CT-100 performance mods

[miniman82]

This will be a thread containing circuit mods which may increase picture performance on the CT-100 (CTC-2) chassis. They are basically 'updates' to the old chassis, same ones RCA themselves made to later chassis having to do with how HV regulation and vertical height were controlled. Both these pertain to the B+Boost line of the flyback transformer, which goes unused on the stock CTC-2.


First thing you will notice on the CTC-2 schematic is that grid bias for the 6BD4 shunt regulator tube is in the string of resistors that make up the convergence bleeder. During testing of the proto CRT, it was found that current draw going to the convergence grid tended to bleed voltage away from the HV pot. This allowed CRT anode voltage to creep upwards any time convergence potential was increased, so in effect convergence always lagged the anode no matter what we tried.

The fix is to take the HV control out of the convergence string, and tie it to B+Boost instead. It's a very simple mod, just lift the wire coming from the 50 meg resistor, move it to ground. Then take a wire from terminal 1 on the flyback (B+Boost), run it though a 2 meg fixed resistor, then to the open terminal on the HV pot. This arrangement gives you 420v on the high side of the pot, 270v on the low side. Pretty close to what I measured originally when it was still in the convergence string, which was 468v high side, 303v low side. During testing, the HV regulator works just like it should. I am able to adjust HV throughout its range from 20kv, to low enough to make the 6BD4 start redplating. Here's what things look like in the cage.




The effects of this mod should be a more stable anode voltage for the CRT, which should make the picture more stable and less prone to brightness changes from scene to scene. RCA did this exact thing to later chassis, because B+Boost is an accurate representation of what the horizontal section is doing and it doesn't require a pair of high megohm resistors to implement. In other words, it's just a different way to control HV that doesn't involve the convergence string anymore.

Next modification will be moving the source of vertical height voltage from B+ to B+Boost. Again, the result should be a more stable picture. This time instead of brightness, picture size is what I'm after. By fixing vertical height to B+Boost, vertical height should follow changes in horizontal width. I think RCA made this change in later chassis because with vertical height tied to B+ as it is in the stock CT-100, changes in the horizontal section would not affect vertical. The result of this was that the picture would shrink/expand horizontally but not vertically, which could be objectionable to some viewers. If vertical height is tied to B+Boost, when the picture shrinks/expands horizontally it also does vertically. This may seem like a bad thing at first, but when you take into account the fact the the raster overscans the dot plate by about an inch, the effects are much less noticeable than if the picture only changed size horizontally.