Crumar Musical Instruments User Group.

Here's a bit of an update:

I've traced the pitch CV path back through the DAC, the adder for the
arpeggiator and the keyboard scanning ICs.

I found that the keyboard scanning clock oscillator wasn't quite running
at the specified 20kHz, so I went and tweaked it (not that I guess this
should matter *too* much). The odd thing is, if I have my multimeter probe
on the oscillator output, everything works. However, if I then take the
probe off, you only get one more key press before the scanning stops
working again.

This is nothing to do with the extra loading perhaps detuning the osc, as
it runs over quite a wide range of frequencies. Strangely enough, the
multimeter probe trick only works if it is in frequency measuring mode!
Not in voltage measuring, or anything else! Peculiar!!

I can only guess that there's something slightly wrong with IC1 (the 40106
multi inverter chip used to form the oscillator), or IC5 (the 4081 multi
AND gate chip) that generates all sorts of other signals, on the keyboard
PCB, P-1016.

The "OSC RUN" signal is high, so that shouldn't be stopping the
oscillator. Hmm, I think I'll have to check if that diode 41 is OK.

Any clues anyone?

Cheers,
Matt Nolan.
--
Chief Knob Twiddler - Teeth Records
               Bath - UK