Tom,

Checked the screens of my 'benched' CTC2 and came up with:

B8000194
G2
R 389
G 344
B 345

As you can see the levels are quite a bit higher. Although there are six feet of extra wire on the 15G, the 'bench' operation is normal and did not require a basic set up. Also, I dug up an unfinished CTC2 setup procedure from 2009 that never got put on my CT-100 site before it went down. Added some stuff and posted it here on AK: http://www.videokarma.org/showthread...98#post3093798

Pete

Also agree that the CRT is not the problem. I don't recall, have you replaced all the original white inductors in the chassis? Every single one of those was bad in mine, even one that tested good at the beginning of the resto went bad after the electrons started whizzing around. Don't trust them, they are always bad!

Have you tested the 6BC7 resto tube, or done any subbing yet? The schematic doesn't have 'drive' controls listed, so I assume you mean the video gain pots on the little phenolic boards. What are your actual G1 voltages doing with the chassis driving the CRT? If the voltages are wildly different, I'd expect to find trouble in the output circuits of the individual matrix channels. Not saying they are going to be exactly the same voltage, but the fact that you're having to mod things points to circuit troubles. Sams has the inductors on each matrix output at around 5 ohms (L54/55/56 respectively), but they are typically wound on high value resistors because they make a quick easy former. If a coil has opened up in the matrix output, there could still be some voltage getting through on just the resistor. Food for thought, ohm them out to check.

This is very likely a DC voltage issue at the G1 terminals and not a video issue, the fact that the brightness isn't working correctly is a clue. All it does is set the DC offset on the G1's of the CRT, thereby varying their relative intensity. You could look at it as an adjustable cutoff control, if you like. Find your missing DC voltage, you'll likely find the problem.

Thanks to both of you for the tips and measurements. I'm not sure why, but my CRT seems to need some extra help to produce a good picture. I wonder if it has just a touch of gas in it?

I was able to make quite a bit of progress after reading your posts above, and taking some time to understand the DC restoration and brightness circuitry in this set. Bottom line is that I had to make a modification to get the brightness into a usable range. I changed the value of R282 on the Sams schematic from 1.8 K to 900 ohms (put another 1.8 K in parallel) and this brought the brightness control into a normal range (or perhaps it even goes a little too bright at the top now).

http://www.videokarma.org/attachment...1&d=1390724208

As you can see, the brightness control is at the bottom of a big voltage divider that starts with the screen controls connected to B+. The CRT cathodes are connected to the junction of the 1.8 K and 4K resistors, and were previously running about +135 volts. The brightness control couldn't bring the grids high enough. Changing the 1.8 K to 900 ohms brings the top of the brightness control closer to the cathode voltage (and lowers the cathodes to +124 V, more or less what the schematic says it should be).

This allows me to run with normal brightness and not need excessive contrast just to make the picture bright. More on that in the next post on the function of the DC restorer.

Other mods I previously made are shown here:

http://www.videokarma.org/attachment...1&d=1390724208

To coax enough red out of this CRT and to tone down the green and blue, the blue and green gain control ranges have been modified (to allow further reduction) and the green background range has been modified. This provides good grayscale from black to white, as long as the contrast is not set too high. Not sure if the green background mod is really still needed at this point (I should try removing it).

The following scope pictures illustrate the behavior of the DC restorer in the CT-100, and also show why my picture turns green if the contrast is too high.

These are all shots of the red CRT grid signal. The brightness control and DC restorer (a separate diode for each CRT grid) set the level of the bottoms of the blanking pulses on the video.

With the brightness all the way down, the bottoms of the blanking pulses are at zero (bottom of scope grid):

http://www.videokarma.org/attachment...1&d=1390724644

The scope vertical scale is at 20 V / div. Increasing the brightness a little just moves the whole waveform up. In this case, so the bottoms are at 40 V:

http://www.videokarma.org/attachment...1&d=1390724644

Now the brightness is set to a good point (bottoms of blanking pulses at 60 V), but the contrast is still fairly low. Note that the red cathode is at 124 V, near the top of the screen, so things are still pretty dark overall:

http://www.videokarma.org/attachment...1&d=1390724644

Now I increase the contrast control, and you can see that the bottoms of the blanking pulses are still at 60 V (held there by the DC restorer) and the contrast grows the waveform upward for a brighter picture:

http://www.videokarma.org/attachment...1&d=1390724644

Here is the point of maximum usable contrast, and the picture is quite good, with good greyscale color tracking. The tops of the waveform are now at or slightly above the cathode voltage, so this is the threshold for CRT grid current starting to flow:

http://www.videokarma.org/attachment...1&d=1390724644

If I turn up the contrast more, grid current in the CRT pushes the whole waveform down. Since there is a 100 K ohm resistor going to the DC restorer cathode, the action of the DC restorer trying to maintain the bottoms of the blanking pulses at 60 V is now overcome by the lower impedance "grid leak" biasing of the waveform down by the CRT grid current:

http://www.videokarma.org/attachment...1&d=1390724644

What this means is that a lot of the red content in the picture is now being pushed toward black, even though a few of the brightest red areas are at maximum brightness. The effect of this is to cause the whole picture to shift toward green and goof up the color balance. Since the green phosphor is capable of higher brightness at lower current, the green is no where near this kind of grid current situation, so the green keeps getting brighter while we start losing red. The contrast control can be advanced further still, driving the bottoms of the blanking pulses all the way down to zero on the red (and still topping out at around 130 -140 V), giving an extremely green picture.

I suppose I really should have captured some screen shots along with the last two scope shots, but I trust you can imagine the problem.

So with these modifications, I now get a well behaved picture if I turn up to the contrast just to the point that things start turning green, and back off a little. Good greyscale tracking, and colors stay pretty true from dark to bright. The picture is not all that bright, and is best viewed in a reasonably dark room. Not very viewable in a sun-lit room by any means.

So I'm not sure why my CRT seems to need these circuit mods to work. There may yet be something wrong in the circuitry, but it looks from various voltage measurements and scope traces that the circuitry is doing what it should.

By the way, the tip on the white coils was probably a good one. I do still have a few of those white coils in place, but I checked them again and they're still OK for the moment. In the video output stages, there are no white coils left. L51 and its brethren were bad when I got the set, so those have been replaced. L54 and its brethren are not white coils, but red ceramic coated ones that seem to be fine (maybe were previously replaced?). A few white coils still left in some other circuits.

I also swapped the 6BC7 DC restoration tube for a brand new one, but that didn't have any positive effect.

For completeness, here are some voltage measurements with the set running well at its optimal contrast point:

Screen voltages:

R: 448 V
G: 275 V
B: 306 V

Cathodes: all at 124 V

Grid voltages (varying with scene, of course, so don't pay too much attention other than the general range of these):

R: 87 V
G: 88 V
B: 95 V

One question that naturally arises when it seems that modifications are needed to generate what should have been possible with the original circuitry is "What's different now?"

One thing that has been apparent all through the restoration process is that the B+ voltage has been quite high on this set. The original selenium rectifiers had been replaced with silicon rectifiers at some point in the past, and that actually caused at least a 10% overshoot in the voltage. The RCA service manual shows the main B+ as 400 V, and Sams shows it as 380 V. I was getting 440 V.

This played some role in why the brightness control range wasn't right, since it resulted in increased bias by virtue of having the CRT cathode about 10 volts higher than it should have been. This, combined with the fact that my red gun cuts off at about 50 volts instead of 68 volts like the other guns (and no doubt what the red gun did originally) was causing some problems.

The previous power supply work had a pretty sloppy appearance, with the diodes and a bunch of power resistors added in the space where the seleniums had been. The original ballast is gone. I had left the previous work in place up until now since it was functional. But now that the high B+ is causing some problems, it's time to get the power supply producing the correct voltage.

The main change, of course, was simply to add an appropriate power resistor in series with the AC feed to the silicon rectifier voltage doubler, as shown here:

http://www.videokarma.org/attachment...1&d=1390796811

Using a value of 8 ohms brought the voltage down from 440 to 399 volts. I also put the fuse back into the circuit, since the previous modifications had removed it. If RCA felt there was a good reason to have a fuse in this circuit, I thought it would be best to put it back in there.

The previously installed power resistors weren't quite the right values. In place of the 315 ohm ballast resistor, they had a 280 ohm resistor (two 560 ohm in parallel). In place of the 800 ohm ballast, they had 750 ohms (two 1500 ohm in parallel). These lower resistances further increased various power supply voltages.

I put in the correct value resistors using combinations of power resistors to hit the values exactly. The 315 ohm needs to handle substantial dissipation. I have three 10 watters in series, and they get rather hot -- dissipating something like 25 watts total.

The 8 ohm resistor also generates quite some heat. I have a 30 watt chassis-mount resistor in there, and the whole vertical sheet metal structure gets pretty hot. Those seleniums must have made pretty good spacer heaters as well as the big ballast tube used originally.

"Before" and "after" shots of the power supply wiring are shown in the attached images.

Lowering the B+ back to the correct target value should keep things running cooler in this set. With my solid state tripler, etc., the flyback was running pretty hot, so I'm glad to cool things down a bit.

I was able to put the brightness modification back closer to the original, with the 900 ohm modified resistor I showed above now at 1100 ohms (originally 1800 ohms) and plenty of upside brightness range.

An unfortunate side effect of lower the voltage this much, however, is that many adjustments were thrown out of whack. Convergence had to be redone among other things, but the second time around was certainly much easier than the first. I also had to lower my focus regulation zener string from 3000 V to 2800 V to get the focus range adjusted to the new reality.

Picture quality is now very good, with good greyscale color tracking. The only real complaint one might have is that the picture is not very bright, but I understand that is par for the course with the CT-100. It serves best as a night-time TV. :)

Tom, thank you for your continuing detailed notes.

Well, this set is basically done. To cap things off, here are some shots of the set in its current state. Thanks to the kindness of some VK members, I have some of the front panel controls now. If anyone has the rest of a channel selector knob they can spare, I can put it to good use!

I've been enjoying watching it in the shop for the past week or so, and this evening I'll be moving it into the house so it can have a better climate controlled environment and stay cleaner (as well as being easier to enjoy watching).

And a few more pictures.

Thanks for all the help, everyone!

Tom

Beautiful! I should take some daylight pics of mine. Second thought, my back would protest so upstairs it will stay!

Tom, Fantastic job. Looks like your just missing the plastic channel # insert on the channel selector knob. Hopefully someone out there can supply you w/one.

-Steve D.

Absolutely gorgeous! Congrats, Tom!

-Clark

Someday I hope 3D printer and some sort of 3D scanner will be able to replicate these parts. Cause I need one of those for my 21CT55.

Dave

Tom,

Here's a 1-1 scale scan of the channel #'s paper insert. Not sure it's of any use to you, but good for reference.

-Steve D.

Some further followup on this set -- The CRT red gun has always been fairly weak on this set. Seems it died a natural death with a worn-out CRT after years of usage way back when. No problem with loss of vacuum on this CRT; just not much brightness from the red gun. You could get an OK picture on it in a dark room, but it's no fun to watch in such a state.

Adding a brightener helped a lot, but I still noticed another problem: The DC level of the CRT grids is set by the DC restoration circuitry and brightness control. This works fine, unless you try to push the signal to the point that the grid goes positive with respect to cathode. Then grid current flows, and the DC level of the grid gets pushed down as in any grid-leak scheme.

For example, on bright scenes, the DC level gets pushed down, which lowers the red level not just on the bright parts of the picture, but everywhere. So bright pictures were very weak on red.

I've done some experimenting and found a way to boost the red grid signal, allowing it to go positive and draw grid current without any effect on the bias. I added a cathode follower as shown below. It is capable of delivering enough current to the grid to hold the DC level and appropriate brightness.

http://www.videokarma.org/attachment...1&d=1517469546

The CRT cathodes sit at around 125 VDC in this set, and with this cathode follower in circuit, I can drive the red grid well above that while holding the black level at around 85 volts where things look good.

Now the set looks good in a moderately lighted room -- something I've never really had before. Much more fun to watch than before.

Anyone else tried this method to squeeze a bit more life out of a CRT that has seen better days? If this were not a 15GP22, I'd give it a good whack with a rejuvinator, but that seems far too risky with such a valuable CRT.

Thank you for doing this interesting research and for sharing your success, Tom.

Grid current is not recommended according to the data sheet, though I suppose you have nothing to lose.

http://scottbecker.net/tube/sheets/093/1/15GP22.pdf

Thanks for posting the data sheet. That's one of the reasons why I posted this -- to find out to what extent operating with positive peak voltages on the red grid may cause some damage. I see on the data sheet that the max rating for grid voltage is +2 volts. Is there an additional reference on there specifically about grid current? I may have missed it.

One thing I noticed in the circuitry is that while the green and blue cathodes are fed by a fairly stiff voltage divider, the red cathode has an extra 2.7K cathode resistor. This reduces the gain of the red and reduces the positive grid voltage with respect to cathode. Shorting this resistor results in a pretty nonlinear response, where the red beam gets too bright on positive peaks and gets completely out of balance with the other colors. I tried reducing the value of that resistor and came away with the impression that it's best left alone. Intriguing, however, to think about how the red signal could be further boosted in some useful manner that gets rid of the nonlinearity. Maybe some kind of active feedback on the beam current? With a little help from a modern op amp that would be possible to do.

If any of you knows anything further about the risks of boosting the red in this way, let me know.

As miniman82 points out, I don't have so much to lose here, but I also don't want to completely kill the red very quickly by trying something completely crazy.

I didn’t look, but from experience reading lots of data sheets I can tell you that going beyond 0 on the grid will typically result in grid current being drawn. For most output power pentodes this isn’t problem occasionally, my concern here would be overheating the grid cup and warping it. That could cause a permanent short and damage the gun.

My instinct tells me that shouldn't be a problem. The grid goes at most 10-20 volts positive with respect to the cathode (I think it is in fact only a few volts because of the cathode resistor), so the power dissipated in the grid will be tiny. Probably gets way more heat from the nearby cathode than from the actual power dissipated in the grid. I was more worried about whether this will cause some kind of cathode stripping or something like that.

Running it into the positive grid region is probably along the lines of a mild clean and balance on a rejuvenator. Odds are it won't hold it's emission for long...It is probably either going get brighter as the cathode cleans or darker as the remaining cathode gets used up. If brightness improves with use you may want to revert it to stock.

Absolutely stunning cabinet and electronic restoration on this set. The screenshots are the best I’ve seen to date on a 15GP22. I was thinking a 3D printer for the knob as well.

Late to the game on this thread, don’t know how I missed it.

Are sure you weren't looking at Nick's screen caps?

The screen caps in post #150 look pretty decent to me...

Those screen caps were not very bright when I took those a couple of years ago. Always have needed to run this set quite dark to get enough red.

Today, I took a quick shot during the Super Bowl. This is the west coast, so there was still significant light coming into the room from the window at 5 PM or when I took the shot. As you can see, red is decent. I'll take some nicer screen shots after I've had a chance to fine tune things better.

http://www.videokarma.org/attachment...1&d=1517805073

Noting the concerns about positive grid voltage and grid current mentioned above, I made an update to the circuit by adding two 1N4148 diodes in series between the red grid and cathode. I used 1N4148 to have fairly high speed diodes. Two in series since they are only good for 75 V reverse voltage each. This limits the grid to only +1.2 V on positive peaks of the video.

Here's the circuit:
http://www.videokarma.org/attachment...1&d=1517805073

With this circuit, the DC restorer sets the black level ahead of the cathode follower, and the diodes clip the peaks off the video drive to the CRT grid. I'm pleasantly surprised that clipping the positive peaks still lets plenty of red drive through (so I don't really need the strongly positive peaks on the grid signals).

I compared this to the original to reconfirm that this was all doing some good. The difference is very compelling. Without the cathode follower, the DC restorer isn't able to keep the black level fixed on the red gun any time the red signal becomes large. As reported above and on the last page, grid current competes with the action of the DC restorer, and the DC restorer loses. Now with the cathode follower and diode clipper, the DC restorer keeps the black level fixed as it should, and I can get decent red.

Not like a new tube, but pretty watchable. With the clipper in there, hopefully I am not damaging the red gun.

Are you still using the brightener?

I hadn't seen those caps before (maybe three years ago), they do look good. Please keep us posted on how well the tube holds up. My set can be viewed in moderate light (even blinder-inhibited daylight) but could be much improved by your scheme. I'm just going to leave the burn-in period to you. :)

Tom, As it happens, I re read your entire post from 2013 yesterday. The set still looks spectacular. Looks like you're still looking for the plastic channel selector insert. Thought about powering up my CT-100 for a Super Bowl photo but got caught up in the game. Also on the west coast and a lot of light streaming in from my west facing windows.

-Steve D.

Yes, the brightener is still on there. I'll try it again soon without just to compare, but I have a hunch it needs to be there.

That's quite a reading assignment you gave yourself! Thanks for the nice comments. Still a work in progress, trying to get this tired old CRT to give us a nicer picture than it wants to.

I had a set that had a very weak red gun and I bought these boosters they look like brighteners but have no transformer in it I have one left for the green gun but can be used for any gun you just have to reverse the green wires to the red wires and this unit has an adjustable pot to adjust the red, and it does work. I had posted about this and no one ever seen or heard of this. This unit is for the base of the 21fbp22 or any color tube with the same base. It is a plug and play has both male and female ends. I had taken one other I had apart and all that's in it is a pot and a resistor and wires tied together.

I remember those...
http://www.videokarma.org/archive/in.../t-258583.html
Look near the end of the page for how they work.

.

Yes I seen it and maybe it does just increase the bias for the given gun but when you have a tube that has a weak gun this is the answer, plug and play rather then make changes to the existing circuit when the tube is going down hill to begin with. As I was told by lots of VK members awhile ago is that these black and white or color sets will never be perfect. But we all seem to try to get then perfect and find mistakes from the original designers, but that's ok.

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.

http://www.videokarma.org/attachment...1&d=1527379373

http://www.videokarma.org/attachment...1&d=1527379373

http://www.videokarma.org/attachment...1&d=1527379373

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.

Really looking nice. I think if you took some closeup shots in the dark the images would look spectacular.

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.

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...

Nice work!

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.

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

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.