Ambika voicecard repair

Hello!

I built an Ambika some time ago, and I’ve been using it for a long time…only one of the voicecards (6x4P) never really worked. I have lots of projects, so getting in and repairing the last card was low on my list (since I’ve been happy with 5!).

Now, in my lockdown state, I decided it was finally time to take a look. The data light has always worked when I enable the card, so I know it gets MIDI signals.
First, I swapped it with another B voicecard (it was in slot 6), and the problem moved with the card. I checked to make sure all the cards were enabled and not muted.
Next, to make sure I really had a mute card, I ran it through a preamp and gave it some gain. I can hear a slight pop at keypress, and a bit of very faint noise thereafter…which might actually be from the excessive gain.
I took the IC out of a known good card, and proceeded to swap them in and out, looking for a quick fix. No dice…and I have tested each and every IC. I left the 328 from the good card in there, as the one that came out works fine in the other card.
I’m a good boy, so I visually checked every component, and verified that there are no wonky values. I inspected every solder joint, and reflowed anything that looked vaguely suspect.
I’m not an EE, but I work with electronics a lot, so I broke out my scope (which I am pretty noobish with, but I can find signals) and started probing, using some info from past threads to guide me, and a printout of the 4P schematic in front of me.
I tested the crystal, and everything there checks out (328 pin 9 and 10)

It looks like I’m getting the right signals out of the MCP4822 in the right places.
I’m not sure what I should see from the V2164, but it seemed like the signals were similar from card to card.
When I started looking at IC3 and IC5 (the TL072 chips) I started to see weird stuff. For instance, when I probe a good card with a note on, I see a waveform on pin 1 of IC5…on the bad card, nothing on pin 1, but when the probe slipped, there it was on pin 2. And apart from ground and VCC, it appeared all the pins were sort of shuffled one down for signal. IC3 seemed normal, but IC1 also has some shuffled values.

I’m thinking it might be the 3.6 V zeners, but I don’t have any of those kicking around to swap out (I don’t really want to desolder 2 cards!)

Anyone have any ideas? When I’m looking at the schematic with the signal output, there are taps to LP1, LP2, LP3, and LP4…and I might be delirious from being inside too long, but I cannot figure out where those go to, so I have trouble tracing that part of the circuit.

Here’s a pic of the component side
and one of the solder side
also, here’s a convenient link to the schematic

Thanks, anyone who might be able to spot something!

–Flounderguts (I also posted about this on MuffWiggler…)

A couple of comments:

  • You can safely remove IC2 for troubleshooting – it controls only the resonance and VCA gain. Focus on getting the filter cascade to work without feedback/resonance.
  • IC1 pin7 should be between 2V and 0V as cutoff increases - that’s the filter cutoff CV. Check that the same voltage is present on all controls pins of the 2164 (3, 14, 11, 6).
  • The 2164 is current in, current out, so pins 2, 4, 15, 13, 10, 12, 7, 5 should be at 0V. If it’s not the case, the 2164 might have been damaged and you’ll have to understand why before inserting a new one.
  • If you’re suspecting something is off with D1 and D2, you can safely remove them, they play a minor contribution in taming the resonance.
  • Finally, the LP1, LP2, LP3, LP4 labels are just that, labels.

Thanks for your help, Émilie!

I finally got back to this (my garage took heavy damage in an earthquake yesterday, so I got sidetracked stabilizing the brick)

I am measuring -0.251 to 1.652 V on Pin 7 of IC1.
The 2164 seems fine.
I shouldn’t do this stuff late at night…as soon as I opened the schematic this morning, I saw that the D1 D2 diodes were a clipping circuit. I’m not sure why I puzzled over it before!

As I was writing this, I decided to try to tune the VCF, and I cranked my volume to do that.
I’m hearing output! It’s faint, but now I can start focusing on issues with gain.

This looks good, the range is about 0 to 2, (not -2 to 0 as I wrote in my original post).

It might not be a gain issue at all - you might just be picking signals from another part of the circuit through trace coupling for example.

I have been messing with this some more.

Voltages look good, and match the voltages I get with a good voicecard.
I have swapped every IC with a known good one.

When I turn the gain up on my amp, I hear the correct tones, as I would on other voices. However, it is very faint, and each note has a loud thump or crack (depending on the attack), similar to the sound of plugging a patch cord into the amp without turning it down.

Interestingly, the data input light on the voicecard always goes on with a keypress, but the LED on the MOBO that indicates which voice is being played doesn’t go on every note…although the very faint audio still comes through. I’m convinced it’s not a motherboard problem, because putting any other card in that slot works just fine.

It really feels like it might be a simple connection issue, as I have tested and probed every component on this card.

EDIT: Went through the ICs again, and found a suspect leg (looked like the leg lifted a bit in the plastic) on the socket for IC3. Replaced the socket, and reflowed everything on the board. Success!