Answer these questions and you might be able to narrow it down.
Do you have a schematic?
When the picture appears, does the color look good for the second? If so
the tv is probably going into blue screen mute.
Look to the system control (aka microcontroller/ micro) ic that basically
runs everything first.
You will may find OSD outputs RGB going to the jungle ic (term for the big
signal process ic that does most everything). Look at the RBG outputs to
see if the blue is on (more or less) all the time. If it is the micro is
definitely putting the tv into blue screen mute. Then look for the signal
sync going into the micro to see if it is present.

The micro has several must signals in order to display a picture. H-sync
(horizontal feedback sync from deflection) set will usually turn off if
missing right away, V-Sync (vertical feedback sync from deflection) tv will
typically shut down after 2 to 3 seconds, 60Hz clock signal from the ac
power line (different sets behave strangely, could cause blue screen mute),
a serial clock and data handshake from several other ics on the databus
(since manufactures do not provide enough information to actually
troubleshoot this, swap a part until it works, some tv sets have a computer
interface and error code service mode to help), of course the micro needs
the signal sync in order to know there is an image to display, we cannot
forget about the AKB (auto-kine-bias) circuits on the picture tubes which
can blank the video completely.

Simple troubleshooting:
1. Since the menu works, and if the audio goes up and down, or different
video inputs can be selected via the remote control. This indicates that at
least the data buss is functional and the ics are communications. It does
not mean there not an ic that is not sending the all ok signal to the micro
however.
2. Since the video comes on for a second when immediately switching inputs,
the set is most likely in blue screen mute.
3. If the blue screen does not have the 10-12 visible diagonal retrace
lines, this pretty much rules out the video output stages.
4. Try the s-video or component inputs to see if they work as well. If the
s-video works, it means there is a problem before or including the comb
filter. If only the component video works even more of the signal circuit
has been eliminated.
5. Do not forget that this device has virtually all of its functional
parameters (adjustments) set by an eeprom and service menu. Corrupted data
in the wrong location can cause the same symptoms as actual parts failures.

As an added note, I strongly recommend you use an isolation transformer for
the tv set whenever working on it with the back removed. It will provide a
level of safety to yourself and your test equipment that no measure of being
careful can achieve. Even though that set uses a SMPS to provide isolation,
there are still exposed components and traces tied directly to the AC power
line source.

After doing some research, I found out you really are at least a Professor
at a college or university so you might have a PhD afterall, sorry for
doubting your word. Don't take anything personal as it is hard to judge the
character of someone from inaccurate information posted on the web. I have
to go by experience and it has not been a good one with some people, as
indicated with the fire and lawsuit.

I have taught unemployed engineers in a course where they were retraining to
become technicians. They were by far the worst students at "getting it"
when it came to real world troubleshooting and circuit behavior.

It should take about 15 minutes maximum to narrow down a close location of
the problem.
I always check the required stuff before going in very deep. Proper and
clean Vcc and Vdd votlages. All the correct signals to the system control,
60Hz ac pulse, H-sync, V-sync, Signal sync, clean Vcc, etc. Then move to
the jungle ic. If the Signal sync is found missing or small, simply start
tracing it back to its source (near the jungle ic I am sure).
"bharat" wrote in message
om...
I haven't had this many arguments over a posting for a long time!!:-)

I own another 52H72 and have troubleshot the power board without any
problem (it did take me a whole weekend to do it). That set kept
turning on and off. I was hoping to avoid a repeat of my "lost"
weekend. I wanted someone to point out the area I should be checking.
I am not that familiar with the signal board and I believe people on
the net have more experience than I will ever have repairing a TV.
Hence the post. I did not want to post a lengthy note on what tests I
did (I checked all power supply lines, H-sync, and yoke drivers. I
have multimeters and a simple oscilloscope). I went through the
schematic. Went through this newsgroup. A previous post mentioned
video amps. There were amplifiers driving the yokes. I assumed that
is what the previous message was talking about when it said video
amps.

If I was wrong, please point it out and point me in the correct
direction. Pointing out my inexperience in repairing a TV is
acceptable, but not providing help and flaming me is not OK. There
are hundreds of postings each day here. If you can't provide help,
don't say anything. Everyone who reads this newsgroup knows (a) a lot
of novices are attempting TV repair who should not, (b) TV repair is
dangerous business, (c) most of these novices will harm themselves
(darwinian principle at work) and learn along the way not to do it
again (as David's post pointed out about the guy who burned his house
down).

Please do not assume that someone is not capable of understanding
things regarding TV and repairing it. With a little help and some
precautions (and common sense about electrical engineering), competent
people can repair TVs. Newsgroups like this one certainly helps.

I did not mean to offend collectively everyone (I was thinking no one
will read this posting because it was old. And only David and I will
scream at each other for a day and then forget about it). Obviously
that is not the case.

I am going to check yokes and sync signals. Hopefully, that will tell
me more about where to go. Thanks for all the help.

bharat