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.
Thanks, anyone who might be able to spot something!
–Flounderguts (I also posted about this on MuffWiggler…)