Ambika SMR-4 Strange Behavior


I recently assembled an Ambika with 6 SMR-4 voice cards. 5 of them work perfectly. The other…not so much. After having assembled 2 Shruthis and an Anushri with no problems, I was a little annoyed, but I commenced troubleshooting.

According to the LEDs, the card receives data from the MOBO as well as MIDI. At first, I thought that it had no output, but I found that when I turned up the volume all the way I could hear a little bit of sound. I can tell that the VCA is working, because I can hear the notes click on and off with a little bit of very low noise while a key is pressed down. Also, if I change the ADSR of EG3, the clicking sound produced by pressing keys down changes as it should.

When I play low notes (below middle C), I can hear a sine wave-ish sound. Changing the cutoff frequency does not effect affect the sound, however turning up the resonance raises the volume of the sine wave. If I chance the waveform, I can hear a slight change in the sound. I can also hear the pitch change when using the “range” parameters for each oscillator. Changing the ADSR of EG2 (which to my knowledge is the filter envelope) does not affect the sound.

I have re-flowed the vast majority of the solder joints on the board, but nothing has changed. I have also traded all ICs with those of a working board and the behavior of each board remains the same. I have also tried switching slots for the voice card and its behavior does not change.

What I think is happening is that the filter is permanently all the way closed (or at least stuck at a very low cutoff frequency). Does anybody have any idea on how to remedy this situation?


#1 problem are constantly bad solder joints. If you are shure you placed all parts right (check the Caps!) just take a bit of solder and redo every joint. 10 Minutes and i bet a Tannenzäpfle your Voicecard will work like charm.

Tannenzäpfle ™ is the #1 tool to keep a calm iron.
Happy easter!

As I said before, I had re-flowed almost every joint. Just did it again to every single one. Nothing.
Everything is in the same orientation as the other cards. Really stumped.

checked the boards connection?

Any particular places to carefully check?

the stacking connectors are a quite regular reason for misbehavings. maybe the CV isn’t fed through properly because of a weak joint?

Will check there, although I already replaced the connectors with new ones with no change.

Checking again, I am able to tell that pretty much every parameter control seems to work except for “Cutoff.” Even trying to modulate the cutoff with an LFO doesn’t work.

I also don’t seem to be able to modulate the resonance with an LFO either, however all other available destinations (except cutoff) seem to work.

Now I am getting an SD Card I/O error whenever I try to use the patches. I did nothing to the motherboard, and now this isn’t working. Tried reformatting the card multiple times. No luck. This just keeps getting weirder and weirder.

Just finished replacing every single component on the board. Still nothing. What are the chances that the board is faulty?

0. The boards are tested in factory for shorts/cut traces with flying probes. Have you tried figuring out at which point of the signal chain of your faulty SMR4 things start getting wrong?

I don’t have a scope, so that is a little difficult. What I think is happening is that there is a problem with the cutoff frequency control voltage.

Voltage at IC7 pin 12 when you set cutoff to 0 and then cutoff to 127?


0V when the cutoff is 0 and 2.45V when it is at 127.

I had the similar trouble with SMR-4 board. The reason was bad resistor R17 4K7 instead of 47K.

Some errors more .
I had two times damaged IC sockets . I realy don’t know what can happend with the sockets , i have replace all IC’s , all solder points are good , and after a few hour i replace the socket … Works .
Can be that there was a solder short under the socket.
Than , and this was a realy bad mistake : One SMR4 soundcard had no bass.
Behind the 4,7uF couple cap i had only high frequency. I replace this cap , the same mistake.
Hours of bugfixing .
No success. In the end i replace this cap a second time . Works . Seems that i had a bad charge of 4,7 NP caps. and i solder 2x a bad cap into the PCB.

Hope this helps


You could try this: solder a 4.7uF NP cap to the tip of a 1/4" jack. Connect the 1/4" jacks ground with the Ambika ground. Plug the jack into a mixer and turn the volume down. Then use the unconnected lead of the cap to probe different parts of the circuit. You should check if there is the raw oscillator sound going into the filter. Then check the signal after the filter (cutoff 127). That should help to identify where the problem is.

A typical error is also to have ceramic caps swapped. They all look the same and the letters on them are tiny…