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VT320 CRT Replacement -(Mostly) Successful

Slob

Experienced Member
Joined
Oct 20, 2017
Messages
172
Location
Kentucky USA
So, I was playing with my VT320 terminals that I painted and cleaned a few years ago. One worked very well, but one was dim even when turned up to max brightness. Both had burned amber CRT's. I thought that it was now or never on the CRT's, so I searched around for replacements. I ordered a few (in paper white, they had amber and green also) for $55 ea and shipping. The shipping was a little high but they did get to my house in good shape. I REALLY, REALLY don't like fooling with CRT's, but I've done it when I needed to, and I haven't been zapped yet.

Replacement went as close to drop-in as possible; the mounting ears fit perfectly. The only problem I had was that the faceplate mounting strap buckle interfered with one of the tabs at the bottom of the electronics base (the original tube had the strap buckle on the side). A little nibbling on the plastic took care of that. This is a cleverly engineered but VERY tight chassis.

I had to make a fairly radical adjustment of the G2 (screen) pot to get the tubes to show a display at all, due to differences in the electrical properties of the new tube (the CLINTON/PHILIPS jumper made little difference). Either I had to turn it up or down all the way. The unit that was dim before (I thought, due to the tube wearing out) was still dim. But the actual image quality of both displays was great. The replacement tubes don't seem to be as tinted and therefore as contrasty as the originals, but, well, I'm just glad to have replacements at all. Who knows how long they will be available.

My knowledge of 1's, 0's, and voltages up to about 24 volts is pretty good but I'm iffy on high voltages/flyback circuits for television tubes. I am considering changing a resistor value a bit (R212 or R224) on the upper right of page 12 of the K-TC-VT320_Schematic_Jul87.pdf that is available on the Internet. The "stock" circuitry does not seem to have much travel for brightness anyway, even on the original tubes. Due to the tightness of the chassis, if you make a change you basically have to take it all apart and put it all back together just to test a change. Does anyone have any ideas?
 
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