I’ve just finished building a Polivoks filterboard and the filter cutoff is not doing anything at all.
No sweep, no attenuation, the sound comes out as it goes in.
The weird thing is that the resonance seems to work fine as I can hear the volume dip and the sine-y tone as I turn it up.
All the parts are from mouser and I’ve built quite few shruthi boards over the years which worked as soon as I plugged them in so the only thing that could go wrong is the lm4250.
I got 20 of these chips from a russian eBay seller and they’re marked “1407”.
They seemed more trustworthy than the chinese equivalents.
Tried every single one of them and I always get the same behavior.
Maybe this IC is not a straight replacement for a LM4250?
I’ve never worked with this chip before but I understand it needs proper current biasing to work.
Anybody has any suggestions?
I can probably get a NTE888 from a reputable source, but I’m a bit unsure if this IC requires any resistor changes…
No it doesn’t.
It’s not a self oscillation sine tone it’s a like a constant swoosh which starts at resonance setting > 35 and stays there until about 63.
Cutoff has no effect at all.
I can have a drone (let’s jam!) playing at full volume with the cutoff set to zero.
Resonance works though, which is just feedback so I don’t think it has any correlation to the vcf behavior in my case.
Is maybe my "lm4250"s biased too high/low/wrong?
Replaced the R15 with a 50k trimpot wired as a variable resistor and played around.
The circuit is insensitive to the current (Icf) flown thru Q3 to the opamps’ pin8.
Voltage stays around -4V regardless of the trimmer setting.
I don’t know what’s going on, maybe the chips are wrong or whatever.
Since the voltage at pin8 is steady at -4V I don’t see any reason to try harder nothing seems to change.
I’ll try with the nte888 (altough I may encounter biasing issues with this as well).
Another solution would be to get a ton of lm4250s off ebay, maybe I’ll find a couple working ones.
Thanks for the help pichenettes.
If anyone has a tried source of these chips please let me know.
I’ll report back when this is fixed.
Looking at this page : http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks.htm
I noticed that the KR140UD8B which I (probably) got is a uA741 equivalent which is just a single opamp, not a programmable one.
The correct soviet opamp would be the KR140UD12.
Maybe some sellers can’t tell the difference and they advertise the UD8 as an LM4250 equivalent and the UD12 as a uA776 equivalent which is not the case.
So I went a step further and tried to pop two actual 741s in there and I, of course got the same behavior so it’s not that the chips are not working they’re just not the correct ones.
I’ve ordered some UD12s, some LM4250s (chinese) and I will soon have some nte888s just to check if they can be used interchangeably.
Hopefully, I’ll get this working as soon as the correct ICs get here.
The remake (k140yd1201) from Alfa RPAR may work with the Polivoks board as replacement for LM4250 (pin-for-pin compatible).The TO-5 packages seem to fit snugly in a socket such as this fella. I have a couple left over from Erica Synths DIY projects (2 are used in their Polivoks VCF, Bassline, etc. as well), and I’ll try it out.
Note: the version К140YD12Р is a DIP/DIL package, though I’m unable to find these widely available. К140YD12Т is a SOIC-8 version, and will require a soic-to-dip adapter. (I like the TO-5 because they look like little spaceships parasitically connected to the board.)