KV9200 Issues

[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.

[vol.2]

Originally Posted by 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.

I think I want to tweak it and see how it effects things just for fun and as an experiment. I have lots of old TVs, so this is just fun for me, nothing too serious.

Would you mind confirming the tube neck pins for me?

It looks like the heater is 105V. That's very high compared to others I have worked on. At 105V pkpk that's something like 37V RMS? Seems very high for a heater, although maybe that's because it's a low voltage focus tube?

Seems like the G1 is 38.3V, the G2 is ~300V and the Focus is 0-500V?

[old_tv_nut]

It's a 6.3 V AC heater, but floating at 105 V DC so the DC on the heater is close to the DC voltages on the cathodes.

[vol.2]

Originally Posted by old_tv_nut

It's a 6.3 V AC heater, but floating at 105 V DC so the DC on the heater is close to the DC voltages on the cathodes.

That makes sense. So it's a 6.3 VAC heater with a 105VDC offset then?

[old_tv_nut]

Originally Posted by vol.2

That makes sense. So it's a 6.3 VAC heater with a 105VDC offset then?

yes - but double check the AC value because I'm not 100% sure.

[Alex KL-1]

Originally Posted by vol.2

I think I want to tweak it and see how it effects things just for fun and as an experiment. I have lots of old TVs, so this is just fun for me, nothing too serious.

Would you mind confirming the tube neck pins for me?

It looks like the heater is 105V. That's very high compared to others I have worked on. At 105V pkpk that's something like 37V RMS? Seems very high for a heater, although maybe that's because it's a low voltage focus tube?

Seems like the G1 is 38.3V, the G2 is ~300V and the Focus is 0-500V?

Old_TV_nut pointed about the floating heather, and your annotations for other pins are exact.

[vol.2]

Originally Posted by Alex KL-1

Old_TV_nut pointed about the floating heather, and your annotations for other pins are exact.

Thanks.

I'm looking at the schematic and trying to determine how best to lower the voltage of G1.

I could either just tie it directly to ground and see how that looks, or I could try to stick a pot in the circuit somewhere to create another voltage divider and lower the voltage somewhat (maybe halfway to start).

I have been trying to figure out how they derived 38.3V from this circuit.
I can see there's a 68K resistor on the far side of the circuit (which I guess forms an RC network with C705), and then there's the 530V source running through a 500k pot and a 330k resistor.

The middle Focus voltage is 208V, which works out correctly at point "219" on the schematic. (530V through the center of a voltage divider 330k,500k works out to ~210V)

So then I have a 210V source that hits a voltage divider at 330k,68k which works out to ~36V, which is basically correct.

So my plan is to replace R721 (68k) with a pot and take it down to like 10V and see how that looks.

Do you think I will run into any issues with the RC network R721,C705?

[Alex KL-1]

You can experiment to ground for first tests, since is easy to make. 38V sounds like not a extreme difference, even taking into account the low V focus tube. In any case, when you readjust the g2, bright and contrast to compensate, the cathode voltages will change. No big deal, unless video amp are out of range.

[vol.2]

Originally Posted by Alex KL-1

You can experiment to ground for first tests, since is easy to make. 38V sounds like not a extreme difference, even taking into account the low V focus tube. In any case, when you readjust the g2, bright and contrast to compensate, the cathode voltages will change. No big deal, unless video amp are out of range.

Meaning that I'll have to re-white balance the set, correct?

[Alex KL-1]

If it mistracks (probably a little), but watch first if beam focus suffices with 0V, or if needs more voltage (negative).

[Alex KL-1]

Just one curiosity, about this subject (a "side topic" related), if one knows about it:
Some B&W sets have g1 driven and cathode grounded for AC. Some color ones also have, for color difference.
This variable g1 not results in a variable beam size/spot size for these sets? Or signal is too low to produce an appreciable difference?

This subject of this thread makes me wonder about it.
(The TV here have only k driven, so will not have this issue).