First off, apologies - I know there are a few of these but none that I’ve found that pertains to this particular issue.
I’m currently on my first build putting together my first VCO using the CEM3340. I’ve managed to get it to play notes from my sequencer (which is incredibly exciting) BUT, it only seems to (sort of) work when I remove the 10nf capacitor. With the capacitor in place, I get a continuous tone that only changes a small amount when turning the 100k pot.
I’ve shot a video to illustrate what I mean:
Not sure where I’m going wrong? I’ve tried multiple capacitors (ceramics).
That capacitor is part of the pitch control voltage circuit, which also includes the two 100k and one 470Ω resistors connected to pin 15, and the pitch control pot. The capacitor goes from the 470Ω to ground. If it’s not behaving as expected with the capacitor there it suggest there’s a problem either with the CV level or with bad or missing connections in the CV circuit. Measure the control voltages as described here, and if that looks okay, carefully check the components mentioned above and the connections between them. If those are all right, check also the components connecting to pins 1–3 and 12–14. It’s easy when breadboarding to get one of these connections wrong and the result is this kind of bad behavior… I should know, I’ve done it.
They look like tantalum caps, which are polarised - is it installed the right way round? I I have made that mistake before when the circuit called for an unpolarised cap.
It also looks to me that the cap in question is going to -12v rather than ground, but I could be wrong, as it’s hard to tell on the pic.
Definitely looks like the rail you have your cap into is the same as what you have pin 4 if your tlo72 into. It also definitely looks like this should be your ground as per the jumper connecting the two blue power rails of your breadboard. That pin 4 should be going to -12v. It looks like a mix up for sure. If it is indeed a negative rail as quoted, that cap won’t work right.
I googled tantalum caps as thought it could be the issue but, although they look like tantalum, they are in fact ceramic (checked the site I brought them from and they’re listed as ceramic) and there’s no symbols to denote polarity. I did give reversing them a go but to no avail.
It’s not clear from the photo but the far right blue rail and far left red rail are both ground. The left/inner blue rail is my - 12v and the only components I have running to it are the 620r and - 12 power to the TL072. Double checked voltages just to be sure.
It’s not clear from the photo but the far right blue rail and far left red rail are both ground (with jumper connecting them). The left/inner blue rail is my - 12v and the only components I have running to it are the 620r and - 12 power to the TL072 as per schematic. Double checked voltages just to be sure.
I’ve checked voltages highlighted on annotated strip layout.
Plus and - 12v going into correct pins on TL072
0v between ground and 100k resistor for CV in
0 - 12v between 100k pot and 100k resistor for corse tune (6 - 12v if 100k pot isn’t grounded)
0v between pin 12 and capacitor
-7.2 at pin 3
I’ve also gone through layout a few times and am fairly confident all is right…
I swapped out the capacitors, trim pot, resistors, and OP amp with no change. Bit the bullet and ordered another CEM3340 (AS3340) and now works as it should.
I remember the issue arose after I accidentally connected pin 3 on the CEM to the negative power rail. Not sure if this is the cause of the issue but all I’ll say is take extra care when connecting any voltage to the chip!
