Hi,

A neighbor was trashing an older Mitsubishi RPTV because he'd replaced
it with newer technology. He basically put it on a street with a sign
"Free for the Taking"...so I did.

I thoroughly cleaned out the unit, got a service manual and went
through the adjustments, and it now shows a pretty decent picture
(given the price I paid.)

However, I have noticed one problem with the pictu after the set has
first been turned on and for a few minutes afterwards, if there happens
to be a bright-to-dark vertical boundary in the picture with white to
its left and black to its right, there will be an uneven, splotch of
bright blue color all along the right side of that boundary, looking
almost like a bright blue, sloppily-painted thick vertical band of
color.

It will gradually stop happening, and I can live with it, but if it
turns out to be simple/cheap to fix (i.e. not involve replaced a tube),
I might tackle it.

Mainly, though, I'm just interested in knowing what is physically
causing this. Is it more likely to be that the blue video circuitry is
somehow unable to quickly drop down to black level as the beam passes
across the white-to-black boundary, or is this some physical phenomenon
happening in the blue picture tube? Could the phospor be losing its
responsiveness?

Thought someone here might be able to explain this...thanks.

Blue tube is soft and has lower emission than the red and green. Run
your contrast down while it's happening and I'm sure you'll see it
diminish as you adjust. Not much you can do past that except replace
the tube.

Is the blue tube projecting a sharp picture? Or are you seeing
bleedover caused by dirty cooling fluid?

I heard somewhere that the blue tube typically runs hotter, and the
cooling fluid clouds up and eventually degrades the image. Replacing
the fluid is not trivial but a lot easier and cheaper than replacing
and aligning the tubes.

I don't know why it would go away after warming up, unless whatever is
obscuring the picture goes back into solution when it's warm.

Mr. Land wrote:
However, I have noticed one problem with the pictu after the set has
first been turned on and for a few minutes afterwards, if there happens
to be a bright-to-dark vertical boundary in the picture with white to
its left and black to its right, there will be an uneven, splotch of
bright blue color all along the right side of that boundary, looking
almost like a bright blue, sloppily-painted thick vertical band of
color.

wrote:
Is the blue tube projecting a sharp picture? Or are you seeing
bleedover caused by dirty cooling fluid?

I heard somewhere that the blue tube typically runs hotter, and the
cooling fluid clouds up and eventually degrades the image. Replacing
the fluid is not trivial but a lot easier and cheaper than replacing
and aligning the tubes.

I don't know why it would go away after warming up, unless whatever is
obscuring the picture goes back into solution when it's warm.



If the coolant is contaminated you get cloudy splotches normally and it
doesn't change as it warms. I would agree with the other diagnosis that
the blue tube has gone weak, though it wouldn't hurt to have the
emission checked as it's possible there's something else wrong. It also
may be possible to reduce the brightness of the other tubes to get a
decent white balance without the blue clipping.

You could try boosting the blue gun emissions by zapping the cathode -
grid with a high voltage arc, that procedure has been discussed here
many times. It is a temporary fix, and there is always the risk of
actually reducing the emissions, but I would guess it works about 85%
of the time for a period of 6 to 12 months.

H. R.(Bob) Hofmann

Thank you!

Curious: why would *lower* emission result in bright blue color
blotches? Intuitively it would seem that lower blue gun emission would
result in an overall yellowish screen.

Thanks again for taking the time to answer.

Thanks for that suggestion.

Think I'll leave it alone though.

It seems to go away after a few minutes and the picture looks pretty
decent afterwards (especially for a big screen that only cost me the
price of the service manual.)

Thanks again.

Thanks for your reply.

Interesting idea. I've only noticed the effect at vertical
light-to-dark edges, though...the rest of the picture looks fine.
Cheers.

Thanks for replying.

WB is OK, I just get the bright blue splotch that goes away after a few
minutes.

Interesting: the emissions can be measured? Is that done via an
equivalent current measurement?

Regards...

Whoops, I thought that Google would put my replies in the right
places...sorry.

Mr. Land wrote:
Thanks for replying.

WB is OK, I just get the bright blue splotch that goes away after a few
minutes.

Interesting: the emissions can be measured? Is that done via an
equivalent current measurement?

Regards...

You put a micro/milliammeter in series with each of the cathodes and
urn on the guns one at a time, or there are tube checkers that can do
the same thing. Another possibility is to raide the filament voltage
of the blue crt slightly to raise the emissions. Depending on the set,
you can add a few turns of wire to the horizontal output transformer
(HOT) and put them in series with the filament voltage if it is derived
from the HOT, measuring the voltage as you add the turns. IF you get
the connection so that the voltage decreases, you must reverse the
connections. A lot depends on how familiar you are with tv set guts.

H. R. (Bob) Hofmann

You put a micro/milliammeter in series with each of the cathodes and
urn on the guns one at a time, or there are tube checkers that can do
the same thing. Another possibility is to raide the filament voltage
of the blue crt slightly to raise the emissions. Depending on the set,
you can add a few turns of wire to the horizontal output transformer
(HOT) and put them in series with the filament voltage if it is derived
from the HOT, measuring the voltage as you add the turns. IF you get
the connection so that the voltage decreases, you must reverse the
connections. A lot depends on how familiar you are with tv set guts.

H. R. (Bob) Hofmann




Yikes, no not the HOT, you put a few turns of wire around the core of
the flyback transformer. Tricks like this and rejuvenating the guns are
generally not advisable on projection CRTs though, they're already run
very close to their limits and trying to get more life out of a tired
one usually just kills it.

JIm:

You can put a few turns around the Horizontal Output Transformer(HOT)
core, that is the same as the flyback transformer unless terminology
has changed in the last few days. G. I have done this a number of
times, frequently adding a resistor in series with the added turns to
ease the turn-on surge due to the low filament resistance when cold.
It prevents a sudden burn-out on turn-on, but like you say it's of
limited value and I only do that as a last resort.

Bob Hofmann