How to add CV/EXP input to an old EHX The Worm?

[gigimarga]

Hello!

I saw that the new EHX The Worm has an expression pedal input, which can be used as CV input, too. I am curious if the old (big box) model can be easy modified in order to have an EXP/CV input? More clear, I want to use Korg SQ-1 sequencer instead of the internal LFO. It is possible?

Sometime ago, Puretube posted how to add an expression pedal (https://www.diystompboxes.com/smfforum/index.php?topic=73245.msg602406#msg602406), but it seems to me that he replaces the Range pot with an expression pedal.

Moreover, Puretube said in a post that the initial variant of The Worm had Mix and Resonance pots, but they were replaced by fixed resistors.
Looking at the schematic that I attached, it seems to me that R27 (100K) is the feedback resistor, but I have no ideea which ones are the resistors for mixing.

Can anybody help me?

Schematic:




Thank you!

[gigimarga]

 

[diffeq]

The only solution I can think of at the moment is to use 3PDT toggle switch to choose between range pot and expression TRS terminals. But that would make your sequencer driving LDR LEDs directly, which can be fine, or can be not.

Regarding feedback. Seems that "mode" switch function is to switch feedback between different stages. Q1 is an input buffer, so the mixing is done after it, see C4-R6-R7-C4 junction, I think this is it.

[gigimarga]

Thank you very much, Anton!

I want only to replace the Worm's LFO with a sequencer, so I don't need a 3PDT right now. In order to drive the LEDs directly by the sequencer, do you think that is a good ideea to increase R49-R51 of the LEDs to something between 3.3K-4.7K before to make any try? The sequencer outputs 5V, so I think that 3.3K-4.7K is enough.

My big question is how to wire the TRS from sequencer to the Range pot...

[diffeq]

I'll try to analyse the circuit, hope other more knowledgeable members will chime in later. 
The LFO section runs on 10V (15V from regulator, down-regulated by D2 zener). When the switch is in Manual mode, range pot voltage region is from 1.2V (two diodes above ground) and 9.3V (10V dropped by R52). That gives you the idea what current hits the LEDs,
upper value: 9.3V / 1000 Ohm = 9.3mA; lower value 1.2V / 1000 Ohm = 1.2mA;
If the sequencer puts out 0-5V, with stock values of R49-R51, maximum current is 0-5mA, which is within the range of the original, although the sweep won't be as deep and there will be a dead spot with CV values below 1.2V. That's the simplest option. You can mitigate this by installing a small scaling/biasing circuit inside the Worm unit, that would use 0-5V range input and transform it to 1.5V-9V. I can draw a schematic for that. That is a second option. Third option is using 8V/HzV output of SQ-1, selecting the useful range in software, which might or might not work in your setup.

Wiring will depend on what option you choose and what functions you want to retain. If the range pot can go, lift its middle lug and connect Tip of the sequencer (or output of scaling circuit) to R49-R51 junction. Sleeve goes to ground.

For a quick test of the first option, lift the side of R52 that points at pin 4 and connect SQ-1 Tip to that. Switch must be in Manual position, range pot max'd out.

[gigimarga]

Thank you very much, diffeq!

Now the thing is very clear for me. Right now I am out of town, but I will try to wire the sequencer directly as soon as I will come back.

[gigimarga]

I'll try to analyse the circuit, hope other more knowledgeable members will chime in later. 
The LFO section runs on 10V (15V from regulator, down-regulated by D2 zener). When the switch is in Manual mode, range pot voltage region is from 1.2V (two diodes above ground) and 9.3V (10V dropped by R52). That gives you the idea what current hits the LEDs,
upper value: 9.3V / 1000 Ohm = 9.3mA; lower value 1.2V / 1000 Ohm = 1.2mA;
If the sequencer puts out 0-5V, with stock values of R49-R51, maximum current is 0-5mA, which is within the range of the original, although the sweep won't be as deep and there will be a dead spot with CV values below 1.2V. That's the simplest option. You can mitigate this by installing a small scaling/biasing circuit inside the Worm unit, that would use 0-5V range input and transform it to 1.5V-9V. I can draw a schematic for that. That is a second option. Third option is using 8V/HzV output of SQ-1, selecting the useful range in software, which might or might not work in your setup.

Wiring will depend on what option you choose and what functions you want to retain. If the range pot can go, lift its middle lug and connect Tip of the sequencer (or output of scaling circuit) to R49-R51 junction. Sleeve goes to ground.

For a quick test of the first option, lift the side of R52 that points at pin 4 and connect SQ-1 Tip to that. Switch must be in Manual position, range pot max'd out.

Sorry for my delayed post, but I had some problems and I was out of town...I've tried the mod as diffeq described above and it works best using 8V (Hz/V) output.

Thank you very much, diffeq!

[diffeq]

I'll try to analyse the circuit, hope other more knowledgeable members will chime in later. 
The LFO section runs on 10V (15V from regulator, down-regulated by D2 zener). When the switch is in Manual mode, range pot voltage region is from 1.2V (two diodes above ground) and 9.3V (10V dropped by R52). That gives you the idea what current hits the LEDs,
upper value: 9.3V / 1000 Ohm = 9.3mA; lower value 1.2V / 1000 Ohm = 1.2mA;
If the sequencer puts out 0-5V, with stock values of R49-R51, maximum current is 0-5mA, which is within the range of the original, although the sweep won't be as deep and there will be a dead spot with CV values below 1.2V. That's the simplest option. You can mitigate this by installing a small scaling/biasing circuit inside the Worm unit, that would use 0-5V range input and transform it to 1.5V-9V. I can draw a schematic for that. That is a second option. Third option is using 8V/HzV output of SQ-1, selecting the useful range in software, which might or might not work in your setup.

Wiring will depend on what option you choose and what functions you want to retain. If the range pot can go, lift its middle lug and connect Tip of the sequencer (or output of scaling circuit) to R49-R51 junction. Sleeve goes to ground.

For a quick test of the first option, lift the side of R52 that points at pin 4 and connect SQ-1 Tip to that. Switch must be in Manual position, range pot max'd out.

Sorry for my delayed post, but I had some problems and I was out of town...I've tried the mod as diffeq described above and it works best using 8V (Hz/V) output.

Thank you very much, diffeq!

Great! You're welcome, gigimarga.