My Christmas day 9T246 restoration! Almost.....

[earlyfilm]

Originally Posted by Kamakiri

Weird problem though. If I leave it on the channel that was on when I turned it on, the set is fine. As soon as I change the channel, the picture loses horizontal sync. Otherwise, I just watched a full episode of Gunsmoke on the thing.....

Horizontal sync that locks as the set warms up, but won't relock after a channel change is a relatively common problem on the early RCA sets using this and similar horizontal circuits.

The cause is misadjustment of the multiple controls in horizontal sync.

I don't have a factory schematic on this set, so take look at the Sams 74, fol 8:

http://www.earlytelevision.org/pdf/R...-Sams-74-8.pdf

If the .01 Mfd cap on the exposed end of T3 Horizontal Oscillator coil under the chassis is original, you can bet that this set was taken out of service before it was 10 years old. But, you say, this cap is paralleled by a 22K resistor, how could a little leakage hurt? Well it can cause some well-intended person to mess up the other horizontal sync adjustments!

After you have finished recapping the set, follow the alignment procedure on page 18 of that Sams. If you are lucky, you won't have to use a scope.

But if you have (or can borrow) a scope with a low cap probe, and you make the final adjustment and then repeat the full page 18 procedure to even things up, you will find that these early RCA sets were some of the most stable sweep sets made during that era.

Today we take for granted that the TV signal will be stable. Remember when this set was made, when ever the station switched from network to local, there was a slight time-shift in the sync. On the smaller stations, often the film chain would also be slightly off with the studio cameras, and location footage would be even more off. These circuits kept the RCA sets from rolling on scene changes, unlike the sets from many of their competitors.

Re: Mica caps

Try to avoid bending the leads of the old ones. This seems to start problems.


Re: Electrolytic caps

If you decide to keep the original electrolytics, add fuses ahead of them to kill the power when they fail.

I won't join the argument on replacing electrolytics, but I am more inclined to leave a pre-1960 one in for set testing (only after disconnecting it and reforming) than a post-1970 one in any circuit. Either way, restuff or replace, carries a definite risk unless you can find relatively new industrial or military grade electrolytics. I am unimpressed by the quality of currently available electrolytics for tube-era voltages. They seem to be made to last the length of the warranty.

James