This is a strange one, a super rare beast, but I had the opportunity to play with one the other day. The controls are a little tweaky and it's also sensitive to the position it's placed in the signal chain but when you get it right! Wooo, there's some great tones here; super long sustain and really punchy rhythm tones and with the parametric EQ you can really pick out certain frequencies that you want to accentuate or remove.
Personally the compression/sustain/noise gate part of this pedal is the one that I like, the parametric eq can be covered by other pedals but the sustain/noise gate section really is something unique. Flicking the distortion switch on gives you some nice clipped tones that add a nice smoothness to your tone.
There is very little info around the internet for these things and even TC Electronics struggle to provide info. I did discover that David Gilmour used a couple in his rig in the late 1980s though; http://www.gilmourish.com/?page_id=2111
Ok, so here's the description from the TC Electronics website (This description was in place for the reissue version that is also now discontinued!);
"One of the true grails of vintage effects pedals. This one really enhances your tone. Go from subtle compression to extreme sustaining effects at the tweak of a knob. This sustainer will prolong life of even your weakest tone otherwise destined for premature death.
The Classic Sustainer + Parametric EQ gives you studio quality compression in a pedal. Crank it to the extreme and it’ll give your sound that percussive “thud” brilliant for funk or country. And with its parametric EQ you’re in full control of which frequencies to target."
After even more searching I uncovered some gutshots of the pedal, check out the goofy 1980s PCB design;
After some more exhaustive searching and a trip to the WayBackMachine I discovered this;
Yes! A schematic file for the pedal. I'm going to have to study this one for a while until I really understand what is actually going on fully in the noise gate/compression section of the pedal but it's a pretty interesting design incorporating both OTAs *and* op amps in various feedback loop configurations. I'm probably going to have a play around with this design and see if I can distil the compression section into a little project for people to build up.