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  #61  
Old 02-07-2017, 09:07 AM
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dtvmcdonald dtvmcdonald is offline
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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.
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  #62  
Old 02-07-2017, 11:56 AM
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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.
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  #63  
Old 02-07-2017, 12:03 PM
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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.
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  #64  
Old 02-07-2017, 05:16 PM
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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.
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  #65  
Old 02-07-2017, 05:41 PM
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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.
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  #66  
Old 02-07-2017, 06:35 PM
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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.
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  #67  
Old 09-28-2017, 04:38 PM
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Its official, my 15GP22 is bad because of gas

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.
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  #68  
Old 09-28-2017, 05:03 PM
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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.
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  #69  
Old 09-28-2017, 05:34 PM
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Tried that with bens AXP which still has perfect getters, ran it for more than a week still has purple neck.
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  #70  
Old 09-28-2017, 06:14 PM
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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.
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  #71  
Old 09-28-2017, 08:59 PM
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dtvmcdonald dtvmcdonald is offline
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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.
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