Vintage Collins Station - Update (March 13, 2013)
After obtaining the KWM-2A, the S-Line took a back seat for several months. One day I decided to operate it. Both the 75S-1 and 32S-1 were inoperative. The 75S-1 was a bad spot on the RF gain level (inside the cabinet just below the meter). That was easily fixed with a bit of deOxit. The 32S-1 would work, after a fashion, but it took a nearly clockwise rotation of the microphone to get 20 or 30 Watts of output power.
Also, the grid drive was erratic. I suspected some type of ground loop, or a cracked resistor. I loosened and retightened the screws for the final RF cage, all tube sockets, and the three shield cans on the bandswitch assembly. I then started going thru the 32S-1 looking at and checking each resistor. Finally the culprit illuminated itself. A dead spot on the IF mixer adjustment control. Unfortunately deOxit wouldn't provide a cure so I bought and installed a new control (R-27). Now the 32S-1 provideds 120 Watts of output power when the microphone control is in the 9:00 o'clock position in Tune/Lock.
Then I found a really good deal on another 30L-1, a 32S-1 as well as a 75S-3. They have been added to the lineup and are being used on almost a daily basis. Truly my Collins station has taken on a life of it's own.
During initial use of the 75S-3, a few minor issures were discovered. First, the receiver is very anemic on 15 meters. Also, there is an ac hum indicating defective filter capacitors or a leaky filament to cathode in a tube. There is some drift (400 cycles or so) in the first 30 minutes of operation. And lastly, the S-meter readings are excessive. For these reasons, the 75S-3 is out of the operating station and on the bench for repair.
This is not to say I'm unhappy with the 75S-3. The rejection tuning works very good and the 500 Hz CW filter is quite sharp. Specifications of the 75S-3 can be found here Collins 75S-3 Receiver
One of the big limitations of the earlier S-line receivers was their lack of audio power. The 75S-3 only has 1.8 Watts output. If matched into a 4 Ohm speaker, this is just barely adequate. For this reason, I'm keeping my eyes and ears open for a good 75S-3C.