On my flawless 15GP22, the red screen did function just like the other two. Why wouldn't it?

Sounds as though the 2.7K 1/2-watt in the red-gun cathode circuit should be checked.

Perhaps RCA put it there because the degeneration it provides smooths out the red phosphor. Haven't seen it on any vintage set other than the CT-100.

Pete

Benman: why not? because the red gun is sick? I checked again and
the voltage is getting to the tube pin, and the pin capacitance
to other things is the same as the blue and green guns. I was thinking that
maybe it was not connect to the actual electrode, but changing it does
have a small effect. IF in fact there was an internal arc it could
have damaged either the cathode or the connection to the red screen.
But if it were not connected, why would it work at all?

However, given that the blue and green screens run almost all the way down,
and do have only a modest range where they have a big effect, it might be that
lowering the red one further would cause a drop in the red beam current.
The 4K resistor tests OK in circuit, cold .. maybe its changing when hot.
So the next step is checking those two resistors under load by
unplugging the CRT socket to gain access and removing the
horizontal output tube to remove HV for safety, and letting it cook.
McVoy said my CRT test 90% on all guns.

Phil: the resistor measures OK and when running it has the expected
voltage (about 1 volt) across it when the set is showing a
bright screen.

If I were you, my next step would be to acquire a Sencore CR-70 or CR-7000 and check the emissions on the red gun. You have put some time on the 15G, probably not a lot though, and cathode material is finite.
Given that the beam current on the red gun is so much greater than on the other two, I'd want to rule out poor emissions first. The drop off in emissions can be rather fast in a tube that is driven hard. I think the red gun of a 15G would qualify...

benman, I suspect you are right, although there WAS those two
arcing events that I can't prove were not in the tube.
I surely have less than 200 hours on it probably more like 100.
And it was never run for more than a few seconds at a time
with the red gun drawing grid current more than 10% of the time,
as when actually watching I always set it well below the "knee" where
the color rendition gets poor.

I suspect the arcng events.

Its not the resistor chain: rock stable for an hour.

I also vaguely remember asking McVoy about the red screen.
I seem to remember "just run it all the way up and don't worry".

Also ... my camera shows that the brightness limit for true
color accuracy has not dropped more than 1/2 f-stop (i.e. to 70%).

Check emissions to ensure it isn't substantially lower than the green gun. If emissions looks fine, then I'm positive you have damage from arcing. Unfortunately, either way the long term prognosis isn't good. I strongly recommend you really use the hell out of that set while you can. No sense in "saving" cathode material in a dying tube.

I really do hope it's not the tube, but if everything tied to the various elements of the red gun is fine, then that only leaves the CRT.

I have no way to measure the green gun without cutting wires. But I
measured the cathode resistor of the red gun and got 600 microamps ..
which isn't great but that was average with a mostly white screen.

I'm actually beginning to think I'm being paranoid. I will know for sure
after the next ETF convention when I will take several pictures of all their
15GP22s under the exact same conditions as mine ... same camera
settings, same picture of Dorothy.

Just keep in mind, a 15G can go at any time. Ed Reitan lost at least one in a restored set, and both my Uncles lost a couple, seemingly at random....
I'd still want to know what the three guns look like relative to one another on a tester. If Steve said all three guns tested ~90%, and then you find the red gun much weaker relative to green and blue, then I think paranoia is out.

How about posting a photo of Dorothy on your CT 100?

I've already long ago posted a picture of Dorothy that look just fine.
She looks just as good now.

That's not the question, which is "how bright is she now compared to then?"

I just realized I have the old .cr2 file ... I'll compare to a new
one adjusted to look the same!

Answer: a factor of five. The red gun max is down by five.

I am now convinced that an internal arc did it, at the time the
first fuse blew.

So I will just rarely demonstrate my set in a dark room, and
await and pray for rebuilding.

I can relate to that sorrow
 

I am always amazed at all you felas do for those very first color sets. I just hope to see one operate in person one day. It must be wild to see one perform as new.

Mine did. Well, I have no idea about "like new".
"Perfect" was more like it. Not perfect on color bars (ringing) but on
ordinary test static pictures, yes, especially near perfect color.

My avatar is a very easy scene, however.

No chance of getting to Hilliard?

Don't hold your breath on 15G rebuilding. Even of it does become possible, I doubt the yield of usable tubes is going to be terribly high. Subjecting an under vacuum tube to the rebuilding process would be an enormous risk. That said, buying a "good" 15G is a massive risk anyway...

Enjoy that 15G while it's still possible to do so. If the arcing is a gas issue, and I strongly suspect it is, you're saving cathode material in order to light up a purple neck. If you run the S.O.B into the ground, at least you'll have enjoyed the set more. Either way, the outcome is the same: you sit with a dark set and await the rather unlikely rebuild process.

Running a gassy tube also may slow the advance of the gas issue by exciting the gas into moving more and thus being more likely to reach and be absorbed by the gether.

The tube shows no sign of gas. No blue glow and no problem with focus
whatsoever, sometimes I see the scan lines. Nor do the blue and green guns
show gas signs.

I'm in no hurry to get it rebuilt. I'll just watch the super bowl on my
55 inch TV. I just wanted to own such a set .. and I do note that it does
display pictures quite nicely, just dim (with blue and green throttled
back to match red).

Going gassy is a process, not a single event. It's possible to have a tube that's going gassy with no blue neck, no focus problems, etc. Just periodic arcing as the first symptom. Remember, every gassy 15G out there (and there are a lot of them) was good at some point.

I slowly lost a 16AP4 to a leak. The first symptom was (probably, looking back on it) arcing. It became more frequent, then it got very dim and lost the ability to focus. Now that said, it's been a SLOW process. A few years minimum...

Ivery much hope to go to hillard!
 

Thanks for reminding me.

Yesterday I thought of a different test for internal wire breaks or shorts
in a CRT. Its best if the neck is clear of yokes/purity coils, but can be done in place.

I hooked up a 1 MHz signal generator, 25 volts, to each pin of the CRT in turn.
There was an aluminum foil shield, grounded to the purity coil, taped to
the bakelite tube base.

I used a high impedance scope probe and moved it around,
observing the pickup.

I expected modestly good symmetry about the three guns if
all elements were as expected, and a smaller pickup near an element
that had, for example, a bad pin connection. All was as expected if
good connections.

I then found some #6 wire and cut off a 1 inch piece of that,
connected it to the generator, and taped it to the outside of the
tube base, lying parallel to the suspect red screen pin. The pickup on my scope
was, again as expected, still there but down by a factor of
four to six.

So its official, my tube has poor red emission.

Also .. playing with the blue and green screen controls
shows that the higher they are, the lower the tube gamma.
And I had noticed that over time my sets gamma seemed to increase ...
the blue and green screens had been turned down as the red
got weaker. Thus I think it was not necessarily a one-off disaster, possibly.

Very sorry to hear that Doug. Just an idea here; maybe try splicing a small brightener transformer into the 15G filament line and only use the set on very special occasions?
CRTs that are rarely used seem to hold up pretty well on a brightener. I have a *dead* 10BP4 on a brightener in a set I use a couple of times a year. It still produces a nice bright image with the elevated filament voltage. You could probably recover decent looking emissions at ~7 VAC on the filaments. At this point you haven't got much to loose.

I'd recommend putting a CRT tester on it to confirm, before trying a brightener.

There ought to be someone near you with a CRT tester and a universal adapter.

ElectronicM,
He's already confirmed that the red gun is weak. His method of finding the low emission is just more sophisticated than the lowly emission testers we rely upon. I'd be more inclined to trust the results of Doug's test than those of a Sencore for instance.

The red gun is weak. Period.

How does injecting a signal then capacitively picking it up with a scope constitute a form of emissions test? It seems more like a way of confirming none of the internal connections to the gun elements are not open. Okay so the red screen is not open, and the gun bias is correct, so by process of elimination the gun must be weak...In his shoes I'd still get a CRT tester and make sure that all the data that fed into that process of elimination is indeed leading to the right conclusion.

It's a relative emissions test. He's comparing the red gun to the green and blue guns. In a color tube the absolute ability of a gun to emit electrons is meaningless. All three guns must be emitting electrons, and they must have abilities that are fairly close to one another, otherwise correct tracking becomes an impossibility. Just the fact that the red gun is weaker by some factor of four to six is enough: the gun is weak.

Now if all Doug cared about was say producing a uniform red raster, then an absolute test of emissions would be in order. A red gun four to six times weaker than the other two should still produce a uniform field of red. That said, Doug isn't likely to be watching any programming like that, almost all content is a mixture of all three primaries, and thus the relative strength of the three guns becomes the limit on usability.

Its official, my 15GP22 is bad because of gas :tears:

I left it unused for a couple of months, and when I turned on
last night, it arced badly with full HV, maybe twice a second for the
three seconds I left it on. With the socket removed there was no arcing.
With the HV removed there was a tiny ticking sound. Trying it a third time
with both connected I saw an arc near the base, inside it, and immediately
shut it off.

I will now wait the final results of Yurkon's work, unless a working one,
or even another set, shows up for sale.

I seem to recall someone having luck on a mildly gassy tube running the heater alone for a few days(weeks?). IIRC the heat helped the getter action reduce the gas to a usable amount. It may not work, but at this point there is not much to loose in trying it.

Tried that with bens AXP which still has perfect getters, ran it for more than a week still has purple neck.

Did you ever get the CRT socket apart?

Just asking because at one time back in the late 70's I had a Solid State Philco 19" that I bought refurbished from a TV shop, it had a new CRT installed.

It would be playing fine and all of a sudden let out a loud HV snap that would me me jump out of my chair. It would play fine for hours or days but eventually do it again.

After a half dozen or so of these event the picture started looking lousy and a check showed the emissions had dropped into the bad range.

The shop replaced the tube under warranty but it soon started doing it all over again.
I sold it cheap to another TV repair guy and he figured out it was a bad CRT socket, tracked from the Focus pin to the other pins and would intermittently arc and damage the CRT.

Yes, Eric, I was worried about that. That's whY I let it sit quite a while and then
carefully turned it on, looking for gas. It was gas.

BUT ... I most certainly will open up the socket
and check it if I get a good tube.