MFOS VCO Calibration - noob question

Discussion & Documentation for Ray Wilson's synths, modules and noise boxes.

Moderators: Kent, dslocum, synthcube

Post Reply
jordylane
Learning to Wiggle
Posts: 10
Joined: Wed Apr 17, 2019 10:53 pm
Location: Sydney

MFOS VCO Calibration - noob question

Post by jordylane » Mon Jan 27, 2020 5:37 am

Hi Wigglers,

I managed to get my first power supply and module working, have learnt a buttload and now this is my first post. Still so much to learn...

According to Ray's description, the raw ramp signal should be going from ground to 2.2V but mine seems to be going from 1V to 2.2V:
IMG-6395.JPG
IMG-6395.JPG (2.29 MiB) Viewed 971 times
I'm assuming this is what is offsetting the sawtooth wave - with the offset trim (R5) all the way up it still floats below ground:
IMG-6394.JPG
IMG-6394.JPG (2.17 MiB) Viewed 971 times
I replaced R5 thinking it may have been inaccurate but really I'm clutching at straws without the proper knowledge. Would appreciate any help troubleshooting. I'm running +-12V.

Here's the PCB:
unnamed.jpg
unnamed.jpg (940.65 KiB) Viewed 971 times
Thanks a million in advance!
Last edited by jordylane on Fri Apr 24, 2020 9:51 am, edited 1 time in total.

jordylane
Learning to Wiggle
Posts: 10
Joined: Wed Apr 17, 2019 10:53 pm
Location: Sydney

Re: MFOS VCO Calibration - noob question

Post by jordylane » Sun Apr 12, 2020 2:51 am

Hi again,

I realise the above wasn't much to go on. So, I've gone over everything (in fact, built a second one) and remembered there were a couple of differences between the design and the components I ended up using, the most notable being C6 silvered mica/poly .005uF. I instead used a PHILIPS KS 4700P (4700pF 4,7nF) 63V 1% Axial Polystyrene Capacitor after reading the following from here:

"BTW i don't think the value of exactly 5000p is that important. i'd rather chose a stable 4700p, like polysterene, COG or NPO. however, you could change that any time anyways..."

Is this reliable info? Before I buy another, any thoughts on whether this could be the issue?

Cheers,

Jordy

jordylane
Learning to Wiggle
Posts: 10
Joined: Wed Apr 17, 2019 10:53 pm
Location: Sydney

Re: MFOS VCO Calibration - noob question

Post by jordylane » Wed Apr 15, 2020 7:08 am

I've now tried adding a 330pf in parallel with C6 but the RAW signal didn't budge. The solo quest goes on. Waiting on parts to see if I can replicate the issue on a second module.

User avatar
kriaction
Learning to Wiggle
Posts: 6
Joined: Mon Mar 16, 2020 4:05 pm
Location: Manchester

Re: MFOS VCO Calibration - noob question

Post by kriaction » Sat Apr 25, 2020 2:17 pm

Hi, you should probe the output of U4-A and adjust the trimmer R5 in order to see a wave centered in 0V (ground) and with 10V from peak to peak. You were probing the raw wave (before the amplification) but in the second picture all seems fine since you can reach the 0V level, you just need some finer tuning.
The value of the cap doesn't really matter that much when we are talking of 4.7n and 5n.

jordylane
Learning to Wiggle
Posts: 10
Joined: Wed Apr 17, 2019 10:53 pm
Location: Sydney

Re: MFOS VCO Calibration - noob question

Post by jordylane » Sun Apr 26, 2020 5:22 am

Thanks for the reply kriaction, but note what I wrote above the second image - with R5 all the way up, I'm still unable to raise the waveform enough to centre around 0V. There are flow on effects for every waveform, so I believe I have to fix the RAW.

I've got a response from someone in this related topic.

"thats most likely due to a problem in the reset circuit. either the JFET isnt being turned on enough, or not long enough. take a look at the gate of the JFET, and see what the voltage levels are like, and how long the pulse width is."

I'm currently trying to find that pulse. Unsure if it's the cheap oscilloscope or the user that's causing the problem.

Post Reply

Return to “Music From Outer Space”