Ambika Voice Card filter stays open

Hi All!

I’m currently midway through an Ambika build that’s been an adventure, and a card I’ve just finished is having an issue with the filter: no matter what I do, the filter stays wide open.

It’s a standard SMR4 from one of Emilie’s kits (which I’ve just pulled out of storage in my closet after a year or two and resumed). As far as I can tell, there aren’t any cold joints or the like.

  • I’ve tried the card in multiple ports, and it’s had the same issue in each.
  • adjusting the resonance has no effect I can hear in the harmonics, but does drop the volume. Can’t tell if that’s because the cutoff’s maxed and that’s just how adjusting the resonance with the filter open works naturally, or if there’s logic in the card that drops volume when resonance peaks.
  • Playing with the filter tuning pot doesn’t seem to help anything.
  • other cards’ filter cutoffs change just fine.
  • reflashing firmware doesn’t seem to help.
  • I’ve used both the default and the YAM firmware; currently using the latter.

I’m very inexperienced - I’ve only built a Shruthi and a mintsynth - so you may have to be patient with me :frowning: . I haven’t monitored voltages yet, but I’m pretty sure I have the equipment to do it (haven’t pulled it out yet, been a minute, etc).

Is there a diagram around that sort of goes over the different signal paths/ICs on the card, and gives a rough gist on what they’re for? I ask because I can’t see any issues comparing to my other two cards, but it would definitely be helpful to be able to say “okay this is the filter path, it’s gotta be somewhere here.” If there already is this thing and it’s in an obvious place, my apologies for being a dunce >_<

(Also I’ve got a bonus board I can pull parts off of because I seriously fucked up an IC on it, then got a replacement board from OHM [to be clear, the issue right now isn’t with the OHM board]. So, if you’d like me to switch out an IC, there’s a decent chance I can try that. :slight_smile: )

Thanks!

hi,
check the schematics here :

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Awesome, thanks!

To be sure I understand, based on this, I should be looking at the signal path around B7/B8, and starting with R18 and R17, right?

yes, B7/B8 is where cutoff signal start,
first check for the resistor values, r14 r17 r18 r19 r20 r33
ic1 seems to be ok as you got sound and resonance volume drop, but try to swap it if you got a spare,
then i would check Q3/Q4 (how to : https://www.elexp.com/electrical_measurements_part_3.aspx)

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You called it! Q3 had a bad joint on the ground. Thanks a ton for your help, and for the link to the how-to!

ah, nice, that was an easy fix…
now enjoy that great sounding instrument :grinning: !