Odd VCA behavior on voice card

Got my Ambika working after flashing all the chips. 5 out of 6 voices working fine. But the 6th one sounds like there’s a problem with the VCA. Either it does a short thump/pop like the ADSR is zero’d out or sometimes if I flick the board with my finger pretty hard, sometimes it’ll start playing notes just fine but be super loud, like the VCA is wide open - no dynamics - full on. I swapped VCAs with one of the other boards and the problem seems to stay with the board not the chip.

I’ve cleaned the board, re soldered anything that looked suspect, and it seems kind of random flip flop between the two states. Any components in particular I should check that could cause this? Ring any bells for anyone?

Knowing what kind of voicecard it is would help…

Oh, sorry. 4Pole.

Thanks. And what do you mean by “swapped VCAs”?

I removed the V2164D from one card and put it in the card that was exhibiting the problem and it still did the same thing, so it doesn’t seem to be the IC itself.

V2164 is the filter.

The chip responsible for the VCA is the LM13700, so if you suspect that’s because of a faulty chip, that’s the one you have to swap!

Swapped the LM13700s and the problem still seems to stay with the board and not follow the chip. I’ve moved the card to other slots and the behavior follows the card so I think it’s a problem on the card not the slot.

Could having a bad cap at C1 cause this? There was a dicey solder on that which I fixed, and now it seems to stay in “loud” mode. I guess I’ll start comparing readings with the other working cards to see if I can hone in on a difference. Any suggestions are deeply appreciated.

By the way - I turned off the wonky card and have been playing around with the 5 voice setup. What a genius little synth

A picture of your board could maybe help. And generally resoldering all parts round VCA might do wonders.
You could also try cleaning your board to make sure there is no tiny piece of solder shorting something out.

Thanks shiftr, that seems to have helped the flip-flop between states, but now it’s just loud all the time. It does seem to respond to envelopes, but it’s just louder than the other voices by about 10db.

That sounds a lot like a mixed up resistor…

A problem with C1 wouldn’t cause a volume boost. Compare all resistor values one by one with a working card…

Is the signal level identical downstream in the signal chain? For example at each of the 4 stages of the filter.

The last filter stage is IC5 pin 1. Is the signal level OK at that point?

Posting a photo of the board would help!

Here are some pics of the board.

The resistors look like they are all on the right spot… The soldering looks like it could use more solder on a lot of joints.
Did you try changing the voicecard in another slot after the resoldering that fixed the first issue?

SOLVED - After being a bit more generous with the solder, all 6 voices are working perfectly.
Thanks for all the help along the way.

LOVE this synth. Amazing architecture. Really beautifully implemented. Genious.

Good to hear you got it working! :slight_smile:


Couldn’t have done it without you guys. Thanks