Ct-100

[benman94]

Originally Posted by Tom Albrecht

Made some further progress on the CT-100 after some time away from it. Two key findings:

1. I actually get more red with the red screen turned DOWN, not up. I modified the circuitry to lower the voltage range of the screen controls, and this actually helped. I don't understand why this is the case. Anyone have an understanding of how the screen control is behaving this way? I've confirmed that the voltage at the CRT socket goes up and down with the screen control setting as expected.

2. While building the cathode follower into the chassis more permanently, I happened to spot check some components under the chassis, and found the choke on the plate of the Q synchronous detector was open. I'm sure I checked this coil when I first restored the chassis a few years ago, but undoubtedly this coil opened in the mean time. This was confusing my interpretation of what was going on with the red. I'm surprised at how well the color worked with this choke open, but it's certainly much better now. Also replaced some drifted resistors while I had it on the bench. Not a bad idea to pull the chassis on a set like this every few years to catch the newly deteriorated components.

I've now got the cathode follower built in to the chassis, hiding inside the cage where the selenium rectifiers once resided. This leaves the chassis looking relatively original. The cathode follower is worth its weight in gold for enhancing the red on this set. It's officially "daytime watchable" now, which makes it a lot more fun to have around. Here are some daytime viewing pictures. Blinds are closed (with sun directly on the windows, which still lets a lot of light into the room).

Here are some quick screen shots. Didn't have nice reds like these in the daytime until now.

That looks extremely nice Tom! I think your screenshots are the first I've found that look remarkably similar to how a working 15GP22 looks in person. Do you do anything special when snapping screenshots?

This also provides a bit of hope to anyone with a rather anemic looking 15GP22, provided of course it's still under vacuum.

[Tom Albrecht]

Originally Posted by benman94

Do you do anything special when snapping screenshots?

These are simply quick shots with my iPhone. I guess I got a little lucky with the lighting level. I didn't have to adjust the brightness up or down from normal viewing for those shots.

Originally Posted by benman94

This also provides a bit of hope to anyone with a rather anemic looking 15GP22, provided of course it's still under vacuum.

If anyone has a CT-100 with a weak picture tube, I'd recommend this method of squeezing more life out of it. Not sure how long this will hold up, but I don't see any change after a few hours of viewing. I don't watch it all that much, so it will be a while before I'll be able to tell how well it's holding up. Much nicer to have it watchable, and something that can be demo'd without having to have a completely sympathetic viewer.

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

[dtvmcdonald]

Originally Posted by Tom Albrecht

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

I have a theory. I derived this as my red gun was slowly dying before it arced
so bad it destroyed itself (and then arcing became almost continuous).

The theory is that the center of the cathode is completely dead and the emission from the outer ring is, with the screen at normal voltage,
hitting an electrode before it gets out of the gun. When th screen is lower,
the focus changes and the electrons get through. This is of course a very far-out idea. It could be tested using an electron gun field simulation program, if we knew the geometry, or by examining a gun removed
during a rebuild.

[benman94]

Originally Posted by dtvmcdonald

I have a theory. I derived this as my red gun was slowly dying before it arced
so bad it destroyed itself (and then arcing became almost continuous).

The theory is that the center of the cathode is completely dead and the emission from the outer ring is, with the screen at normal voltage,
hitting an electrode before it gets out of the gun. When th screen is lower,
the focus changes and the electrons get through. This is of course a very far-out idea. It could be tested using an electron gun field simulation program, if we knew the geometry, or by examining a gun removed
during a rebuild.

I believe you're onto something here. A weak B/W tube will often give blackened highlights when driven too hard, and show poor, non-uniform focus. Dial the brightness and contrast back and some tubes will behave more normally.

I've seen this effect blamed on a dead cathode center region in period papers. It's possible the same effect that causes this behavior in B/W tubes is causing the strange behavior of Tom's weak red gun.

[jr_tech]

Originally Posted by Tom Albrecht

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

Is the spot size from the red gun larger under these conditions? I'm guessing that with lower g2, much of the beam current is being generated from a larger area of the cathode surrounding the very center, which is likely fairly well depleted by now.

just a WAG,
jr

[Tom Albrecht]

I think you're on the right track. Now that I've modified the screen control circuitry so that the adjustment range of the screen controls is at a little lower voltage, as I go from the lowest screen voltage to about 1/3 the way up, red brightness increases. Beyond about 1/3 up, the red brigthness decreases, and near the top of the range, I think the entire picture (all guns) is defocusing a little. This might be consistent with additional current dragging down the voltage on either the first or second anode, without additional beam current actually making it to the screen (it's hitting elements within the gun).

I don't think I'm seeing a bigger spot size on the red. Picture is pretty sharp overall. Don't know if that's consistent with the idea that the center of the cathode may be worn out, but I'll simply report what I see.

[kvflyer]

Tom, I know that I am late to the party. I did see the pictures that you posted a few days ago. Just wanted to say how wonderful it looks. I must admit that I only recently became aware of the 15GP22 CRT thanks to this forum and the Antique Radio Forum. I will admit to having never seen a real 15" color set (vintage that is).

Very Nice!