No audio from XT

First post here. I just finished putting together a Shruthi XT with Dual SVF filter. The build seemed to go ok (once I figured out about the different firmware versions!) but I’m not getting any audio out. When I probe J1 from ground to OSC I should get a tone, right? I only get the dreaded ‘DC click’ when I touch OSC and it reads -2.46V DC. This is definitely not normal, right?

Can anybody point me in the right direction? I really need a win after several not fully successful builds and repairs.

> When I probe J1 from ground to OSC I should get a tone, right?

Yes.

> I only get the dreaded ‘DC click’ when I touch OSC and it reads -2.46V DC

You’ve probably swapped the + and - probe of your meter, you must be reading 2.46V DC. And It’s a normal reading. At rest (when no note is playing), the Shruthi emits a 39kHz square wave swinging between 0V and 5V, so your meter should indicate something close to 2.5V DC.

Are you sure it is receiving MIDI?

I’m embarrassed. Yes, I must have had my probes reversed even though I checked twice at two different times of the day. Ok, so yes, I do get a the signal at 39kHz (although it swings from +/-2.5V rather than 0-5V). Thank you, sir!

The problem must be that I’m not receiving MIDI signal. I see in the manual that I’m supposed to see a musical note on the display. Can’t seem to get that to show up so that must be the problem and it is probably something simple. I’ll keep searching to see if I can figure it out

A simple check is the orientation of the optocoupler IC (6N137) and the diode next to it.

You would not be the first … :wink:

Orientation is correct on both. Next up, I’ll get scope the IC for MIDI at input and output and check source voltages.

Btw. these 6N137 do not cling too much to their lives. I had 3 of them dying in my hands, as they are pretty ESD sensitive.

There’s a good chance that’s the problem then. The metal tab for a voltage regulator on the filter board sticks up high enough that it touched the pins/legs on some control board components a bunch of times while powered up. I’ve since bent the regulator down a bit but it is directly under the 6N137. I ordered a new one yesterday so we’ll see how it goes. Thanks for the tip

Ok, I replaced the optocoupler and now I have MIDI I can see a little “~” when I press notes on my controller. However I still get no audio output. I’ve checked a whole lot of things (good solder joints, all ICs seated, polarized parts are oriented correctly, supply voltages are correct) and now I don’t really know where to go.

I want to think maybe it’s the Dual SVF that is the culprit but I don’t know how to test it. It has to be connected to the control board, correct? And there’s no way use it with the control board removed? I tried putting an audio signal through the filter with a key pressed on my controller to open the VCA but I don’t get anything.

Any help?

Progress !

At this point it might help to take some pictures of the boards to help spot issues …

By any chance did you possibly forget to solder the bridges aside of the audio connectors ?

I soldered them. I have it configured for analog control. I’ll take some hi-resolution pics of both boards when I get a chance.

I actually wouldn’t rule out a soldering issue. There were a whole lot of places where solder just wouldn’t flow, mostly the pushbutton switches but also some other places. I should have cleaned the board before I started and sanded the pins on the pots and switches because I’ve had to struggle to get good joints and this has never been a problem for me. So I’ve desoldered, cleaned and resoldered a lot of spots and I think I got them all but maybe not. It’s so weird… Certain joints, if I just touch a hot iron to them it kind of sticks to the solder.

So I’d like to do some signal tracing on the filter board to see if there is a point where the signal drops. Can I do that? Just inject a signal from another synth’s oscillator into the _OSC port and trace it?

> Can I do that?

You’ll also need to provide CV to open the filter and the VCA.

A better solution would be to use jumper wires to connect the filter board to the control board.

Could you have cut the long pin-headers connecting the two boards too short? This has been a fairly common problem with the non-XT Shruthi-1 over the years.

Testing with jumpers connecting the boards would tell you if this was the problem (assuming everything works as it should).

a|x

Brilliant! I’ll use jumpers and get to work.

Thanks

@nosamiam Were you able to figure out what was wrong with your machine? I’ve the dual SVF and XT which are exhibiting the same issue. Though my midi is working. I’ve wired mine for analog control, the jumpers are soldered as instructed and I’ve wired everything. …but no audio.

Thanks.

David

I never got it up and running. It’s just collecting dust.

When I powered up mine for the first time I used an incorrect power supply. I used an AC supply instead of a DC supply. I’m going to switch out the chips and see if I can find one that might’ve been damaged by the incorrect power supply. …but i don’t know at this point. i’ll keep you posted…

I’d replace the voltage regulators and electrolytic capacitors too. There’s
a good chance they took a hit too.

Good luck!

Ok, I have signal, I can hear the oscillator when the sequencer is running from the osc pin and the legs of various chips on the board and i can hear the both filters working. I still don’t have output to the 1/4" jack though. Here’s a part of the schematic where I lose signal though.Lost signal I lose signal after the 100K resistor (R14) Maybe that capacitor is bad (C9)? Maybe I ruined it because I used an AC supply when I first powered on?

Help! :thinking:

I’ve found either a dead/broken trace going from R3 to the bottom pin of R2. I kludged the connection as reflowing didn’t seem to work. I can’t see a trace on the board either so I’m not sure it’s even there. I now have audio out but it seems very quiet. …but it’s progress. :slight_smile:

What would cause a low level at the output?