Losing self oscillation on SRM4mkII board after a while

Hi, After a bit of a dodgy start I got my shruthi-1 together and am getting some great sounds of it. Only other hardware synth I have is a JP8000 which has now been relegated to MIDI controller for the shruthi! But…I think I have a problem with it.

I made a mess with the trimmers while assembling it, I put the wrong trimmer on the control board to start with (dumb mistake) and really had to fight with it to get it back out…I have the correct trimmer on the control board now and it is working despite not even being seated in the pcb. The trimmer in the filter board would appear to be working but when I originally tried tuning it I was getting absolutely no scale on the filter at all, each note was going -/+ 2 octaves as i hit the key (almost like a S&H envelope, but there no envelope/modulation) I didnt mind as the filter seemed to sound good but today whilst playing I dialled in the setup for tuning the filter and was surprised to hear it scaling pretty well…almost pefect, but didn’t want to open case just now. Anyway, now I find if I really crank the filters (which I like doing) after this they are losing the ability to self oscillate and losing a bit of character. With the cutoff at 64 I get no sound at all, I need to have it at about 90 before it will have any oscillation but very faint. Prior to this they were whistling away very loud.

If I switch it off and leave it a while I can get it to work again but the second I go distorting it its cutting out again. Is this just some limitation of the board or am I bad using my filters or is there (more likely) something wrong with my setup? Anyone come across this and know where to start troubleshooting? I’ll try and post some audio samples later if no one is sure what I mean. Thanks for now.

FWIW…on the filter board I was missing two resistors, a 220 and a 68, but had some ceramic ones lying around so used them. Sure I read somewhere on the site it doesnt matter about ceramic v metallic and that metallic was only used cos its just as cheap. Would this make any difference?

This is not the normal behavior, the filter should always self oscillate when resonance is approaching 60.

When you get behaviour like this, where something changes over time it is typically down to bad soldered joints, they are acting like a capacitor.

Think my shruthi was getting a bit hot when leaving it on top my JP8000 (the blank space where the logo is, fits the shruthi perfect but is probably just above the power supply for the roland). That area of the synth doesn’t get hot when nothings on top of it but with the shruthi on top of it both were quite hot (bottom of shruthi case was pretty warm) Would heat cause the oscillation to stop? I know temperature would cause drift. Anyway, after unplugging letting it cool and placing it somewhere else, it returned to normal and sounding spot on again…maybe just not a good idea to keep it there? It fits so well! :slight_smile:

Solder joint acting like a cap is interesting…have attached pic of bottom, as mentioned in other thread I had to do a bit of wiring due to ripping copper trace around a wrongly inserted regulator. All the voltage testing seemed to be spot on so was happy that this had fixed my problem but it may come back to haunt me…I’ll just need to a buy one :wink:

Temperature will cause small drifts in cutoff, at worst +/- a few semitones. Not a drastic change in the operation of the filter, and nothing that would cause self-oscillation to disappear.

>Solder joint acting like a cap is interesting

I had it on my PreenFM. It was working perfectly but then after a while it stopped working right. When I plugged it in the sound appeared for a while and then gradually faded out.

I reflowed some joints and it was then fine.

come on are you saying it could be my fault then that once my dual SVF was acting weird?
it hasn’t done that since, but next time i take the boards apart i’ll work over the joints again, i need to do the zener switch anyway.

No worries man, thanks! I’ll see how it goes, must say I am very impressed with how it is sounding and the fact that once complete I just plugged it in an it worked well…dying to build another despite my hatred of soldering! (at least there were no pots or switches I might redo some of the joints then but fact it worked straight away on first boot want to just leave it be and apply some lessons learned on next build)