[sdiy] Ensoniq keyboard calibration advice?

Rainer Buchty rainer at buchty.net
Thu Jun 30 23:28:49 CEST 2022

On Thu, 30 Jun 2022, Peter Pearson via Synth-diy wrote:

> Anyone on the list have insight or advice on getting Ensoniq gear to pass
> the keyboard calibration routine?  I'm looking at an ASR10 at the moment.
> [...]
> Ensoniq was very stingy with schematics but the keyboard section is posted
> here:

Yeah, so you know what to check for ;)

All Ensoniq polypressure keyboards are essentially 16 sections per four 
keys (or, more correctly, 2*8 sections per 2*2 keys) muxed to their 
corresponding high-speed VCO. The VCOs' frequencies are driving the KPC 
ASIC's high-speed (well, at least back then) counters.

Bluntly put, each key is a little Theremin: The distance of the key's 
metal plate to the PCB-etched coil determins the frequency. For 
scanning, the ASIC's counters are reset, the muxes are set, then the 
counters are enabled and read out. Those counter values are processed 
through a bunch of hysteresis and threshold black magic in order to 
properly determine idle (key up) from "key traveling" and "key down" and 
finally "pressure".

As such, there are multiple failure scenarios for keyboard calibration 
errors (i.e. "unable to determine idle frequency").

- on VFX and possibly other machines that got the dual-PCB keyboard it's
   the connector between those two halves; remove it and put soldered-in
   wires there instead.
- PCB haircrack -- unfortunately, that's more than often the case
- pass transistor shot
- multiplexer dead
- defective KPC ASIC (though have never seen that one so far)

If it signals "calibration failure", then you can at least rule out the 
CPU, as "failure" is a dedicated code sent back to the main board.

(At least on the SQ80, the machine would just simply hang in calibration 
if neither "ok", KPC error #1/#2, or "calibration failure" are sent 
back. I somewhat doubt that they implemented a time-out on the later 
machines. Ensoniq somehow never liked timeouts -- e.g. if your Mirage 
after booting seemingly dies, just wait some 10 minutes until it finally 
fails filter calibration of all 8 voices and it most likely will spring 
back to life.)


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