Happy Labor Day on CTC-5
 

Happy Labor Day!

Interesting monochrome shot with color captions on your CTC5. I see a grayscale change from the bottom left compared to the top right. Was that the photo still or the CTC5 display?

(FYI on my CTC5 there is a slight purity error on the top left. I do not recall the manual discussing degaussing. Only setting the permanent magnet edge purity adjustments.)

This tube has a white field purity problem in the lower left that I have not been able to adjust out. The pure R,G,B fields don't look so bad. White field purity issues were fixed by the invention of negative guardband tubes, where the shadowmask holes are larger than the phosphor dots (which are then masked by black surround).
This is a GE-branded rebuild, and I have vague suspicions that the guns may be rotated a tad.

Actually, there are instructions to degauss the tube, BUT they don't tell you that it's imperative to go back to ground zero by retracting the edge magnets before degaussing, or degaussing will actually magnetize the cone opposite to the fields of the edge magnets, making it necessary to readjust them and maybe impossible to reach a good end result on the edges.

Well, given the rarity of these things, are more than worth maintaining it working despite this purity issue.

No question of keeping it working. It is only technical minds always are curious about these things. My CTC5 is very watchable and I enjoy using it a couple of times a week for a couple of hours each time.

My CTC5 has a similar purity issue and is using it's original CRT (date code 56-30 or July 23 to 29, 1956).

Just rotating the tv cabinent in a room 90 degrees means checking purity. It appears best to me to first retract the edge purity magnets, degauss and then readjust the purity magnets, to do a thorough job. As I only have a minor impurity on the top let of the screen, I may simply just tweak the magnet to touch up the corner impurity.

The scan linearity on my CTC5 is now surprisingly good and remains consistent as the set warms. I found a number of the carbon comp resistors in the vertical sweep circuit drift with temperature. And I had to carefully select the metal film capacitors I used to replace the paper dielectric units to maintain good vertical linearity as the set warmed. I have noticed sets not using a vertical blocking transformer and instead just use feedback from the output transformer especially prone to linearity drift with temperature.

Could you explain a bit more about the vertical capacitor selection? I don't seem to have a significant drift in linearity with warmup, but it's nowhere near perfect either. I'd have to look again, but I think when the top and bottom are symmetrical on average, the very top is pinched a bit, and there is a noticeable transition near the center of the picture that's visible with vertically scrolling titles. Replacing tubes helped, but didn't totally fix it.

My set has developed a brightness drift - too high when first turned on, drifts darker over a minute or two.

I discovered in my set a number of resistors that drifted badly with heat. I replaced quite a few with NOS carbon composition resistors and and it addressed a number of issues including the brightness drift you described. Perhaps it was just a bad batch of resistors? The replacement carbon resistors do not have the same problem.

As for the selection of the capacitors in the vertical sweep circuit, I was plagued by quite noticeable linearity and height changes in the vertical sweep circuit. I found a number of drifting resistors here and frustrated with it replaced all the resistors in the circuit. It was much better but there was still an minor annoying change in linearity as the set warmed. I recall replacing C512, C513, C508, C507, C511 and C504 a second time to address it. These were all new DigiKey replacements I was obliged to change again with another brand of capacitor and I cannot recall the specific one. But once addressed, the sweep linearity is excellent and there now no noticable change as the set runs.

It just seemed odd to me that despite the capacitors testing good I could not identify the chacteristics of the capacitor change with heat that affected the sweep.

Capacitors can have surprising temperature coefficients. But, the good ones will be more fair, having near the indicated tempco for it's dieletric.
My guess is caps from not so good maker or bad batch.
Or, it sum and subtract from resistors tempco, then you found ones compensating for that...

Yes I got a bad batch. Probably my fault as I went for the cheapest.

I recall when I was trying to fix this that magically I must have found the right combination because the drift stopped. But it was only after changing the resistors first and then playing with the capacitors.