SMR4-mkII No oscillator sound when resonance fully closed

Hi all!

I just finished building the Shruthi-1 but I have a problem:

The sound of the oscillator section disappers when I turn the resonance fully down to 0 (Sometimes the signal does kick in really loud, broken vca maybe?).

Is there an obvious mistake I did? I initally swapped IC1 (LT1054) with IC2 (TL072P) by mistake when building the power section but after correcting it the +5V and -5V signals looked ok. Is there a chance I fried the TL072P?

Any help would be welcome before buying new IC’s.

cheers,
Robbert

First, check for bad solder joints.

Check that the jumper is correctly installed in one of the LP2/LP4 positions.

Probe the signal with a jack connected to your amp/mixer/soundcard, and find at which stage you lose the signal:

Visually checked all solder joints, they look ok to me.
Jumpers and bridges are all ok.
I lose the signal after measuring point number 2. On all numbers (3-9) there is a very faint signal.
1 and 2 have a loud oscillator signal.

Hope that’s enough info?

What is the voltage at pin7 of IC4 when cutoff is set to 0? And to 127?

With resonance at 0
cutoff 0: 0.01
cutoff 127: -0.10

Ok… Looks a bit weird but could be explained by no calibration of the V/Oct trimmer.

Remove IC6 and IC8.

Put your meter in current measurement mode (mA). You might have to plug the probes differently since it’s a current measurement.

Hook black probe on pin 1. Red probe on pin 6 of IC6

Take a current measurement with cutoff = 0 ; cutoff = 32 ; cutoff = 64 ; cutoff = 96 ; cutoff = 127.

You mean put the black probe on pin 1 of IC6? It measures -0.00 no matter what I set the cutoff to…

Are you sure your meter is set to current measurement with a range of 1mA?

range
2mA

cutoff = 0 ; -.005
cutoff = 32 ; -.030
cutoff = 64 ; -.144
cutoff = 96 ; -.502
cutoff = 127 ; -.502

These looks good.

Do you get signal on points 3, 4, 5, 6 with IC3 removed?

Maybe you could post a high-res photo of your board (both sides) so that we could spot bad parts values or suspect solder joints?

Well, I just tipped all joints with a little bit extra solder and now it seems to be working fine (keeping fingers crossed) :slight_smile: Thanks alot for your time Olivier.

cheers,
Robbert