Shruthi-1: Not recognising MIDI note data. Audio only sine?

Me again (sorry!)

I’ve completed the build of my Shruthi-1 but I’m having problems. These are the symptoms:
Receiving MIDI signals fine but no pitch data.
The audio produced appears to just be a sine. Changing patches does not alter the timbre of the sound at all but the envelope settings are changing detectably.
The test tone is not produced when holding and releasing button 4.

The LCD, LEDs, buttons and knobs seems to be working fine.

I realize now that I was too hasty in completing the assembly of the control board. :frowning:

Any help would be greatly appreciated.


Yay - well done phonodyne :slight_smile:

This forum is indeed a very helpful community.


It works! I resoldered the 220 and the IC7 and everything seems to be functioning as it should be. I was sure that my general ineptitude had killed it.

Thanks very much for your help, both. As a beginner at this stuff, it’s really amazing to be able to get such excellent advice so quickly when working out the kinks. Cheers again!

Those components and soldering look fine as far as I can see.

While you’re trying that swap with the TL074, when you carefully pull it out (lever up one end from the IC socket a little with a flat-bladed screwdriver, then the other end, then the first etc. until it lifts out cleanly) check that all its little legs are still straight - if one of them got bent as you pushed the chip in it can either result in one pin touching another or one not being connected at all - both of which could cause odd problems.


TL074 is correctly oriented.

It can’t be the TL074 or the LM13700s since it’s rare for them to be only half-working (half of IC6 is working since you get signal at point 3 ; half of IC8 is working since you get signal at point 4 ; half of IC7 is working since you get signal at both 3 and 4). If you’re really suspecting something here, try swapping the TL074 with another one ; and try swapping the two LM13700 - if something changes it might be one of those chips.

Otherwise the most likely suspect could be a bad solder joint on the 220 resistor above IC8 - it has one leg to ground and those are always more difficult to solder.

Thanks for the advice, yewtreemagic. This is certainly an educational experience.

Photos of my board below… Please excuse my poor soldering (which might be the culprit!)…

I think the components are in the correct spots, but is it possible that the orientation of the TL074CN is an issue? The notch and dot are at opposite ends of the IC and I went with the notch.

I will investigate whether there is a short somewhere now.

OK - your problem lies in the 3rd stage of the filter then - check for shorts to ground or between traces around the area of C27, R28 and R29, and for correct values components at those locations (C27 should be one of four identical 1nf capacitors - C25, C27, C32, & C34), while those two resistors should both be 10K.

Also, if you can, post a photo of the back of your board zoomed in on that area.


1: Good
2: Good
3: Good
4: Good
5: Non

Then it’s probably something wrong on the filter board.

Probe the signal using the same technique on all the following points and check at which point it starts sounding wrong:

Hi pichenettes. Thanks for your assistance and your patience.

Using an audio probe cable I do get sound at OSC>, it changes from patch to patch, and different MIDI notes are playing.

At the moment you can connect a wire to the tip of a jack, another wire to the sleeve ; hook that to an amp or soundcard and use that to probe the circuit.

Do you get sound at the pin labelled “OSC>” on the digital control board? Does it change from patch to patch?

Ah, sorry: SEM-4 MKII.

I do not have a scope, but I will pick one up if necessary.

Which filter board have you built?

Do you have a scope?