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.


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?


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.