No sound from SMR4 board

I must admit I am at a bit of a loss here. I have working LCD and LEDs but I can get a single sound out of my shruthi. I have checked my sequencer is functioning correctly and that my sound is all turned up and I dont have my sound card muter or anything. I didnt use pots for the volume, just bridges from resistor legs. I am honestly at a bit of a loss here. I feel like im missing something silly rather than having damaged something. Anyone have any ideas?

Have you tried adjusting Cuttoff???

Maybe the jumper on the poles selector header?

Do you see the note icon on the LCD when you send a note into the Shruthi?

Thanks very mu8ch for the quick replies. I have adjusted the cutoff to no avail yet. I have tried the poles jumper in both positions which doesn’t help.

I don’t seem to have an note icon on my screen though. I am assuming I dont need to be in any particular page for the midi in to work? The unit doesnt seem to be reacting to midi at all. Ho would I begin to trouble shoot this problem please?

Thanks guys!

One problem at a time!

For the no sound problem Hold the first switch (the oscillator pages switch) for more than 1s and release it to trigger the test note. If you hear a sound, you’ll know that your problem is with the MIDI in.

For the MIDI in problem: on which channel are you sending notes? (by default the Shruthi receives only on channel 1)? Also check that 6N137 is correctly inserted.

When holding down key 1 I hear a little tiny pop sound, I assume this is cause I haven’t tuned the filter at all. Sorry I was expecting to hear some “wrong” sounding audio rather than nothing. Also im sending on ch1 but I have run through all channels just in case still nothing. 6n137 seems oriented and inserted correctly.

Do you have access to a scope?

unfortunately not.

Still working on it. I have looked over the boards carefully and I dont think I have any shorts. Checked, IC orientations, now going to work on recheck all the resistors. I will get there!

I just ran the same test as the gentleman in this thread ( on my shruthi and got really odd results. For me pin 6 shows -53.1 with the midi off and and random jumping around between -50 and -100 with midi on. Obviously very very wrong. Would this indicate a bad optocoupler or a short under the LCD or something else perhaps?

What are you measuring? Certainly not volts.

Look very closely at the polarity the audio output capacitors C3/C4, my early mistake.

A test I found useful when I wasn’t getting sound out of my filter board (due to a silly mistake that was soon rectified), was to take the osc signal from the digital board and probe the tip of the output jack/socket (you can do that with it plugged in still) You should hear the synth loud and clear, but obviously with no filtering as you have effectively bypassed it.
It will at least help narrow down which board is at fault.

Thanks for the advice guys! Luap, I have sound just fine from the audio in, it looks like the problem is the filter board.

@sealion, I looked and C3 and 4 and I am pretty sure I put them in correctly, is there a way to check the polarity after cutting off the legs?

Those caps have no polarity - so it really looks like what sealion did fixed the problem because of a “side effect”.

C3 and C4 (and C1) have no polarity? They are not radial electolytic caps?

Thanks for the info, I am still really enthusiastic about fixing this, I had a look for a cheap scope today but they seem a little pricey for me at the moment. I will be taking a couple of pictures of my board and uploading them and starting to try and trace the signal. I hope to get this nailed down the following weekend!

@Bitracer: they are non-polarized electrolytic cap. They are designed to couple audio signals. If you put there a “standard” cap with a polarity it will be in the wrong polarity 50% of the time, because audio signals swing on both sides.

@pinchenettes: I didn’t even know there was such a thing, I’m learning new stuff every day.

Its in the Building Instructions…Step 9 states:“Next, the capacitors. For these ones, polarity/orientation does not matter:”. Maybe you missed some other tiny important bit?