Ambika - Analog VCF silent on one voice

I finished the Ambika build early January with 6 SMR4 voices. Everything seems to function; voices, test voltages, LEDS, outputs, display, programs, etc., EXCEPT a single voice card filter VCF is silent. Also I’m unable to tune the VCF on the voice card in question since the analog filter is not producing a self oscillating tone. (Note: Freq and Reso are both set to 63, Env2 and Lfo2 are both set to 0, and both Waves are set to None.) Any suggestions?

Might as well ask for a picture of the card in question ^^

Work your way through the signal chain:

  • IC6 pin 6
  • IC3 pin 8
  • IC2 pin 8
  • IC2 pin 9
  • IC3 pin 9
  • IC5 pin 7

thx for the responses, i’ll give it a go. here is a photo in the meantime:

pichenettes when you say “work your way through” the signal chain do you mean test the voltages with a meter or listen to them with a coaxial cable and a speaker? what am i looking for? with what program/voice settings?

Looks like Q3 is soldered backwards.

> pichenettes when you say “work your way through” the signal chain do you mean test the voltages with a meter or listen to them with a coaxial cable and a speaker? what am i looking for?

Open the schematics, look at what is the CV chain and the audio signal chain. Listen to important points in the audio signal chain (op-amp or buffer outputs); and measure voltages on important points in the CV chain (op-amp outputs).

oops, careless mistake, thanks! working on it now…

ok, i removed Q3 and flipped it to its correct position. this took a looooong time and my work looks a little sketchy…

i’m still having the same problem as above. with Freq and Reso both set to 63, Env2 and Lfo2 both set to 0, and both Waves set to None, the VCF filter does produce a self oscillating tone. here are my voltage and audio observations of the signal chain:

IC6 pin 6 = 2.04 V (hearing slight click w/ midi note send)

IC3 pin 8 = -9.3 mV (hearing slight, faint, toned click, but no sustain)

IC2 pin 8 = 3.2 mV (hearing slight, faint, toned click, but no sustain)

IC2 pin 9 = -30.2 mV (hearing slight, faint, toned click, but no sustain)

IC3 pin 9 = 6.2 mV (hearing slight, faint, toned click, but no sustain)

IC5 pin 7 = 3.8 mV (hearing slight, faint, toned click, but no sustain w/ midi note send)

also note, listening to the above points with the 2 Waves set to Saw does produce sustained tones.

i’m not familiar with schematics, any help with the next step would be much appreciated!

With the oscillators on you should hear the normal synth sound out of IC5 pin7 … If your signal is there but not out of the output of the mainboard you problem is maybe in the connectors or in the mainboard. Did you set the A/B jumpers right?

> and both Waves set to None, the VCF filter does produce a self oscillating tone.

> listening to the above points with the 2 Waves set to Saw does produce sustained tones.

So what’s wrong then?

Also:

What about putting two voicecards, the working one and the damaged one side-by-side, and trying to spot at which point they start behaving very differently?

shiftr- thx yes with oscillators ON i hear the normal synth signal on IC5 pin7. the A/B jumpers are set correctly and the problem seems to be within the voice card only. when the oscillators are off and VCF set to self oscillate there is only a slight clicking sound with a slight tone, no sustain.

pinechettes- yes i have tested side by side with two other voice cards and the voltages and audio differ. the voltages all read 2.04V on IC6 pin 6 but differ on all points from then on. the VCF does not self oscillate with midi input, just makes a click sound on the problem voicecard but the other voicecard VCFs produce tones. maybe the problem is within IC6 or before? i can’t pinpoint the problem, any ideas?

problem SOLVED, 100% functional w/ 6 voices and filters working. thanks for your help

What’d you do to fix it?

after fixing Q3, i realized Q2 was also flipped around. silly mistake, simple repair. i apologize for wasting time on this forum.

Ain’t no waste of time if it’s documented for someone else to not make the same silly mistake. Good on you! :slight_smile: