RCA CTC-5 "Wescott" restoration

[bandersen]

Proper fuses arrived today. I'll give them a try tomorrow night.

[bandersen]

We are back in business!


I did some more adjustments by eye and it's better but it's time to go through it methodically from start to finish.
However, I think I should replace the original 3A3 HV rectifier and HO tube before that. I say that because I'm having trouble filling the width and the image blooms/loses focus if I don't set the brightness and contrast just right.

Even so, it is watchable.

[Electronic M]

One thing that's worth doing both for blooming and improved HV is to convert it to the CTC-7 HV regulator circuit. The only difference is one of the CTC-5 fixed resistors was replaced with a pot and a fixed resistor on the CTC-7. This allows you to adjust the HV regulator as all previous and subsequent RCA color sets did. Setting it so it just starts to limit at the max HV spec on the schematic is usually a noticeable improvement over what fixed resistors and whatever emissions of the tube has presently arbitrarily achieve.

[bandersen]

Yes, I do plan on making that mod - thanks for the reminder.

I'm slowly improving the convergence and color hues, but a new problem has cropped up.


After running about 10 minutes, I get a dark band, then the image fades out, then the screen goes dark.
A few seconds later, it comes back fine for a minute or so, then the cycle repeats.
Probably temperature related, just seems odd that it recovers so quickly. Not much time for anything to cool down.
Not sure yet if the problem lies in the HV supply or the video drive.
Here's a short video showing the problem.
https://youtube.com/shorts/ODEe6L3QyIY?feature=share





Rather annoying as it's playing fine otherwise.

[bandersen]

Seems to be another bad ground connection. This time on the video amp board. Reseating the tubes and tightening the mounting screws has eliminated the problem for now.

While troubleshooting I did some voltage checks and found that the HV regulation is non-existent. Should be about 22.5kV. I get 19.5kV with brightness all the way down and only 14kV with it up all the way. The raw HV output must be too low for the shunt regulator to work with.

I've already swapped out the 3A3 HV rectifier, 6BK4 shunt, 6CB5 HOT and 6AU4 damper with little change. I'll try the 6CG7 hor. oscillator next. If no improvement, I may just need to pull the chassis again.

I should anyway to make the adjustable HV mod mentioned earlier.

Curiously after sitting overnight, the HV regulation issue improved considerably. Swapping out the 6CG7 help a bit more. Holds fairly stable at about 19.5kV.
Yes, I do need to adjust the efficiency coil. So nice of them to place it behind the HV box between the damper and HO tube. I'll need to fabricate a long adjustment tool.

Just in time for Saturday morning cartoons.

[etype2]

Excellent progress. On my CTC-5 Chandler, Mike found charred tube sockets from heat which contributed to nasty problems.

[Electronic M]

Sometimes the HV regulator without an adjustment circuit will get stuck turned on and lug down the HV.

[bandersen]

Interesting. That could very well explain it.

[Yamamaya42]

You let Simon Bar Sinister tune your TV?!

[bandersen]

LOL.
The linearity/efficient coil slug is frozen. I'll see if I can free it up next time I pull the chassis. Meanwhile it continues to perform well.
I'll touch up the converge once the HV gets up where it should be.