The Set: Pete Deksnis's Site about the CT-100
Restoring a Vintage Color Television Set
Thank goodness for newly manufactured versions of vintage CT-100 parts.
The last of my three RCA vertical convergence transformers (part number 1104314-1) failed. One is used in a CTC2, the CT-100 chassis. It is mounted on the rear of the high-voltage cage.
Sunday morning June 18, 2006, the fuse protecting the H-V cage blew on turn-on. Unlike my two other failed transformers, however, which still display correct dc resistance, this failure wiped out the secondary. The tap is completely open.
There are about 12 Meg between the top and bottom of the secondary where about 12 k-ohms should be. This part has an extremely high failure rate. Probably second only to the 'white' peaking coils that rot open.
For the repair, I tacked in one of John Folsomís newly manufactured replacements I've had for about a year now. Seen below, it was static-tested at about 4 kV across primary-to-secondary for a few hours when it arrived last June, and so I expected a no-issue installation.
On the bench, the H-V ran a steady 21 kV with the new transformer in place and the H-V control set wide open with a 115-Vac line. The focus voltage, which passes through the secondary and is the reason for all the stress on the transformer, is steady at 3.5 to 4 kV, depending on the focus control position.
Pictured below is the new transformer on the rear of the H-V cage.
In the future, Iíll remove the failed core from the transformer housing and use the original case around the new transformer.
The failure of a vertical convergence transformer is particularly insidious because a common failure mode causes a chain reaction that burns out the focus pot. Too much focus current is drawn if the secondary shorts to ground; the 1X2B focus rectifier plate goes cherry, and the pot can literally blister on the outside and disintegrate internally.
Returned the chassis to the cabinet today, cranked the H-V control back to the specified 19.5 kV and while I havenít reconverged it yet, it's perfectly watchable.
I performed a full convergence on the set four days ago, and the newly manufactured vertical convergence transformer is doing a fine job. The procedure improved convergence dramatically and, while not absolutely perfect of course, I am very pleased with the overall 15GP22 image.
Here are the two minor gremlins: On the far right of the screen the three beams diverge: blue sloping downward and green up. On the top of the screen there is a linear displacement of the red beam that puts it below the properly converged blue and green beams. After you move back about four feet from the screen, these pesky anomalies are unseen with normal eyesight -- if/when program content exposes the flaw.
A minor inconvenience is the weather. This is the second time in recent history that a spell of humid weather affected the high voltage. Not certain where the damn humidity planted the unwanted load, but it's highly unlikely that the new transformer is a factor. The last time this happened was a week after replacing the transformer. A three-week hiatus with no power applied to the set brought it back: the high voltage recovered after the weather turned dry.
With the remnants of a hurricane passing through here recently, there's been a lot of rain -- the grass is so green it looks fake. So what better time to again check H-V under these extended high-humidity conditions.
With a 115-Vac line, I pulled open the H-V lead then applied power. The H-V swung right up to 21 kV (about right for a CT-100 unloaded H-V supply) and stayed there.
Plugged the CRT back in and powered up again. Sure enough, there was some mild audible 'frying' and then the H-V settled at a slightly low 17.5 kV, but in a few minutes it was back up to 19.5 kV.
I believe the culprit is leakage from the flange; last year after restoration I had to remove and clean the CRT and rubber insulator that wraps around the flange to arrest the same frying/leakage.
Not looking forward to pulling the CRT for another cleaning. Not so much for the work involved, but I don't like potentially messing up this satisfactory alignment.
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