CAT SL-1 Reference repair

best to call CAT in upstate NY & have them repair it. They know their products well & can fix any of their old products.
You will never find a sch of any of their products on the web as they never release any of this info. All of their topologies are proprietary & many of the internal components are also proprietary. If you’ve opened the box you’ll see that none of the caps have any values on them - just a catalog number. The CAT factory has the decoder key to match catalog number to cap value.
Let the factory take a look & tell you what’s wrong....
excellent preamp, if i may say so.

 if you're onto DIY,
first, check if tubes light up on you phonostage.
2nd do visual on phono-board by checking any signs of burning or caps leak with magnifying glass
3rd if tube(s) light up, you have to measure bias current or voltage accross bias resistor of each tube. bias resistor can be found in grid circuit and the grid pins can be identified via internet just by querrying your tube ID.
4th, if you're local enough to raleigh nc, i can accept your unit and make it work and if not, unfortunately i don't do shipping turnarround. 

Assuming your TT was working fine in your previous preamp and both channels are equally affected, do you have a digital source hooked up to the line stage? If so, turn it off and then run the phono section. If you get the same thing then it has to go for repair if changing the tubes didn't help. The only thing common to both phono channels is the HT supply to the output 6922 and it looks like CAT uses n-channel mosfet regulators to the plates.

Thank you all for the replies. I tried both MC and MM, neither works properly. I checked up tubes, all of them light up. Thank you czarivey, I'll follow your steps to check the bias.

Yep check values of bias resistors by removing them from circuit and if one found out of spec, replace resistor and tube. Usually bias resistors goes bad when tube does. Replacing DC caps in the chain of grid bias supply  would also not hurt preventative.
Also would retouch existing solder in case if I spot cold joints. With tube electronics it's very common and often done before any troubleshooting begins.