Hi all
I've got a pulse that i picked up broken S/H about 10 years ago.
I fixed the initial problem of a failed opto isolator on the midi input.
Also another issue was a failed 4051 multiplexer that was causing the pan to misbehave.
Latest problem is a bit more harder to pin down.
There's an odd noise that sits behind any of the 3 oscillators when triggered, the noise source seems to be un-effected but needs more testing. (its very faintly audible in the background..)
It more obvious once you start pitching down through the octaves on the oscillators below 0, ie -12 / -24 / -36 when set to triangle waveform.
Setting a longer release time seems to mask the problem, the decay setting has some effect on the nature of the noise.
Noise will move up and down in volume with the mix level for each oscillator, so with everything set at 0 = no noise when triggered.
Its hard to measure on a scope as well though have have just found some settings that do emphasize the problem.
It seems to beat against the main signal from the oscillator at lower frequencies.
I've looked at the multiplexers again and replaced both of those, also the S/H op-amps though only replaced one of those so far. (i ran out of IC sockets...)
Also had a look at the caps around the S/H circuit as well.
Seems like the control voltage is modulating the oscillators audio output. l'll test some more and see if any other things the multiplexer controls is effected.
MP3 of the problem:-