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VCO tuning issue
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VCO tuning issue

by Alexwfm on Fri Oct 23, 2020 5:45 pm

Hello,

I was trying to calibrate the VCO of an adafruit xoxbox I bought second hand and I found a small problem.
I can tweak TM5 to get an octave between two C's but then the other octaves are quite a bit out. I've regulated TM6 to get 5.333v and then measured the CV signal coming from the R2R network.
These are the voltages for the different C's, before and after the IC11B opamp:

C1 1.000 1.011 V
C2 1.995 2.020 V
C3 2.963 3.000 V
C4 3.955 4.003 V
C5 4.950 5.007 V

there's a bit of a weird interval between C2/C3, if it's 80mV for a semitone I'd guess those 20mV difference might be the cause of the problem?
I've checked that IC9P outputs on the R2R are actually outputting 5.333 so I'm not sure what I'm left to try! I'm assuming the 1% resistors wouldn't just fail?

Thanks in advance for any help!

Cheers,
Alex

Alexwfm
 
Posts: 3
Joined: Fri Oct 23, 2020 5:25 pm

Re: VCO tuning issue

by Alexwfm on Mon Oct 26, 2020 3:10 pm

I've ended up cutting and measuring all the resistors in the R2R network.
They were mostly around 202K but R90 that was 203.5K. It's not a huge difference, though the fact that it sits on one of the most significant bit line of the R2R could make it the reason why there would be a lesser voltage for the 3V step.

Tomorrow I'll get new resistors so I'll try swapping them.

Another thing I was wondering is how are they all 202Kohm? were they selected from a big batch or is my meter off by 2K? on the bills of materials for the xoxbox it's not specified that they had to be hand matched, so I assumed the 1% tollerance was enough to have a precise enough ladder..

Alexwfm
 
Posts: 3
Joined: Fri Oct 23, 2020 5:25 pm

Re: VCO tuning issue

by Alexwfm on Sat Oct 31, 2020 6:09 am

Fixed by replacing all the 200k resistors and making sure they were all by not more than 500ohm different in measured resistance.

After adjusting TM5 the xoxbox is finally in tune!

Alexwfm
 
Posts: 3
Joined: Fri Oct 23, 2020 5:25 pm

Please be positive and constructive with your questions and comments.