2. amps-preamps
  3. 123537-8471

CAT SL-1 Reference repair

I recently bought a CAT SL-1 Reference preamp. The line stage is great. But the phono stage doesn't work properly. I crank up the max volume but it doesn't give reasonable listening level and it became too noisy. The sound is dry and week.  It doesn't balance well either.  I replace all the tubes in phono stage but without luck. Anybody has a schematic of this preamp? I didn't find anything on internet. Any suggestion is appreciated. Thank you.
cygnus_859
gs5556
1,234 posts
 
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.
cygnus_859 OP
20 posts
 
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.
czarivey
5,946 posts
 
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.



cygnus_859 OP
20 posts
 
Thank you czarivey. I checked the bias voltage last night, all the plate voltages are very low <20V. I looked into the power supply and found a intermittent connection on high voltage regulator transistor TIP50. I solder it. Now the CAT is alive!  But one thing i notice that V1 / V2 (6DJ8) plate voltage is about 90V instead of 180V that someone suggested. Is this normal?
czarivey
5,946 posts
 
plate voltage in 6dj8 must be 90V.
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