Problem with Ambika SMR4 Cards

Hi All,

I just finished building my Ambika this week and it has been an amazing journey so far. I sourced all the components, flashed the chips with the usb tiny and could make it mostly work. Thanks all for support so far.

From the 6 voices cards, 4 are working without any problems.

One them work, but when I tried to tune the filter I get almost no tone out of it at all, completely different behaviour from the other boards. If I use it normally with a patch/program it seems to work ok and make the same sound as the other voice cards. Any idea why it’s like that?

The other one sound a bit more tricky (or not). I flashed the chip on this board directly and both leds react accordingly to being assigned to a part or playing a note - but no sound come out of it at all. I tried changing the voice card position on the main board and move from A to B positions and re-checked all componenents/soldering to look for anything obvious but didn’t find anything really. Any ideas on how is the best way to debug it?

Thanks in advance.

> Any idea why it’s like that?

Could be a wrong resistor value somewhere. Could be that one of the caps in the filter is badly soldered, or has a wrong value, so the filter sounds like a 3-pole instead of a 4-pole and does not provide enough phase-shift at cutoff for self-oscillation to occur.

> Any ideas on how is the best way to debug it?

Check if there’s signal at the DAC output, then at each stage of the VCF, and then at the VCA.

Check the VCA control signal output.

i’ve had the same problem with the tuning of one of my smr4 cards. turned out that after resoldering all joints it worked (and i didn’t even could spot any bad solder joints with my eyes).

Thanks for the advice Pichenettes and Aerror. I will go over that tonight and see how it goes.

So yesterday I went to do some work on the problem.

I re-checked them again for components and soldering, and re-soldered points that were not that good. The filter problem got fixed - I could tune the filter and it is happy with the other 4 SMR4s.

The other one still has the same behaviour. So I got Eagle to see the schematics on the board and the schematics PDF and started to debug it down. As both leds fire correctly I think the 328 is ok. Then I saw the 328 sends output to the MCP4822. Looking at the chip specs it looked like the output is the pins 6 and 8. So I tried to simulate playing a key and checking volts with the multimeter on this output, but could not be that confident I was measuring something as my multimeter (UniT 61A) stays floating. As a last test I switched the MCP4822 between the bad voice and a good one. The good kept good and the bad kept not working. Then it was 1:30am and had to shut off the debugging as work waited for me this morning.

So some questions for now. Is the pin 6 and 8 of the MCP4822 really it’s output? If a note is played what should I see on the multimeter? A oscillospe can help somehow?

Any other comments based on what I’ve done so far?

Will come back to it tonight again.

Thanks a lot!

Since resoldering fixed a problem a suspect more resoldering (maybe at hotter temperature) will fix more problems…

> So some questions for now. Is the pin 6 and 8 of the MCP4822 really it’s output? If a note is played what should I see on the multimeter? A oscillospe can help somehow?

The schematics are quite explicit about the fact that pin 8 is the VCA control signal ; and pin 6 is the oscillators.

So pin 8 should raise to a few volts while the key is pressed (depends on velocity), and go down to 0 when the key is released (depends on release time).

Pin 6 is an audio signal, your multimeter won’t tell anything interesting about it, you need to either listen to the signal to a mixer/audiointerface, or probe it with a scope.

Hi Pichenettes/Shiftr,

Thanks for the info.

So after some more troubleshooting with the scope and making a wire jack to probe directly the sounds I could find the problem. The IC3 LM13700 was not working. So I switched it to a new one and now have all the 6 voices perfect.

Ambika is done! Listening to some midis here and it’s sounding awesome.

Will build in the case and post some photos soon.

Thanks a lot. It has been a great journey in learning all that.