couldn't find an adequate vero version. I don't have experience with board etching. I had already ordered some LM13700 and LM13600 but they never left my stash. What are you using it with, guitar or synth?

[ChrisKelly]

There's a decent perfboard layout here

http://effectslayouts.blogspot.com/2016/08/korg-ms-20-filter.html?m=1

Mine's for a synth. The oscillator is a 555 atari punk console and the LP resonance struggles (HP is fine).

The LP tends to only work at certain parts of the pulse width modulation - with an APC you're changing the PWM around. I think that has something to do with the problem but not sure why.

[anotherjim]

Check what the switch to choose between LP and HP does. When in LP, does it connect the 680p cap in the second OTA to Vr? When it HP, does it connect the input cap to pin4 to 0v or Vr?

[ChrisKelly]

I've got it set up so that pin 4 goes to GND in HP mode

Same as in this picture

http://s756.photobucket.com/user/storyboards27/media/Mobile Uploads/E9B94169-591E-4021-A7C4-D929F7F7BD6E_zpsdxarmfs2.jpeg.html

Sorry - I should have used this pic in my original post

[ChrisKelly]

http://s756.photobucket.com/user/storyboards27/media/Mobile Uploads/E9B94169-591E-4021-A7C4-D929F7F7BD6E_zpsdxarmfs2.jpeg.html

Hope this link works 
But to answer your question better:
In LP mode - input to pin 4 and 680pf from 2nd OTA goes to Vr
In HP mode - pin 4 tied to GND (from 0.1u cap to GND) and input to 680pf cap of OTA 2