National NC-TV7M 7" TV

This set was given to me many years ago, but after sitting around, it became apparent that it needed a lot of work before it would ever play again. One of the more obvious clues was a paper cap which had split open. It's the black one at the top left of the picture:

Paper caps in any electronic equipment need to be replaced. Even if they're good now, they will fail later, or at least degrade the performance. There were four 40 MFD 450V caps which I replaced with new ones. Normally I would reform them, but these were pretty bad off. There is also a can type electrolytic on the chassis, but it's relatively new, so I just brought up the B+ slowly.

Carbon resistors usually hold up well, if they're the Allen-Bradley type, but the ones in this set were mostly of lesser quality. Most of them were just a small amount out of their tolerance, but a surprising number were at least double the marked rating.

The horizontal sweep output tube is the one near the center with the mica trimmer. This stage operates in push-pull, and each plate load is a 47K resistor. In this particular set, they were each replaced with two 100K 1 watt resistors (a perfectly good substitute). It would seem that a 47K load wouldn't draw that much current, but the output stages run on +445V, and return to -260V, which is over 700 volts. That translates to a rather large amount of power. Of course, the 100K resistors charred slightly even though they were the right ones. I used some larger carbon film resistors as replacements.

Just about all of the pots were frozen in place, presumably the result of contact cleaner residue, or the grease which was originally used. I don't believe they intended the set to work for 55 years. I was able to disassemble most of the pots, clean out the old gunk, and apply a film of Lubriplate. The vertical hold pot, and both size pots were replaced. The focus and centering pots have long plastic shafts, and the focus pot was so bad off that I replaced it with two fixed resistors. The horizontal centering pot budged enough to center the image, but the vertical centering pot just happens to be frozen where it needs to be.

Here are all the parts I replaced:

It didn't take long to replace the parts, especially since it's very relaxing to work with real components after working with surface mount parts.

After all the parts were replaced, I did a quick ohmmeter test of the B+ circuits. The lower B+ showed about 1K to ground, and after a little probing, I found a capacitor ground lead touching the sound IF plate supply.

I monitored the B+ and B- supplies, and got a raster on the first try, but no sound or video.

The signal circuits were dead, and the tuner & IF filaments were dark. Looks like an open filament choke, but there was 6 VAC on all the sockets. Wiggled the tubes, and they all came on, one by one.

That's when the set came to life - great sound a pretty good picture, considering that this was before the days of keyed AGC and synchroguide;

This set doesn't have a flyback transformer or a yoke. Instead, it's electrostatic deflection, just like an oscilloscope. In fact, it is a scope with an FM radio tacked on.

One of the neat things about these sets is that you can watch the picture on the back of the screen just by looking into the side of the tube;

Here's what the chassis looks like with new parts;

Real radios & TV sets glow in the dark.

This picture shows the tube filaments. The white light is the CRT screen, and some of it is reflected off the focus grids in the neck of the CRT.

In the high voltage cage, the orange glow from filament of the 1B3 can be seen, and the blue glow is caused by residual gas in the 12AT7 which drives the HV transformer.