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From: hoffman@pitt.UUCP (Bob Hoffman)
Newsgroups: unix-pc.general,comp.sys.att,sci.electronics
Subject: Re: Bouncey-Bouncey
Message-ID: <3785@pitt.UUCP>
Date: 24 Aug 88 17:13:41 GMT
References: <593@proxftl.UUCP>
Reply-To: hoffman@pitt.UUCP (Bob Hoffman)
Organization: Univ. of Pittsburgh Computer Science
Lines: 31

In article <593@proxftl.UUCP> aaron@proxftl.UUCP (Aaron Zimmerman) writes:
>My 3b1's monitor (you know, that ugly GreenScreen...) has a sore case of the
>bounceys. Sometimes it will decide to sort of vibrate up and down.

One possible cause may be similar to that which afflicts many of our
Zenith and DEC CRTs:  bad solder connections.

The symptoms are that the display will begin to vibrate or fold over
itself.  If left alone, the condition worsens until you have a single
vertical or horizontal line, or no screen image at all.

What is happening is that the solder holding a joint cracks, creating
a high-resistance connection.  Heat is generated by this resistance,
causing the crack to widen.  Eventually, the connection begins to arc,
causing even more damage.

Look at the solder side of the PC board which contains the high-voltage
sweep components.  The problem seems to affect the larger components on
the board, particularly connectors, coils, and transformers.  The bad
connections look like a dark ring around the component pin.  Also note
any discolored areas of the PC board. These indicate excess heat in
that area.  It's not a bad idea to take a soldering iron and some good
solder and re-flow the joint at each of the pins of those components.

I have repaired many "dead" CRTs this way and have not had to replace
any components.  Simply re-flowing the solder connections fixed them.

	---Bob.

-- 
Bob Hoffman, N3CVL       {allegra, bellcore, cadre, idis, psuvax1}!pitt!hoffman
Pitt Computer Science    hoffman@vax.cs.pittsburgh.edu