Ct-100

[Tom Albrecht]

I've been enjoying watching this set over the past few days, but there was one major shortcoming -- the focus stability. I could adjust the focus for a poor compromise: slightly out of focus for a dark scene, nicely in focus for a medium bright scene, and completely blown-out of focus for a bright scene.

Measurements showed that the problem was not insufficient HV anode voltage regulation, but rather unstable focus voltage. The focus supply is completely unregulated, and unfortunately it seems that the current drawn by the focus grid of the CRT varies quite a bit with scene brightness.

Looking at the circuit, I got the idea of using a high voltage zener diode to stabilize the focus voltage:



I replaced the resistor below the focus pot with a 3000 volt zener. Or more precisely, a string of 15 Z4KE200A 200 volt 1.5 watt zener diodes. Here are a few pictures:





Somewhat to my surprise, this is a fantastic fix. The focus goes from being completely problematic to rock solid. I can turn the brightness and contrast up all the way, to the point the picture looks very bad in other respects, but the focus stays sharp under all circumstances!

Now I realize this isn't really "preservation," but given that I have had the experience of seeing how the set works in its original state, I would say I have experienced that enough, and now I'm ready to watch a set that really works.

Given that my high voltage cage doesn't look particularly original any more because of the big voltage tripler module, this little insertion of a string of small zener diodes is hardly noticeable, and of course is easily reversible.