[sdiy] Non-DIY, but looking for repair advice...

[John Ames]

On Mon, 13 May 2024 14:18:13 +0100
S Ridley <spridley1 at gmail.com> wrote:

Excellent tips, thanks.

> - See if the output is hissy as well as hummy - that would suggest the
> fault is further back and allowing VCF and/or VCA noise to be heard.

There's a little hiss, but it's lower-level than the hum and not really
audible at all under normal circumstances. It *is* affected by the VCA,
though, so it's coming from the synth itself and not the sound card on
my workstation.

> - Is it low level in all filter modes?  A dirty filter mode switch is
> a possible cause of level drop.

I've definitely had issues with the filter switch being crackly, but it
seems stable now, after I worked it a bit. The level is about the same
in LPF mode with the cutoff all the way up as it is in HPF mode with
the cutoff all the way down.

> - Turn up the VCA initial level - that would rule out a low envelope
> level fault.

EG's working fine AFAICT - it drives the filter cutoff at what I'm
certain should be normal levels (zero cutoff to nearly full
passthrough, full cutoff to outside human hearing range) and affects
the VCA similarly; it's only the total output level that's an issue.

> - Does an external signal and/or noise come through full level?  If
> so, that would point to a VCO or buffer fault.

This is an interesting one. Noise comes through at about the same level
as the oscillator, but if I patch one of the oscillators on my kARP
2600m directly to the external in, it is *much* louder, drowning out
the CS-15's VCO almost entirely. Hmm...