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From: wtm@uhura.neoucom.EDU (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: big screen went out
Keywords: big screen
Message-ID: <1991Feb02.032921.2427@uhura.neoucom.EDU>
Date: 2 Feb 91 03:29:21 GMT
References: <16231@sdcc6.ucsd.edu>
Distribution: inet
Organization: Northeastern Ohio Universities College of Medicine
Lines: 37

Loss of the picture can be caused by a number of problems.  I
recently had an experinece with a friend's Zenith big screen TV.
There was a cold solder joint on the power supply board that caused
the sensing of the CRT anode voltage to be incorrect; this in trun
caused the flyback pre-regulator to shut down -- the presumption
that the customer would be spared from being fried by an over
abundance of X-rays or possible damage to the CRTs.  According to a
local service tech; cold solder joints are an endmic problem on
Zenith 46" chassis power boards.

Loss of scan or improper scan will also trigger a shut down of the
high voltage on most sets -- especially big screens where there are
three CRTs that can suffer from burned phosphor.  Any number of
things can cause scan problems.  Look for bad electrolytic caps in
emitter circuits in the vertical output or in interstage coupling
in either horizontal or vertical sections.

Another one I've seen several times on small (13 inch)
line-operated Korean made sets is that 130 volt regulator takes a
power line hit and shorts out.  This increases the power supply up
to around 150 volts and once again triggers the X-ray safety
circuit.  This is easy to spot.  Put the set's input on a variac
transforer and reduce to about 90 volts; the set should come on.
Look for thick film hybrid module with a marking like "STR-135" on
it.  Good parts houses should have these.

I hate working on TVs even more than programming in Ada.


==Bill==


-- 
Bill Mayhew      NEOUCOM Computer Services Department
Rootstown, OH  44272-9995  USA    phone: 216-325-2511
wtm@uhura.neoucom.edu   ....!uunet!aablue!neoucom!wtm
via internet: (140.220.001.001)