The Set: Pete Deksnis's Site about the CT-100

Restoring a Vintage Color Television Set


restoration log

7-2-99 In pursuit of my goal to restore THE SET to full operation, I formulate mini- or sub-goals as incremental steps in the overall process. Whaat?

Okay, I got this plan to bring up sections of the CTC2 chassis separately. First thing I did was isolate the power supply and fire up the filaments. That done, it was clear that my next job was cleaning. That's still in progress, but I got itchy to move ahead with a plot to restore the electrolytics. Now apparently some guys make a living replacing paper and electrolytic caps in vintage tube stuff, and more power to them. But my goal is to replace absolutely nothing that doesn't clearly need it. (Who hasn't heard, "If it ain't broke, don't fix it.") So, using the CTC2 schematic from Sams, the production schematic, I color-keyed all references to a given power bus and proceeded to find out what ain't broke.

The 275-volt bus supplies the last five stages of video IF, the two audio IF stages, and the two audio amplifier tubes. These areas are the least complicated and the one I expect to restore to operation first. Much of the remaining low-level circuitry requires an operational flyback -- keyed AGC, chroma burst amplifier, and others such as convergence circuits all require horizontal-rate pulses taken from small secondaries on the flyback transformer.

A 285-volt bus is derived, as is the 275, from a ballast resistor in series with the main low-voltage bus -- the 380-volt output of the voltage-doubler. This 285-volt bus supplies many of the color and Y-video signal processing tubes, the sweep oscillators, and convergence circuits. This supply also feeds the tuner, where the first IF stage resides.

7-3-99 IN PROCESS THIS Fourth-of-July HOLIDAY WEEKEND... Catch up time. I started with the two 200-uF, 250-volt electrolytics in the main voltage-doubler power supply. With the ballast resistor assembly removed, capacitor C1 is effectively isolated from the rest of the circuit. So I fired up a 0- to 400-volt regulated supply (tube, or course, circa 1960, with parallel 6L6s as the pass tubes). I connected leads from the supply to the terminals of C1. The metal canisters of both C1 and C2 are insulated with a heavy paper sheath manifesting the danger to life and limb when working around and with tube stuff. I mean, who hasn't gotten bitten by an inadvertent brush with a hot circuit.

Anyway, starting with zero volts across C1, I brought up the PS slowly while watching the current meter for spiking. There was some, but I managed to get both C1 and C2 to 100 volts with a quiescent current of 5 mA. Capacitor C2 wasn't quite as well isolated, however. There is a clear path that put a couple of backward volts across a 40-uF electrolytic in the screen grid circuit of the 6CD6 horizontal output tube. Six or seven short-term reverse volts probably aren't going to kill the cap, but...

Later, I did a better test. At least the 40-uF had some forward volts across it. The doubler caps C1 and C2 are connected in a series-aiding configuration. What would happen if I popped a couple hundred volts on top of the series combo C2 and C1 and carefully monitored the individual drops on the way down to ground, which includes the filter choke that develops the minus 30-volts bias? It worked out nicely. With 260 volts on top, the total quiescent current is 5 mA. Seemed OK to me.

[4-23-2005 update: 40 uF, 200V electrolytic [3C229(RCA); C19(Sams)] and 50 uF, 50V electrolytic [3C314(RCA); C20(Sams)] can cause the preceding test to fail.]

Next I attacked the 275-volt bus. It is well isolated from circuitry using other buses. Fortunately everything went extremely well. I took about thirty minutes to bring the bus up to the full 275 volts, watching, as before, the current meter for unusual spikes and excessive current. You can actually watch the forty-five year old electrolytics reforming. Where twenty-five volts once caused 10 to 15 mA of quiescent current, 150 volts are needed for the same level only 15 or twenty minutes later. Once I hit 150 volts and the current slid down to 5 mA (that seems to be the benchmark), the run up to 275 was a piece of cake. At 275 volts, the quiescent current is not more than five milliamps.

Happy with those results, I pressed on. The 285-volt bus is also easily accessed at the octal socket for the ballast resistors. This time there was a variable. There are several resistance dividers directly to ground on this power bus. Most draw insignificant amounts of current. Two draw a mill or two. The result is about five mills of built-in current load. Added to the benchmark quiescent 5 mills, we're looking for about 10 mA of 285-volt quiescent bus current. Didn't turn out that way. Best I'd achieved at first was a whopping 25 mA at only 150 volts.

[5-31-2004 update: Test the 380- and 375-volt buses together. Plug in the cross purity coil (be sure it's in the correct socket and not the field neutralization socket). Connect the power supply to the 380-volt line. Crank up the voltage in 25-volt steps watching for current surges. Back off the voltage, then gently 'nudge' any point where current surges occur. Once you've reached 380 volts, look for a steady current draw of about 30 mA.]

7-4-1999. Okay, here's where Sams is wrong. The vertical convergence output transformer, T12, does not go to the +375-volt bus as their schematic claims. It goes to fuse M5 which is connected, via one jumper acting as a safety interlock on the yoke connector, directly to the main power supply output (380V). The RCA schematic shows it correctly -- except that it bypasses fuse M5. So neither was 100%.

Also found another anomaly. Both the RCA and Sams schematics and the Sams parts list call for a 27,000-ohm resistor in the plate of the vertical convergence amplifier (1/2 a 12AU7). In actuality, THE SET has a 270,000-ohm resistor there. Perhaps as a consequence, the 0.22 uF, 400V coupling cap (C113 on Sams / 2C206 on RCA) connected to that plate is seriously leaky. It needs replacement. This was all discovered while troubleshooting the 285-volt bus problem, as noted above. [9-4-99 Update: Received e-mail from another collector restoring a CT-100 -- that CTC2 chassis also has a 270,000-ohm plate load resistor.]

7-6-99 Spent a few hours cleaning components around the bottom of the chassis. About 1/3-the time was spent investigating the strange 12 k-ohms between the 285 and 380 busses. More to come, my day job is calling...