CRT autopsy question: What finally killed my monitor?

[Mxsmanic]

Today I bought a new monitor. My eight-year-old Sony 20" Multiscan IIe
finally scanned its last raster on Thursday, after some 30,000 hours of
faithful service.

I find myself wondering nostalgically what finally killed the CRT. Over
time, of course, it had gradually gotten darker and a tiny bit fuzzier.
The convergence and geometry and color balance were remarkably constant
right up until the very end. But the brightness and contrast gradually
diminished, and during the last 90 days or so, they started to decline
fast, with frequent shifts in the RGB output.

My assumption is that the electron gun itself was simply worn out, no
longer able to produce a copious supply of electrons for the tube. I
don't recall if this tube had three guns or one (perhaps three, if the
RGB balance shifted randomly). Eventually the screen got so dim that I
had to turn down other monitors and turn off lights in order to read it.

The real mystery for me, though, is why it finally failed abruptly. A
few weeks ago there was a power failure, and the CRT was off for two
hours and cooled down completely (instead of just being on power-saver
standby). It took 2.5 hours for it to come on again, but when it
finally did, the image became visible quite abruptly, over a period of
about 2 seconds. What I don't understand is why the screen was dark for
2.5 hours (no trace of an image), and then _suddenly_ the screen came on
and looked okay.

Doesn't a failing tube simply get darker and darker? Wouldn't a tube
that takes a long time to warm up just fade into visibility very
gradually? Why did the tube stay dark for hours, then abruptly produce
a visible image?

It's mostly academic now, but I've been trying to figure out why it
behaved this way. Any ideas?

 

[Bob Myers]

Doesn't a failing tube simply get darker and darker? Wouldn't a tube
that takes a long time to warm up just fade into visibility very
gradually? Why did the tube stay dark for hours, then abruptly produce
a visible image?

A failing tube DOES get darker and darker, but that
process is not nice and linear, especially near the end of life.
What's going on here is basically a loss of emitting surface
in the cathode, through the "wearing away" of the emissive
layer and the "poisoning" of that layer by other materials
within the tube (ions which are attracted to this surface and
wind up bonding to it). The factor controlling cathode aging
is not just the beam current, but the current per unit area -
and as bits of the surface are knocked out of commission,
that same amount of current (since you're adjusting the
"brightness" control, trying to keep the thing usable) gets
crammed into an ever-diminishing area, which then hastens
the process in that area. Sometimes, small areas of the cathode
can temporarily "rejuvenate" themselves, by essentially blasting
the accumulated crud off (an oversimplification, but it's basically
the same action that the old drugstore "tube rejuvenators" did,
back in the Olde Days) - but then they quickly fade again, as
the current now crowds into THAT area. That sort of thing
may account for your sudden burst of brightness at the end, or
it may just have been an intermittent connection somewhere in
the video signal path. Hard to tell at this point, and, as you noted,
mostly academic.

Bob M.

 

[Mxsmanic]

A failing tube DOES get darker and darker, but that
process is not nice and linear, especially near the end of life.
What's going on here is basically a loss of emitting surface
in the cathode, through the "wearing away" of the emissive
layer and the "poisoning" of that layer by other materials
within the tube (ions which are attracted to this surface and
wind up bonding to it). The factor controlling cathode aging
is not just the beam current, but the current per unit area -
and as bits of the surface are knocked out of commission,
that same amount of current (since you're adjusting the
"brightness" control, trying to keep the thing usable) gets
crammed into an ever-diminishing area, which then hastens
the process in that area. Sometimes, small areas of the cathode
can temporarily "rejuvenate" themselves, by essentially blasting
the accumulated crud off (an oversimplification, but it's basically
the same action that the old drugstore "tube rejuvenators" did,
back in the Olde Days) - but then they quickly fade again, as
the current now crowds into THAT area. That sort of thing
may account for your sudden burst of brightness at the end, or
it may just have been an intermittent connection somewhere in
the video signal path. Hard to tell at this point, and, as you noted,
mostly academic.

Thanks for the assessment. Whatever happened, it sure was a nice
monitor. Sony monitors merit their high prices. This one went well
beyond the call of duty.

 

[Mxsmanic]

Well, at least they DID merit it. Too bad everyone is
in the process of bailing out of that market...sigh.

So I've heard. They even stopped making their excellent Artisan
monitors. But I presume they'll still be making broadcast television
monitors for a long time to come.

 

[Bob Myers]

Thanks for the assessment. Whatever happened, it sure was a nice
monitor. Sony monitors merit their high prices. This one went well
beyond the call of duty.

Well, at least they DID merit it. Too bad everyone is
in the process of bailing out of that market...sigh.

Bob M.

 

[chrisv]

Thanks for the assessment. Whatever happened, it sure was a nice
monitor. Sony monitors merit their high prices. This one went well
beyond the call of duty.

Yeah, nothing beats a Trinitron for image quality. I love my F500R,
and will be truly sad when it can no longer serve me.