4PM filter tuning, some experimenting

Changed one of the over-sized diodes, no difference. Olivier, can you tell me if there are any specific components I should check that would influence the ability of the filter to self-oscillate?

Schematics

IC2A and all parts around it generate the current that gets into the resonance VCA.

IC3OTA1 and all parts around it are the resonance VCA (OTA).

D2, D3, D4, D5 and the 4053N (IC10) that switches them on/off are also responsible for shaping the resonance.

Thanks Olivier, this is an interesting puzzle.

A common cause for weak resonance is one pole in the filter too away from the others. Bad solder point on a TL072 pin or the integrator cap.

As far as I can tell, everything is okay. Obviously it’s not but I think I checked all of the joints, the resistors, the caps and tried another LM13700 and TL074 and swapped all of the TL072s. I re-flowed most of the joints, I just can’t see anything odd. Figured I’d post some photos, maybe it’s one of those, “Can’t see the forest for the trees” situations. If anyone gets a few moments to meditate over these photos, I’d appreciate it.


Are the 4.7uF caps non polar caps? They look like normal caps.

(but it wouldn’t make a difference regarding resonance).

They’re polar. You can make out the white stripe on the side if you look carefully.

Maybe another hint, looks like the resonance is working but the ranges is screwy, watch (and listen to) the video:

Vid

I tried adjusting the Range trimmer but the oscillation disappears as the pitch lowers.

You might try cleaning the bottom of your board: if any of the residue is conductive that could be a problem. Andre’ from TubeOhm has a nice video on how to clean a PCB aber auf Deutsch. Hint: spiritus = denatured alcohol :slight_smile:

@mmarsh, thanks for the suggestion. I’ve never found the flux from the solder I use to be conductive before. I generally do not clean the flux from my boards, it just seems like a solution (no pun intended) looking for a problem.

But there could be small conducting solder traces in the flux invisible for the naked eye.
I recently build a hexinverter NEINOHNEIN bassdrum that had problems. It was fixed after cleaning the board.

YMMV, but I’ve found that cleaning the board makes it much easier to spot issues.

I suppose a small bit of solder is possible but that has never happened to me. I checked the bottom of the board with very strong glasses on, it is fine. Every time I clean a board I seem to cause new issues. Given that I am getting some resonance sound but it seems to be in the wrong place, I’m thinking the problem lies elsewhere.

I swapped the Range trimpot with a new one, noticed the middle solder pad had come loose. It looks like the middle and outer pins on the trimpot are connected so I ran a jumper from one to the other. I also re-heated solder joints that were connected to both trimpots, mostly around the TL074 it seems.

When I compare the same patch with the other 4PM, the sound is close but there is less resonance on the newer 4PM. On my old 4PM, using patch 16, Liquid, the res is set to 38. To get the same sound using the same patch on the newer 4PM, I need to set the res to 46.

Using the filter tuning process, I’m getting some self-oscillation in the upper range of the keyboard and the octaves are in tune, but it just seems like the resonance is shifted up by two or three octaves.

This still isn’t working and I’m a bit lost for new ideas now. Just for fun, I refreshed the firmware and the factory patches, which provided something new. Some of the factory patches will not load. 1, 2, 4, 6, 8, 10, 12, 14 and 16 are junon. I cannot load anything else into these. So, I swapped the Atmega, and the factory patches load fine. Still have the resonance issue 'though.

Looks like I might need another Atmega.

I gave up, put it together and decided to use it as is. Funny thing is that it doesn’t sound too much different from my supposedly working 4PM. Maybe a bit less resonance, but I removed the cap that’s part of the “resonance mod”, which seemed to bring the two 4PMs closer in sound. Still can’t actually tune the filter but it sounds okay for now.

I would still like to figure out what’s going on here, and still have the screwy Atmega to figure out. if anyone can read the chain events and offer up some suggestions, I would appreciate it.

Hi mate! :0 Know this is a bit of an old thread but i am wondering if mebbe you can help shed some light on filter and resonance tuning on a 4 pm Shruthi I own …

…When I I I put the liquid and wobbly modes into self resonance with cutoff at 0 the lowest frequency i get is 496 hz and when I crank up the cutoff the frequency goes well pâst 20,000 before fully open…

When I switch to The Ms 20 or Whacky modes I lose about 15 dbfs of gain and the lowest cutoff frequency is about 637 hz …and goes well past 20 000 before fully open again …

Any light on this wopuld be greatly appreciated ! :slight_smile:

“Know this is a bit of an old thread …” Wow, I had forgotten about this stuff. I couldn’t even remember my password. You might want to ask this on an open or new thread, or even on Muff Wiggler. I haven’t even thought about this in quite a few years.