Wiebe Cazemier wrote in
b.home.nl:

On Saturday 26 April 2008 19:40, bz wrote:

So, what is different about 'just turned on'?
Components are cool.
High transient currents occur.

Look for places that those can make a difference.

(snip)

From the symptoms, I would start by looking at the PSU outputs.
Troubleshoot by 'divide and conquer'.

Divide the device logically into two halves, and localize the problem
to one of the two.

Continue until you reach the bad part.

I've been reading the repair faq at
http://www.repairfaq.org/sam/crtfaq.htm. At some point it says this:

Occasionally, small conductive flakes or whiskers present since the day
of manufacture manage to make their way into a location where they short
out adjacent elements in the CRT electron guns. Symptoms may be
intermittent or only show up when the TV or monitor is cold or warm or
in-between. Some possible locations are listed below:

One of them is: Heater to cathode (H-K). The cathode for the affected
gun will be pulled to the heater (filament) bias voltage - most often 0
V (signal ground). In this case, one color will be full on with retrace
lines. Where the heater is biased at some other voltage, other symptoms
are possible like reduced brightness and/or contrast for that color.
This is probably the most common location for a short to occur.

That appears to be exactly what's happening. The blue gun is turned on
completely, also showing the retrace lines. Also, remember that I have
been able to reproduce the fault a couple of times by tapping the
cathode assembly.

If you are SURE that no vibrations from your tapping traveled elsewhere,
that is a strong clue.
Otherwise it can be a misleading clue.

I have played the 'taps' game and won. I have played it and lost.
It is best when you can make smaller and smaller taps while getting closer
and closer to the problem.

If the tap will only occasionally trigger/fix the problem, then it is easy
to be misled.


I will try the "put the monitor on it's face and try tapping it to
dislodge the short" method first. If it works, it would be a very low
tech, but effective solution

The "blow out the short with a capacitor" method also seems fun, but
also a little risky

When I had a TV repair shop, in the early 70's, we had a tester for picture
tubes.
It had a short indicator and a 'remove short' button that discharged a
capacitor through the short.
Sometimes it would fix things. Sometimes it would make things worse.

Good luck with your experiment. Hopefully, it won't drop something into the
center of the shadow mask.

Be aware that with 15 lb of air pressing on each square inch of the CRT,
you are playing with a live bomb.

I once took a picture tube out, laid it on its faceplate, got about 50 feet
away and tossed rocks at it.

Woomp. Dust and dirt and glass flew everywhere.

Chunks of the face plate (glass about 1 1/2 inches thick) landed about 50
feet BEHIND me.

Now, I would want a thick sheet of lexan between me and any CRT that I was
tapping upon, and safety goggles, gloves.




--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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