Just picked up a whole flock of 721 sets from another VK member. Got the console 721TCS, the tabletop 721TS, and an additional chassis (minus CRT) from a TS. Also picked up a nice fairly complete Crosley 9-408 which I put aside for a future project.
Pulled the chassis and drop the cabinets off at another shop where I will tackle the cabinet work. First up onto the electronics bench is the 721 TCS chassis.
Pulled all the tubes, and ran them through tests on a TV-10 tester. Found a few weak 6SN7s and 6AG5s, a 1B3 with essentially zero emission, and a 5V4 damper tube with an open section (cathode tab broken internally). Rest of the tubes looked good, and put aside. CRT tested good for emission, but lousy cutoff according to my CR31. Have a really good spare 10BP4 here, so no problem.
Seeing no oozing or other signs of damage to the electrolytics, I plan to bring it up slowly to see if there is any chance at them reforming long enough to at least evaluate the rest of the set. I knew that the voltage divider resistor string is a usual problem here, so I popped open the resistor box, and sure enough, the 1125 ohm section of the upper resistor is wide open. A quick transplant from the donor chassis fixed this problem pretty easily. The candohm resistor checks good on all sections. Resistance checks to ground show no dead shorts of any filter caps.
Putting an ohmmeter across the AC input showed no continuity at first. A good shot of deoxit and working the power switch a dozen or so times broke down the insulating "patina" from the contacts, and restored proper operation. Looked like the transformer primary was good! Took the time to clip out the 2 micamold caps from the AC power inlet to ground. Have had these things go "bang" from time to time when hit with line voltage, so just wanted to avoid pyrotechnics. They will be replaced with proper XY rated ceramics. The removed caps (.01/400V) test around 10 M leakage with a multimeter, FWIW.
Connected a DMM across the 5U4 socket plate pins (tube removed), and plugged the set into the variac. Brought voltage up a bit at a time to 115V, where I had ~750VAC across the rectifier plates, 6.3 on the heater lines, and no hums, smoke or other unwanted drama.
Put all the tubes back in just for the heater load, and let the set run at full line voltage for about 30 minutes with no unpleasantness. All tubes lit properly, and gave off enough heat to release some of that wonderful "hot dust, beeswax, and rosin" smell that we all love.
While things were cooking, I grafted a replacement width coil from the donor chassis to replace a broken one, and put some heatshrink over the brittle and cracked ion trap magnet leads. Ion trap coils tested good on ohmmeter. Have to do similar heatshrink job on the wiring harnesses going to the speaker and the pilot lamp, as well. The speaker field coil is used as a filter choke, so those leads need to be insulated well before trying to bring the B+ up. Will get some long lengths of color coded heatshrink tomorrow.
More to come...