KV9200 Issues

[Alex KL-1]

Yes, you can connect one end to F1.
From your link, it seems to be a 2k pot. Too low value; will load too much the 530V supply.
You can choose a pot >1M ohm, since the focus grid almost don't drain current.
In fact, I use a 12M pot I found in a local electronic store when optimizing focus.

[vol.2]

Originally Posted by Alex KL-1

Yes, you can connect one end to F1.
From your link, it seems to be a 2k pot. Too low value; will load too much the 530V supply.
You can choose a pot >1M ohm, since the focus grid almost don't drain current.
In fact, I use a 12M pot I found in a local electronic store when optimizing focus.

Thanks Alex,

I looked on Mouser and Digikey, but the highest thing I can find in stock is a 1Mohm pot. It's a 2W bournes unit: https://www.mouser.com/ProductDetail...76JZfFDA%3D%3D

There's others in stock, but they all seem pretty similar.

Do you think 1M would be fine?

[Alex KL-1]

Seems to be perfectly fine.

[vol.2]

So I finally got the 1 MEG pot to try fine tuning the Focus Voltage.

I hooked up the Focus (F0) to the wiper, and then I hooked up one end of the pot to the F2 terminal (500V), and the other end to F1 (Ground).

To my surprise, the focus looks the clearest when it's all the way at 500V, but it looks better with the pot hooked up this way than it does with the Focus (F0) hooked up directly to F2

I'm assuming here that this is because I have essentially have a 1 Meg resistor hooked up between the 500V source for the Screen pot and GND

This seems to have the effect of lowering the overall brightness quite a bit (meaning I then have to turn up the Screen to compensate), but it then gives me a lot more headroom for the Picture control, and I can turn up the Contrast (Picture) more before the whites start to bloom.

Is there some reason not to just hook up a 1 Meg resistore between the 500V source and ground at this point?

Thanks

[Alex KL-1]

The potmeter was there for factory adjustments to compensate for CRT variations, but it seems that you got a CRT with optimum focus voltage just right with the supply value, so is not really necessary if is working well at the moment. The issue will be only in the future, if CRT changes a little, then will not have how to readjust it.
That said, probably the CRT stands with the current value for years.

[vol.2]

Originally Posted by Alex KL-1

The potmeter was there for factory adjustments to compensate for CRT variations, but it seems that you got a CRT with optimum focus voltage just right with the supply value, so is not really necessary if is working well at the moment. The issue will be only in the future, if CRT changes a little, then will not have how to readjust it.
That said, probably the CRT stands with the current value for years.

I got a little fatigued on this project and put it aside for awhile, but I have a moment to look at it again, and I realize that the focus could still be better if I have to admit.

I looked into other ways to improve focus, and I ran across a thread on this forum talking about lowering the G1 voltage (ideally making it negative) to improve focus.

Looking at my schematic, it looks like the G1 is set to 38.2V, and the G2 is set to 307V. The focus range is 0-530V, but it's got those "steps."


Do you think perhaps I could improve focus by sticking a pot on the G1 and lowering it closer to ground?

Wondering also if it would damage anything to do so.

[Alex KL-1]

You can use the lens effect and reduces Vg1, but you can need to compensate raising the Vg2 (screens), and also needs more video amplitude, if you wanto to maintain same contrast ratio. Perhaps a little loss of contrast can be compensated for taste sake with the sharper picture.

For me, for example, my delta-gun TV, I swears that it have more bright when it are critically focused. I not measured it, but I have a DMM with luximeter that I already measured some TV brightness, including that. Someday I measure it.

But I assume that being a side product (impression caused) of the focusing.