Desire to underscan Horizontal

[Opcom]

I have a need to greatly underscan a tube type monitor chassis. The reason for it is because I have replaced the 110 degree 8" 8HP4 rectangular tube with a 10" 55 degree round tube, a 10SP4. I have not changed the yoke yet. Maybe it will be done but it is worth it to try without it.

I've tried going to a lower tap on the H out. transformer but it's not working out too well. I noticed in another thread about a color set where it was suggested that an unwanted or excessive overscan could be reduced by changing a tap on the yoke. The function of this change merely put a 560pF capacitor in series with the windings.

On my chassis, a Conrac CNB-8, there is a width adjustment but I am at the end. I have tried putting a small inductance in series with the H winding and then to correct back down the overall inductance necessary for the circuit tuning, placed an inductor across the H yoke winding.

I have not tried a capacitance in series yet. Has anyone had experience adjusting tube set widths to unusually narrow settings or with using a tube having a lesser deflection requirement?

[old_tv_nut]

Mathematically, it looks like you want two more inductors:
Added inductor 1: same inductance as yoke, wire in series with the yoke winding;
Added Inductor 2: twice inductance of yoke, wire in parallel with the yoke/inductor 1 combination.

The net inductance will be the same as the old yoke, but the current in the yoke will be 1/2 of the old value.

[Opcom]

Thanks. I had gone down that road but my inductors may have been saturating causing the poor linearity issues shown. This confirms I should revisit it with better quality inductors. They are not the easiest things to find.

[Opcom]

I was able to buy some other yokes, 70 degree ones. The one in that set is a 110 deg.

I am sure the results will be better with the 70 degree yoke. The tube is a 50 or 55 degree tube, but that sort of yoke is older and not as easy to come by. neck size 1-7/8". still looking for focus coil(s), same diameter in "wanted' section.

[ChrisW6ATV]

Look at C613/C613A. If the "47k" values mean 47,000 pF (therefore .047 uF each, and about .1 uf total), you could also try reducing the value, or even disconnecting one of them temporarily.

[Opcom]

They are 0.047uF each as you said. It won't harm anything to try it.