The Set: Pete Deksnis's Site about the CT-100
Restoring a Vintage Color Television Set
Symptom: Search: Solution:
Well, the cantankerous beasties that they are, your CTC-2
chassis in your CT-100 will require newborn-like attention
to care, calibration, and overall healing therapy.
CT-100 with Over The Air video
High voltage drifts below 19.5 kV during setup and alignment with known good vertical convergence transfomer.
Always be aware of the line voltage applied to an operational CT-100. High voltage, brightness, focus, and convergence adjustments vary with a drifting line voltage.
Know the parameters under which your CT-100 operates. Antenna input, line input, and user controls affect the final video presentation far more than with today's digital, microprocessor-controlled television circuits with switching power supplies. For actual power measurements of an operational CT-100 using two 1N4005 diodes in place of the original selenium power rectifiers,
[11-7-05 Pete Deksnis]
Two minutes after a cold start the screen goes dim. Video is there but weak with low brightness and contrast. Color is depressed but present.
Monitor the AGC at a convenient point such as the tuner AGC terminal for a steady AGC level at the interface between normal and anomalous operation. The AGC circuit requires a horizontal pulse taken via a dc path from the first video amplifier. Since the AGC circuit continued to operate after the failure interface, video amplifier two became the prime suspect.
Replace the second video amplifier tube 6AN8.
[11-7-05 Pete Deksnis]
With a 75-percent red baseband signal modulated on VHF channel 4, a red screen is seen. With a 100-percent signal, the screen turns mud-orange-green.
Click here to continue.
[11-19-05 Pete Deksnis]
…it takes about 2–3 minutes for the color to pop in from a cold start. Afterwards I have to set the fine tuning all the way to one end to get a color pix; I would like to see the fine tuning knob to be closer to the middle of the range. Because I have problems with my cable overloading old tuners, I am running it through a VCR/DVD player and going into channel 3 of the set.
The color problem I was having was the tuner. I put the fine tuning in the middle of the range and adjusted the tuner strip slug closest to the front and the color came right in.
[11-22-05 Steve Kissinger]
Large horizontal chunks of the screen turn solid blue or solid green or sometimes both blue and green areas of color appear.
Video quality recovers in 0.5 – 3 seconds. Afterwards, color, brightness, contrast, and sweep geometry are all immediately normal, although particularly large chunks of the intense overdriven color are followed by high-voltage regulator recovery.
This anomaly is unaffected by brightness or contrast control setting. The effect was still visible with the brightness and contrast controls fully counterclockwise. Sharply defined edges of the anomalous shapes coupled with the very solid colors suggest dc coupling rather than RC network involvement. In other words, it 'looks' like a short; it is not characteristic of a video signal being processed by amplifiers and reproduced on the screen.
A look at the schematic shows two possibilities. Either the 15GP22 is intermittently shorting internally somewhere, or the 6BC7 triple-diode that serves as the dc restorer is shorting. Light tapping of each tube during an episode does not aggravate and therefore pinpoint a perpetrator.
I replaced the dc restorer, but unfortunately the problem has recurred. T'shooting continues... [1-15-2006 update: unfortunately, the consensus is that the CRT is the source of the problem. Fortunately, it has been three weeks since the last episode. Case closed for now.]
[12-27-05 Pete Deksnis]
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