1948 Motorola VX101 brightness changes picture size
 

I am restoring a 1948 Motorola VX101. It's fully re-capped, all the tubes are tested and good, and at this point it basically works. But it has a strange symptom. The picture is very dependent on the brightness and contrast. Too little of either and the picture goes dark - not that surprising. But too much of either and the picture "blooms," i.e. expands rapidly out of control and then disappears. If the brightness and contrast are just right, the picture looks great, but it does not seem as if the brightness or contrast should affect the raster size, and it does not seem as if the picture should be so sensitive to having just the right brightness and contrast settings. Any suggestions?

Some blooming is part of the design of some sets.

Excessive blooming is often the symptom of a failing HV rectifier.... basically the higher the gun current the higher the HV current which causes marginal rectifiers to pass less HV. Less HV means darker picture, and also slower current in the electron beam which means bigger deflection because the slowed electrons in the beam spend more time in the influence zone of the yoke.

Other potential issues are bad H output and or damper, issues with the flyback, H Osc or power supply.

Some blooming is to be expected, and if the set is 50s or older you may want to set brightness for just slightly above the minimum for good viewing in a room with subdued lighting. Many sets that old would fall on their face if you asked them to make a watchable picture in the sun...

Lots of posts here say don't trust a tube tester to tell you if the high power / high voltage tubes are OK. I would try tube substitution first.

+1 I should have said that too. I've had testers miss sweep and HV rect tube failures that substitution caught several times before.

Hi All ... Thanks for the input! I was also thinking that perhaps the HV rectifier was at fault. Of course, it tests OK on a tube tester, but as you say, that does not tell the whole story.

As it turns out, however, yesterday (the day after I posted my message) I ran the TV for about 6 hours, and it did not show the same symptom. A slight amount of blooming with the brightness or contrast, but only what I would expect. I think I will need to wait until the problem reappears before I can do any substituting and/or testing. Do you sometimes find that these sorts of problems are self-healing, i.e. go away after the sest is "burned in" a bit?

On another note, I found that all six of the tubes in the IF receiver train were making poor contact with their sockets. These are all of the smaller variety, i.e. pins coming directly out of the glass with no plastic base. It was easy to test - I just put my finger on each one in turn and wiggled it a bit. None of the "octal" tubes showed any problem in this regard.

In the past, I had never had this problem before, and so I had not been cleaning all the tube sockets as a matter of routine - but I think I will in the future! What seemed to work was to spray some contact cleaner into each pin receptor, followed by inserting the tip of a toothpick into the pin receptor and twisting it. Dental picks did not apply enough abrasion to clean the socket. I also lightly sanded the pins on the tubes, just to be thorough. Voila, the set no longer reacts when I wiggle the IF tubes!

If it is self healing it probably was moisture in the flyback. Set runs a few hours and it boils and or evaporates the moisture and the flyback returns to working to spec.

Great input - thanks!

By the way - my bad! I meant to say that I'm working on a 1948 VK-101, not VX-101 (doesn't exist).

Well, the problem did reappear, and so I swapped the HV tube (1B3). Since then, I have operated the set for about 8 hours over two days, and it is rock steady. So, thanks everybody for your help!