SVF Shruthi: VCA not opening?

Just finished my Dual SVF Shruthi build, and I’ve hit a snag. It appears that the VCA isn’t opening up properly. At first, I thought I was getting no audio, but I noticed that with every note sent (via MIDI or jam mode arp/seq), there’s an extremely faint impulse (about -60 dB).

Looking at the BOM, I realized that I installed polarized electrolytics in place of the NPs (I remembered the tantalum, though). I made the same mistake, though, on my 4PM build, and that one is working fine (I’m ordering NPs to fix both builds anyway).

Any ideas on where to start probing?

Do you have a clear sound on IC13 pin 4 (filter 1) and IC12 pin 15 (filter 2)?

If I plug something into the audio input and don’t have the control board attached, would there be audio at those pins, or do I need to have the control board attached?

Also, thank you for the quick help!

You need to have the control board attached. Both 1x8 and 1x6 connectors.

I soldered together a test probe, and am able to hear sound on the “Osc<” pin on the control board. Unfortunately, I do not hear anything on IC13 pin 4 or IC12 pin 15.

So it’s not the VCA, but the digital switches routing the audio.

Photos of your filter board?

Solder on back:

Hmmm… Looks like there’s a short between 2 of the 2164 pins.

I checked that out, but it’s more of an illusion from the camera angle. I reflowed it just in case, but I’m unfortunately getting the same behavior :frowning:

Do you have sound on IC12 pin 12, 13, 1?

IC13 pin 12, 13, 1?

I get a crackling on IC12 pin 1, but not sound like the OSC< pin. Nothing to speak of on the other pins.

Hmmm… So the filter doesn’t sound at all…

What about IC2 pin 1?

Nothing there. Should I swap out IC2? I have plenty of backups.

Wait… Are you doing the tests with the control board connected to the filter board?

Yes, with the control board set to jam mode so that I can get a good, constant signal.

Hmmm… check all the soldering joints around IC2. You can try swapping it with another TL072 too!

No luck yet. I reflowed one suspect joint and replaced the TL072. Is it possible that the headers are faulty? I get a signal on the “OSC” pin on both the filter board and the control board, but I’m wondering if the VCA pin has a poor connection. Any good way to test the VCA pin?

Why do you suspect the problem is with the VCA?

You don’t even get audio at the first mixing stage, signal isn’t even going through the filters!

Try to fix the first mixing stage…

Do you get +/- 5V on all chips?

Hi Olivier,

My apologies for that. You know the circuit way better than I do, and it was rude of me to ignore your earlier assertion that it isn’t the VCA!

I’m going out of town for the holidays, so I’m going to let me sick Shruthi sit for a few weeks and come back to it with fresh eyes. I’ll test the voltages then.

If you end up making it to NAMM this year, I definitely owe you a drink as thanks for your help with this and with our modular class.

Finally came back to this after getting a lot more DIY experience.

IC2 is receiving +5V, but not -5V. Any suggestions for fixing this?