Ambika issue on only one voice

I need some advice on an Ambika.
I have built three of them with SMR-4 filter boards
On the second Ambika I am assembling, everything works as expected…execpt voice 6 has a “birghter” sound if compared to all the others, like the cutoff frequency of the filter is different, and the filter is “more open”.
Data and Note leds are OK, I get sound out of all voices.
I suspected the LM13700s, swapped them with voice 5, they work perfectly on voice 5 but voice 6 still has the same issue with the other LM13700s
The filter board all have the same components…what should I check? the TL072 and TL074 as well? the DAC (MCP4822)???.
Or maybe I have simply misplaced some component like a capacitor or resistor in the wrong place or wrong orientation of an electrolytic?
what’s your suggestion?

Might be a misplaced resistor or capacitor, possibly C10 or one of the 1nF capacitors close to the LM13700s. The DAC is most likely not the problem.

Alternatively, check R18 connecting the reference voltage by the zener diode to the frequency input opamp, and R19 and R20 while you’re at it. Those are involved in the cutoff voltage-to-current converting assembly.

Have you tried trimming the trimmers? Shouldn’t make such a huge impact but maybe you can get closer with that.

Thanks a lot for your reply, and …nevermind :slight_smile:

After checking the board for the 10.000th time with a bigger lens I discovered that the central pin of Q3 (a 2N3906) was not soldered at all.

I am an idiot, I can only say that after soldering 18 boards I was probably a bit tired :wink:

Happens to all of us!

yeah, so true… :wink: