CRT question

Is there an astigmatism adjustment? That's what's usually used to tweak display for even focus.

Presumably the electrode voltages aren't staying the same as the thing sweeps across the screen.

NT

On almost any element the wrong voltage should affect the whole screen pretty much equally. One exception would be Scheimflung but that is not what you describe.

What I think is the drive to the deflection plates is not properly balanced.

Also, guy from Poland was telling me how crappy Russian CRTs are, maybe it is just defective ?

Yeah, there is an astigmatism adjustment. And I adjusted it along with the focus and brightness to get the sharpest spot. What is weird is that the display is sharpest on the left hand side of the screen and then it gets more out of focus on the right. This out of focus is gradual, with each number being a little more out of focus. Eric

Any idea why this voltage change would happen? And if the voltage was increased could the out of focus become less? I figure there must be some sort of maximum sharpness the CRT is capable of. If this sharpness is voltage dependant then maybe higher voltage will fix the problem? Eric

Nope. That's not how astigmatism is suppose to be adjusted. It's done with a horizontal trace across the screen. You adjust the astigmatism knob so that the trace is consistent across the screen. If you just adjust for the best dot, you end up with the focus and astimatism controls fighting each other, especially since you can't see what's happening outside of the dot area. Try it using a line and see if it's any better.

Well, there might be a component failure, but let's play with the knobs a bit more first to see if things improve.

I'll need to see if I can get thing to make just a straight line. If not I can lock the display to just showing digits. Then I can adjust the astigmatism and see if the digits on the right hand side of the display get sharper. I can also swap the X axis wires, which mirror images the display. I know because I already did this accidently. If the focus problem mirrors too then I wonder what that would mean. I'll try the astig. adjustment first later today. Thanks, Eric

Do they use true differential deflection signals, or unipolar (ie one plate of each deflection pair is held at a constant voltage)? If unipolar, it can cause this type of trouble.

Jon

I dunno Jon. How would I tell? Measure voltage to ground and see if one or both changes? Eric

If you physically rotate the whole device 90 degrees, does anything change? Possible magnetic flux from your shop!

Paul

Greetings Paul, Yeah, the CRT is especially sensitive to magnetic flux. But the thing has the out of focus problem no matter the rotation or where I stand in my shop. It is always less focused on one side. But thanks anyway. I know almost nothing about tubes in general and CRTs in particular. Eric

High Voltage on CRTs tends to use very high resistance. An 11 meg meter could load down the test points and give faulty readings. A high Voltage pr obe is often around a gigohm load and cause much less disruption in the rea dings.

In theory, voltages shouldn't have any affect on the focus across the face of the CRT, but in practice, all sorts of variables happen.

It's indeed possible that your problem will go away if you get the CRT close to it's designed operating voltages. At least, that's what I would do before condemning the CRT as defective.

Is there a schematic of what you built on line?

No, there is no schematic. Paul Drahn suggested to me that the CRT may need to be degaussed. I have a demagnetizer designed for demagnetizing tools. I'm sure it would work to degauss a CRT. Eric

A Weller heavy duty soldering gun is also useful as a small degaussing tool if you don't have a proper open coil one handy.

John :-#)#

If they have a centering pot that drives one deflection plate, and some amplifier that drives ONLY the other plate, then that is single-ended deflection drive. if they have differential amplifiers that drive both plates in opposite directions, then that would be differential.

A scope could show that both plates are driven, but not sure a DVM would give a definitive reading.

Jon

Posting us a schematic would help, but you don't have one. Stating he obvious here you should look at any variation in focus voltage at the horizontal sweep speed.

Generally speaking Russian CRTs are perfectly capable.

NT

Well, I got time this morning to fuss with the CRT. The astigmatism adjustment works but I cannot get the display to be as sharp on one side of the tube as the other. I did degauss the thing too but there was no change. Later today I'll swap the X axis wires and see if the out of focus swaps too. If that happens then it must be the circuit and not the CRT, right? Thanks, Eric

I tried swapping the X axis connections to the CRT and the out of focus conditionn swaps too. So it is not the CRT. I'll contact the contact the kit supplier and see what he says. Thanks, Eric