Sharp 25R-M100 27" CRT, from 2002

Got it last week through work, from someone who broke off the antenna
and tried unsuccesfully to fix it. They already had a better one
anyway, so they were going to junk it.

I fashioned a new coax jack from a connector and a twist tie (for the
center pin) and soldered them in place.
For several days it worked 100% (well, it could use a de-guassing, but
its minor)

Then, all of a sudden, this morning upon turning on, it made the usual
noise and flash of a CRT first turning on, and then immediatly shut off
again.
The power strip it plugs into is shut off at night to save electricity,
so there is no chance that any part was too warm or that it needed to
be reset via disconnecting the power.

I have found that this only happens when the antenna is plugged in
(actually the coax line is from the ReplayTV, which in turn is plugged
into a 12v amplified roof antenna via coax)
If it is on channel "input" then it shuts off if there is a signal at
the composite input.

With coax and composite unplugged, I can go through all channels, I get
the normal blue screen, and I can access the built in menu. Therefor
there is clearly nothing wrong with its capacity to display an image.
But as soon as I plug it in to an incoming signal, it shuts off. I can
generally see an instant of what would be on the screen, and for that
instant it looks normal.

Once it has shut off, it must be unplugged for a few seconds before I
can turn it on again. Once I have done that it will operate from
either the front controls or the remote (assuming I unplug the signal
again).

I have searched the forums, and while there are several entries on
spontaneous shut down, none mention that it only happens if the antenna
(or cable or whatever) is plugged in.

Hopefully that condition will make it much easier to narrow down the
problem.

Thanks