 The magazine of the Melbourne PC User Group The Big Picture - A Monitor Resurrected John Mackesy mack@melbpc.org.au |
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Some time ago, I had a 19-inch monitor given to me, free, gratis, no charge,
just take it away. I can just hear you thinking "Some people have all the luck! There's got to be a catch
somewhere." Several catches, in fact.
The first was that it was thought to have some obscure fault in the vertical deflection area, the second was
that it had been severely damaged by a falling object while the case was removed. The upshot of all this was
that it was deemed to be beyond economical repair, had been written off, and was mine if I wanted
it.
For one shackled by the surly bonds of poverty but possessed of a few "technosmarts", this was an offer too
good to refuse. When the time came to collect it, the sheer size of the thing came as a bit of a shock. And
the weight! The weight of monitors seems to increase exponentially with size. This baby tipped the scales at
31 Kg - too heavy and too awkward to carry any distance.
Now, repairing monitors and power supplies is not for the faint-hearted or the technically disinclined. For
one thing, they're usually plastered with prominent "No user serviceable parts inside, enter here and you
die" notices. Once you get inside, there are more warnings like "Danger! 27500 Volts!" and "X-ray
precaution!"
Having spent a few years around things like multi-thousand horsepower aircraft engines and the more grandiose
variety of industrial electronics, I wasn't too concerned by any of this. I'd also fixed a few less imposing
monitors, and a number of TV sets in the distant past when they were black and white and chock full of
valves. It was a while ago.
For those of you who have never looked inside a monitor, a peek within will reveal that it's mostly picture
tube. Surrounding the tube will be one or more circuit boards - power supplies, amplifiers, logic and that
sort of thing. Mounted on the neck of the tube itself will be an assembly quaintly referred to as "the yoke".
This is an array of electromagnets, ("deflection coils") driven by the horizontal and vertical amplifiers.
The varying magnetic fields deflect the electron beam within the picture tube... Perhaps I'd better stop
there.
Assessment
The first step in the repair process was assessment - ie. was it worth fixing? Surveying the damage I found
that a small (75 mm x 75 mm) printed circuit board mounted on top of the picture tube's deflection coils had
been smashed into three pieces. Most of the small thicket of wires attached to it had been broken off or
pulled away.
Repairing this was going to be an exercise in both handicraft and industrial archaeology. The verdict? Able
to be done, but an interesting challenge and at the end I would know the nature of the original problem. In
its favour, the device appeared to be solidly built (very solidly built!) to a high standard. Before I went
any further, I would need a service manual, for this was a seriously complicated item, well endowed with
obscure features.
Eventually, a manual was acquired, $35.00 for a rather slim and very ordinary photocopy. Hmmm - very
interesting. Pages of circuits and adjustment info, but nothing on the smashed circuit board. Oh well, it
couldn't be that complicated. And that's where it stopped, while other, higher priority, jobs were taken care
of. The months slid smoothly by until quite suddenly it was January 2000. Decision time -it would either be
fixed or turfed. This was a serious space occupier and it was becoming a big dust gatherer.
Preparation
Before starting the job, I made a drawing of the damaged area and labelled all the wires. By comparing the
lay of the broken wires and the broken ends I was able to divine what went where. The next job was repairing
the shattered circuit board. This turned out to be quite easy - the pieces were glued together with
industrial-strength cyanoacrilate adhesive (superglue) and the breaks in the tracks soldered together.
Interestingly, one of the components on the board was found to have a poor soldered connection (a "dry
joint"). Once that was out of the way, I turned my attention to recreating the wiring loom - about 15 wires,
all of which had to be soldered to the existing stumps and tacked down with Silastic; neutral cure, of
course. It sounds straightforward enough, but in practice was very fiddly. When that was done the printed
circuit board was glued to the yoke with Silastic and the work of reattaching the wires begun.
Quite suddenly, the job was finished. We won't talk about the sheet metal fabrication work - an internal
panel had been lost during its previous existence. I had a cup of tea, checked and double checked my work,
then connected the 15-pin high-density D by 5 x BNC video cable and power cord - attached it to my old 386
and switched everything on. It worked! Well, sort of- the picture wouldn't stand still and was folded
over.
Laborious probing revealed there were no horizontal synchronizing pulses, a problem I traced to a test switch
that had been thumped during the original trauma. Reset to "on" position, it worked like a dream. I've been
using the monitor ever since. No problems so far. I suspect the original problem was the previously mentioned
"dry joint", which would have been fairly difficult to get at.
I guess there would have been about 20 hours labour expended on this job; typically I'd spend an hour to an
hour-and-a-half on it every day. Having it set up on a card table in a corner of the lounge room meant it was
always accessible and in a pleasant, air-conditioned working environment, but invariably that sort of thing
requires some negotiation with one's partner. There were also occasions when that extra pair of hands was
useful.
Total cost was $35.00 for the manual, $32.00 for the video cable and about $5.00 for sundries. Equipment
used: small hand tools, bright light, industrial quality soldering iron, multimeter, hot air gun,
oscilloscope and of course, reading glasses.
Reprinted from the March 2000 issue of PC Update, the magazine of Melbourne PC User Group, Australia
