MFOS VCO Problems

[kjackman]

I'm trying to breadboard the VCO before building it. I want to understand it well enough to troubleshoot if something goes wrong. I built everything below except for the linear input, sync input, and HF compensation. I get some kind of wave on the output. but it's 0.8mv p-p, and the frequency (8k?) never changes, no matter how I twiddle the coarse and fine freq knobs. Also, the wave looks more like a very much rounded sawtooth or something.

In the diagram below, I show what the output of the summer stage and the first op amp look like before I attached the later stages.

I did not build any other parts of the circuit from other schematic pages. Does that make a difference?


VCOProblems by kjackman, on Flickr

I'm pretty sure all the components are connected correctly; I double-triple checked everything.

Any help would be much appreciated, thanks!

[prgdeltablues]

Two thoughts:

Pin 2 of U1A is a virtual ground, so you should read 0V there, and the range you report at that point isn't right for the coarse/fine controls - try measuring on the other end of R8 - you should get a range from about +7V to -10V. You should measure about 20mV change at pin 1 of U1A for every 1V change in the input.

You say you omitted the HF compensation - not clear whether you have anything in the feedback loop of U1B (pin 7 U1B to pin 7 U3) You need the diode and about 20k resistance, even if you don't use a trimmer.

hope this helps.

Peter

[jordroid]

Whoops this one might be my fault, from your last thread, i was writing too fast and not thinking everything through... prgdeltablues is right (about the feedback loop), sorry! I always mean to add a disclaimer stating that anything i say could easily be wrong and should be double checked by someone more experienced, but sometimes i forget to. Good luck with the VCO,

jordan

[kjackman]

[kjackman]

Fixed the problem with the coarse/fine tune knobs, sort of. The 47k and 10k resistors were swapped, so I put them in the proper place. I get something closer to the desired range (+4.12 to -9.59), but it's still not quite right. Enough to dial in usable voltages, though.

I put the HF compensation and feedback line in place.

I discovered I had hooked the 100ohm scale trimmer trimpot up to U1 pin 2 instead of pin 1 (how did I miss that for so long?). So I fixed that.

The circuit behaves somewhat differently, but still wrong. I get a rounded sawtooth that changes amplitude (but not frequency) when I adjust the knobs. I tried to measure the current feeding U2-A pin 2, but got zero micro-amps (?). Huh.

I unhooked U1-A pin 1 from the rest of the circuit to double-check that I get that inverted 20mV per 1V amplification you mentioned before. I disconnected R16 and R19 from pin 1. Then I disconnected the CV input chain (pin 2 of R8, the input voltage) from the op amp so I could measure the incoming voltage there, then plugged it back into pin 2 of the op amp and measured the voltage at pin 1 (the output voltage). Here's what I got:

[kjackman]

I think the expo converter is working, too. It looks like I'm getting the kind of exponential current increases Ray describes on his VCO notes:

[kjackman]

That is all