Electrostatic Deflection "Bunching" - Solved

[Tom Albrecht]

For years, I've had problems with sets that use electrostatic deflection with a CRT like the 7JP4. At high brightness/contrast, the vertical deflection would tend to show "bunching" of scan lines in bright parts of the image. To get rid of it completely, I would often have to run at a lower level of brightness and contrast than the CRT seemed to be well capable of.

I've noticed this problem particularly badly on a Sentinel 400TV and also my latest set on the bench -- the Airline version of the same set (basically identical; manufactured by Sentinel).

In the past, I put quite a bit of effort into trying to make sure the HV was stable, the voltage supply to the vertical deflection was stable, etc., by experimenting with adding extra HV filter capacitors. All to no avail. Today, I figured out how to solve it definitively.

In the schematic below, the vertical deflection coupling capacitors are circled:



Increasing the capacitance of C73 and C74 from .0047 uF to .01 uF solves the problem. Now the brightness and contrast can be run much higher, without picture distortion.

I suspect there is some difference in properties between the modern ASC 6000 V tubular capacitors and the original 6000 V paper capacitors that were used. I'm not sure exactly what is happening when the vertical deflection gets distorted, but it must have something to do with charge from the electron beam building up on the vertical deflection plates in bright parts of the scene. Adding more capacitance in the vertical deflection capacitors either increases the leakage (very slightly) or simply makes it so that it takes more charge to distort the scan.

An added bonus is that on this set, the picture was trapezoid shaped (top and bottom parallel, left and right sides sloping inward toward top) until I increased the capacitors. That also went away with the change.

And one more bonus -- the amount of available vertical deflection is drastically increased by the change. With the .0047 uF caps, the vertical deflection was not quite sufficient to fill the mask. Now there is way more than enough vertical deflection available, and I can adjust the linearity much better.

So I guess my advice would be to intentionally use larger vertical coupling capacitors than the set originally had, and you'll be much happier!

I'll post some pics later to show the effect.