Restoring CT-100 #B1111566 - Page 4 - Videokarma.org TV - Video

ChrisW6ATV · #46 06-05-2012, 03:10 AM

Quote:

Originally Posted by miniman82

You can do what I did and unplug the anode lead and regulator top cap, then measure HV with the probe.

Did you remove the HV regulator tube entirely and power your set without it for your tests? One thing I have not yet done is confirm that the high voltage is 17KV (or less) on the 15GP22; my measurement was on my test jig, which does not necessarily have a yoke matched to the CTC-2's circuits.

miniman82 · #47 06-05-2012, 10:53 AM

Yep, pull the 6B(D/K)4 out then measure HV. If adjustments need to be made, HV will remain low. It should make more than 19.5kv with the regulator out, then reg should pull it down. I do this with the chassis connected to the original yoke, just pull the anode lead to the CRT and measure.

ChrisW6ATV · #48 06-05-2012, 09:47 PM

Thank you for the tip. I tried it, and I get about 19.5kV without the 6BK4, and the same with it installed and the HV turned to maximum (which it had not been before). These measurements were with the brightness and contrast at minimum as specified for HV measurements, which I also didn't do before. The raster also seems plenty bright and sharp as well (but the drive line is still there), too. I am running the set through a Variac to give it 110V rather than the 120-122V that my outlets provide. Next steps: Test all of the resistors in the horizontal and HV circuits that I did not already replace, and measure voltages to compare to the service information.

miniman82 · #49 06-05-2012, 10:15 PM

Well, there's your problem! 110VAC is not enough, RCA specified 115VAC as the input voltage, but 117 wouldn't be unreasonable. Bump the line voltage up, HV should also rise but I would fix the drive line ASAP. I'm a little foggy, but I think mine put out 23kv with no regulator. Excessive drive can cause horizontal cathode current to spike, so run through the horizontal setup procedure. Adjust H-lin, width, drive and HV then see what the picture looks like. You'll probably have to do them several times through, since they affect each other to some extent.

Hint: before you measure across the resistor in the cathode of the horizontal output tube to get the current, measure its resistance with an accurate meter. I promise it's not the same value called out in the schematic, due purely to age. That will cause your current calculation to be skewed. Once you have the (actual) resistance, read voltage across it and plug those values into ohm's law. I got something like 140-160ma on my set.

ChrisW6ATV · #50 06-06-2012, 03:05 AM

I know that resistor is good, because I just replaced it as part of my restoration work. It had been missing and the cathode was shorted to ground before(!). I am going to check all of the other resistors in that area as well, and do the alignment.

miniman82 · #51 06-06-2012, 10:20 AM

'Good' is one thing, but you need an accurate resistance measurement to properly calculate cathode current. Taking it 'on faith' that it is what it says it is led me to artificially high or low readings, which is why I suggested reading it out first. I'm anal about horizontal setup, because a flyback failure means doom for these sets.

ChrisW6ATV · #52 06-09-2012, 01:30 AM

The cathode resistor is 12.2 ohms. On Saturday, I am going to buy replacements for all of the other resistors in that area that were previously replaced with miscellaneous parallel and/or series combinations, then install them, align, and see what I get. I will also make sure I have a lower-value resistor available for the drive circuit as needed.

ChrisW6ATV · #53 06-13-2012, 10:59 PM

I installed proper resistors and aligned the horizontal circuits; I had to replace the 47K resistor in series with the drive control with a 22K and now I can eliminate the drive line. HV is 15-16kV maximum when the drive line is gone, though. Horizontal output cathode resistor voltage was about 1.6 volts. This set also has some intermittent issues with the video circuits. (Originally, I feared that it was problems with the blue gun of the 15GP22, but it is doing similar things on my test jig's CRT.) My main goal with this set right now was to confirm the condition of the 15GP22, and it is good. The set still needs to have a lot of other resistors re-replaced as well as a number of peaking or other coils. I have other non-TV work that needs to get done, so I am going to put this set off for a while until I get caught up with some of those projects.