(*SIGH*) 3b1 monitor question.

Rob Stampfli res at colnet.uucp
Tue May 21 12:52:48 AEST 1991

>>The most common problem with 3B1 monitors is a failure mode where the right
>>side of the screen becomes increasingly nonlinear.  Once this starts, it
>>proceeds at an exponential rate until the monitor fails.  The problem is
>>a bad capacitor in the flyback circuitry.  If it is caught prior to the
>>actual failure of the monitor, this is all that has to be replaced.  Once
>>the capacitor fails, it takes out many hard-to-replace parts, like the
>>flyback transformer, effectively rendering the monitor unrepairable at any
>>reasonable cost.
>is there only one capacitor in the flyback circuitry, or is there a
>particular one to replace? if so, what are the specs?

In response to your query, and numerous others, here is the original article
posted some time ago:

Newsgroups: unix-pc.general
Path: cbnews!res
From: res at cbnews.att.com (Robert E. Stampfli)
Subject: Re: Monitor problem leading to failure
Organization: AT&T Bell Laboratories
Distribution: na
Date: Fri, 16 Nov 90 06:25:39 GMT
Message-ID: <1990Nov16.062539.25894 at cbnews.att.com>
Summary: progressive monitor non-linearity problem leading to failure resolved
Lines: 57

I have witnessed the loss of more than a few Unix-PC monitors to a phenomenon
where the right side of the screen becomes increasingly compressed
(nonlinear), at an exponentially increasing rate, until the monitor fails
completely.  Usually, this occurs over a period of months, and it usually is
observed after the monitor has been removed from service, stored, then placed
back into service.

We had a monitor on the verge of failing on one of our machines recently,
and, not having a spare, I coaxed one of our hardware gurus to have a look
at it.  This guy is really good at finding hardware glitches and he found
and fixed the problem -- which turned out to be a failing non-polarized
electrolytic in the flyback circuit.  For those of you who are experiencing
this problem -- and I believe it is quite common -- I am reproducing his
notes.  Beware, however, that this particular problem was caught before the
monitor failed completely.  We believe that an actual failed monitor would
take out an associated transistor and the flyback transformer, unless the
monitor is fused, presenting a much more difficult problem to fix.

Here are the notes of Harry Maddox, who found and fixed the problem
(thanks, Harry!):

A 4.7 uF 25V non-polarizing electrolytic capacitor (C411) is bad.  C411
developes a high internal resistance and gets quite hot, which further
causes it to deteriorate.  It may explode -- use caution.  C411 is
located between L402 and T401 inside the monitor proper.  The value and
voltage rating of this capacitor is not critical, "ESR" is however.

Replace C411 with a mylar capacitor, 4-6 uF @ 25V or more, such as
AT&T 535GA (4.22 uF @ 100V) or equivalent.  Mount on end with insulation
on top lead.  Dress away from adjacent parts.

Replace CAP ASAP before further damage is done.  Replace fuse if blown.
Check TR402 for Collector to Base shorts if set was not working.  Also D403
for short.


1. Remove CRT Cover (2 screws in back).
2. Remove Speaker Assembly (2 screws).
3. Remove RF Tape from bottom cover (shield).  One tape is hidden at
   front of bottom shield.
4. Remove bottom shield.
5. A small part (I think a thermal sensor) is found on the bottom of
   the PC board just under C411.  It is covered with black tape.  Remove
   the tape and bend sensor out of the way.
6. Remove C411 and replace with Mylar capacitor, 4-6 uF, 100V.
7. Replace sensor and tape.
8. Reassemble monitor.

Note: the bottom shield is tricky to remove and replace if you don't unmount
the monitor from its swivel mount, but it can be done.  If monitor has failed
completely, then check fuse (if present), TR402 (C-B short), and D403 for a

Rob Stampfli	/ att.com!stampfli (uucp at work) / kd8wk at w8cqk (packet radio)
614-864-9377	/ osu-cis.cis.ohio-state.edu!kd8wk!res (uucp at home)
Rob Stampfli, 614-864-9377, res at kd8wk.uucp (osu-cis!kd8wk!res), kd8wk at n8jyv.oh

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