VCO won't track properly..

LektroiD · Posted: Fri Sep 16, 2011 9:55 am Post subject: VCO won't track properly..

I'm just building my second VCO (I'm delighted with the first, it works brilliant).

I have an issue with the tracking though, even on the widest scale setting, it is not wide enough to give a full octave.

Readings as follows (trimmed to widest scale setting):
1V = 100Hz (base frequency)
2V = 194Hz (should be 200Hz)
3V = 386Hz (should be 400Hz)
4V = 772Hz (should be 800Hz)
5V = 1554Hz (should be 1600Hz)

I was wondering if changing R1 for a higher value (1K or something) would rectify the problem. I'm running it on 12V.

fonik · Posted: Fri Sep 16, 2011 1:05 pm Post subject: Re: VCO won't track properly..

bubzy · Posted: Sun Sep 18, 2011 1:12 pm Post subject:

im also getting range problems with mine, i have tried several ic's including mc14046, a national 4046, and the XNP HEF4046BP im getting a maximum of 3.8k hz (roughly) with a very slight variation between chips.
ive tried jumpering R2, doesnt seem to do much for the overall range

**edit** AHEM......i seem to have used a 100nf cap in C3....... Very Happy

sorry. and ty JovianPyx for pointing me in the right direction.

LektroiD · Posted: Thu Sep 22, 2011 5:29 pm Post subject:

My problem appears to have stemmed from absent mindedness; I didn't allow warm up time.. Left it switched on overnight, and now it tracks perfectly. I now have all 4 built and they sound magnificent!

fonik · Posted: Fri Sep 23, 2011 1:37 am Post subject:

JingleJoe · Posted: Sun Nov 13, 2011 7:55 am Post subject:

fonik · Posted: Sun Nov 13, 2011 8:46 am Post subject:

valid point!

widdly · Posted: Wed Nov 14, 2012 11:22 pm Post subject:

Sorry to dig up an old thread but I am having the exact same issue. I built my x4046 over the weekend and I could only get 11 semitones per volt tracking. I tried a TI and a national 4046 and got the same results. I used 1% resistors through out.

After lowering the 100k resitor to about 90k on the 1v/oct input I was able to get 1v/oct very easily over at least 5 octaves. Nice!

The other thing I found was I was able to trim the VCO offset without an oscilloscope by using a Multimeter set to DC voltage reading.

fonik · Posted: Thu Nov 15, 2012 2:25 am Post subject:

the TI part is known to not meet the criteria. the national, however, has been used in the past with success. you are the 1st one to have problems with the national. alas, i used the NXP part only, so i cannot say anything about the national...

anyways, glad you solved it. five octaves is not the best result, nevertheless, it makes this VCO usable for almost all musical applications, i believe.