I finally got to the point of having to pull my C4000 out of the rack and go through it.
The Hologram was making ugly noises when the volume was past 9 o'clock.
There was hiss, some low level distortion and also some hum from the power supply's rectifier current switching pulses.
Initially, my aim was to improve the power supply, so that the rectifier pulses were not getting into the noise floor. The use of snubber capacitors and some careful rerouting of grounds netted me more than 6dB improvement in s/n ratio. (This was brought on my the need to play my Ultimate Fireworks Video, which has over 85dB of dynamic aperture between ambient sounds and explosions).
Later, I started looking into improving the filtering and bypassing of power on each of the circuit boards. I was finding a lot of audio garbage on the Vcc +/- lines on many parts of the preamp. So I started added bypass capacitors to several cards that had none whatsoever. It was a wonder the preamp didn't oscillate at all manner of frequencies with only capacitors at the power supply and nothing on the circuit boards!
The Bandpass card was a weird exception... Instead of +/- 15V, there was only 11.63V on those busses, and there was almost a half a volt of audio signal riding on the power lines! I traced that to a couple of 56-ohm resistors in series with the Vcc leads.. there was only 4.7uF filtering those.. ultimately, I bypassed the 56 ohm resistors with 4.7uH chokes and changed the caps to 25uF each. That cleaned up the audio bleeding into the power on that card.
I still have a bleed through problem with the volume at zero, and the Tone controls engaged. I think I have traced that to an improper ground return for the Bass and Treble pots. Their ground is on a different PCB than the rest of the circuitry, so that ground may have audio junk riding on it. Bleed goes away with Tone off.
Many of the 4138 op-amps were oscillating at 30-40MHz, and some of them in the Hologram B board were doing so at portions of the audio cycle, just before 270° and just after. Down stream, this resulted in some funky looking distortion. The chips were badly tarnished (pins) and this is a common problem with TI chips from the late 1970s. The fix was to run a pencil erasor across all the tarnished pins, removing the conductive carbonized buildup that was producing a semi-short between pins, causing the chips to become unstable. A followup check of the Hologram revealed that the distortion was eliminated.
I've added chokes in series with the AC power to the transformer, more chokes in series with the rectifier bridge and poly caps bypassing the electrolytics in the power supply. Any PCB that has electrolytics also got poly caps across those, for added HF suppression.
There's more to do, but the preamp is quieter, massively cleaner sounding, and oscillation is gone. Later this week, I plan to attack the ground reference problem on the tone card, to fix the bleed through and cross talk between L & R channels.
Just thought I'd share that with ya'll.