SMR-4 Board No Sound


I’m attempting to build a shruthi-1 synth from schematics; not using the kit.

My board is in a semi completed state.

All the components are soldered in, except the following which are on bread board:

3*10nf parallel ceramic caps (couldn’t find enough 33nf caps)

2*33nf Film Caps (The only 33nf film caps have too big a footprint)

1*2.2k resistor on the digital board

No switches are installed, but I can short the contacts with a piece of wire.

No LCD is installed.

The firmware is definitely loaded, turning the encoder makes the LEDs change and stuff.

I don’t know about the factory pre-sets, I played the midi file from Qtractor; but I don’t know how to confirm whether it was successful or not.

Starting the synth up and shorting switch 1 for 1s, does not produce any sound, nor does playing a midi keyboard into the synth.

The pole selecting jumper is definitely installed.

What’s the next step, start testing stuff with a scope?


I hope you don’t find my post too funny :wink:


First step: check if it’s a problem with the digital board/MIDI or with the filter board.

Temporarily connect the output of the oscillators (>OSC pin) to a mixer/speakers. If you have sound it’s a problem with the filter board. If you don’t have sound it’s a problem with the digital board and MIDI.

If it’s a problem with the digital board, scope the MIDI output (the line at the output of the opto, pulled-up by the 10k resistor). The signal is at 5V at rest, pulses down on MIDI bits.

If it’s a problem with the filter board, poke the signal chain with a scope to find where it’s broken

Oh and you don’t have to solder the LCD to test it. I don’t talk much about this trick because it’s hard to master (it works 50% of the time so I don’t want people complaining that their LCD is broken just because they couldn’t do this trick), but if you solder the 16 pins connector on the LCD and then insert it on the board and keep it at an angle in such a way that there’s a connection between the LCD pins and the boards, you can test it (and see the MIDI note on the LCD to monitor MIDI input). I always do that when I assemble a kit and want to compare various LCD colors…

Okay, I connected the output of OSC against ground to a scope (I don’t want to break my flatmate’s amp haha), and it makes a perfect square wave; even without midi being connected; is this normal?

I’ve connected the LCD and nothing appears when I play something into the midi, but it is working otherwise.

What’s next?



The square wave at 39kHz you see is the PWM carrier. If MIDI was working, you should see it wobbling.

So there’s a problem with MIDI. First thing to check (I spent 1 hour getting mad with this): your MIDI cable :slight_smile: Now let me take a break and go on thomann to order new cables…

Ok, the next thing to check is the MIDI input. What do you see at pin 2/3 of the opto, and at pin 6?

At pin 2/3 you should see a horrible ground hum with spikes at MIDI notes. At pin 6 you should see a +5V signal temporarily pulled to 0 when there are 1 bits in the MIDI messages.

Unfortunately, I have no way to test the midi cables (no other midi devices); and I can’t be sure they work, they’re new, cheap and from ebay lol.

I attached pins2/3 to a scope against my supply ground, there was a lot of noise like you said, and occasional dipping, but I cannot be sure that it was correlated to me sending midi notes; and I didn’t note the amplitude, but I doubt it was 5v.

Is there an easy way to connect a midi cable to an analogue/digital scope to test if that works first?



you could do an easy connectivity test with an ohm meter

Sorry, I should have explained I’m using a USB->MIDI adapter; when I said test, I didn’t mean continuity, I meant testing for midi data.

well, to verify the cable’s ok a continuity test is sufficient imo. as long as there’s no connection between different pins. i don’t know much about the midi signal so i tried it myself. i’ve sent midi note messages from a sequencer at pretty high speed via usb->midi to the scope and adjusted the settings so one can see the periodic changes which appear to be very short negative pulses. i verified it being the signal by changing the gate time between 10% and 90% (i used the Klee2 software seq., but that’s not in the video) and the changes were visible too.
i can’t say much about it, but i know that my midi cable and the gm5x5x5 interface work, so that’s what it looks like:

here’s a short video
changing the time settings on the scope.

Wow thanks for your detailed post, very helpful. I don’t have a chance to get into the lab at the moment because I have other issues with my synth, but I’ll check it sometime next week.



Okay, new midi cables and connecting a scope across pin6 of the opto-isolator against ground shows a 4.82 volt signal being pulled down to 0v, when there’s a midi playing.

And there’s still a constant square wave coming from the OSC pin; midi or no midi.

However still no sound output; is the next step testing the analogue board?

Is it possible it’s caused by my transistors?

I’ve still used PNP/NPN where appropriate but not the model listed.



I probed my analogue board, it seems to go wrong after testing the 1st pin of the bottom TL074P, the osc shows a rising DC voltage, no oscillation, even when midi or a test note is played in.

Also, the 1st pin on the TL047P (on the left near the header) displays a slightly distorted square wave, instead of being slightly slewed; no change with midi: [img]//[/img]

What’s next?



Sorry for being unclear, I was in labs and very rushed.

As far as I know, MIDI works, the opto-coupler pulls down the voltage as it should. Unfortunately my LCD is not connected at moment, so I cannot confirm fully; when I do connect my LCD, on which character block with the note appear?

I haven’t connected the raw oscillator output to a speaker ( I presume you mean the >OSC pin). I have however connected it to a scope, and it shows a perfect square wave; which appears not to change with the presence of MIDI. Is it essential for me to connect to a speaker?



From your messages it’s not clear whether:

  • MIDI works. Do you have a note icon lighting up on the LCD when you send a note?
  • The raw oscillator output works. Have you tried connecting it to an audio mixer/amp?

No need to try to fix the filter board if there’s no precise answer to those questions.

> I haven’t connected the raw oscillator output to a speaker ( I presume you mean the >OSC pin). I have however connected it to a scope, and it shows a perfect square wave; which appears not to change with the presence of MIDI. Is it essential for me to connect to a speaker?

No, but from this it seems that there is no sound coming from the digital section ; so it looks like there’s a problem with the MIDI input.

Can you observe a change on the F> output when you adjust cutoff?

Not soldering the LCD is a good idea at this stage - since you might have to rework some bits of the board that will be later covered by the LCD. However, while it’s tricky to do, you should be able to get the display to work without soldering - just hold it in place, at an angle, with the 16 pins inserted in the Shruthi PCB.

I’ve connected the LCD, and no note appears on the screen when I play midi.

Am I looking in the wrong place?

Is it possible that my LCD doesn’t know how to display a note?



I’ve checked the continuity of the midi-in section; and it’s perfect. It must be a faulty opto-coupler?


Ordering a new opto-coupler is a few days away; is there a quick and temporary fix I can do with ordinary parts?

No, the note is a custom character sent when the firmware boots. Even if there was a problem here, you would see a weird character.

Could you post an image of a MIDI byte at the opto output? (this is easier to do with a digital scope!)

I took a video yesterday of the scope, pin6 against ground; while my sequencer was playing a bunch of random notes, it’s crap quality so I’m not too sure:


I’ve just bread boarded my opto isoltator; and for some random reason midi in is now working, it shows a little note on the LCD.

However still no sound from main audio jacks, what’s next?

Could it be my substitutes for transistors?



Obvious point, but stumped me for a while. Make sure you’re using the right midi in jack. It is the right one( looking from above), not the left like I had assumed…