Some time in the late 1980s, my C4000 began producing what I would describe as "bleedthrough distortion" when the hologram was engaged. This bleedthrough was constant, regardless of master volume setting. With the volume at zero, the bleedthrough was the only signal audible, when input signal was present. Normally, this bleedthrough would cease after a short 5-10-minute warmup and it was fine. I lived with that for many years.
This past year, however, I've been noticing sporadic and occasional 'crackling' that would be audible on louder peaks of input signal, but then would clear up after a while. For some time, I thought the problem was dirty pots on an EQ in the ext proc loop, and other times, it seemed the switches on the C4000 were the culprit. Nearly ALL of the pushbuttons are noisy and will impart crackling noise in the output if touched in the slightest manner while playing.
But this past week, I think I narrowed it down to the Sonic Hologram hardware itself, as I was observing a particularly nasty kind of distortion on louder musical peaks and with the input levels at higher values. If the input level from signal source is low, there is no audible distortion--this only happens when louder portions of program appear at the C4000's inputs. I was able to RELIABLY repeat the injection and removal of distortion by engaging and disengaging the Sonic Hologram during a particular piece of music where the distortion was particularly dominant.
Now I think the problem is specific to the Hologram card itself. Has anyone confronted this particular gremlin in the C4000 and if so, have you been able to locate the component that was at-fault? My intuition tells me it's probably a semiconductor flaw, possibly ultrasonic oscillation triggered by signal levels above a certain threshold and not a capacitor issue, but I'd like to get a better idea as to what it might be before I tear the unit apart and start working on it.