Bill Mazlar said:
I've heard of simple fixes to CRT monitors that involve
little more than identifying a busted capacitor or tweaking
a potentiometer (though most of the labor is probably in
disassembly/assembly). I took a "free" monitor left outside
of college library, which gives no video signal whatsoever
when I plug it into my laptop. It makes a low scratchy-staticy
noise whenever it is on...any telltale indications of the
problem with this CRT?
I assume you know something about basic electronics, i.e., know how to
use a volt-ohm meter, are good at soldering, and have common sense
about safety.
One of the best starting places for information is
www.repairfaq.org
or a book about TV repair. Never open a monitor without first
unplugging its AC power and letting it sit for at least 30 minutes,
and as long as you take this precaution, risk of electric shock is
small,the chances of electrical shock are small, unless you remove the
thick cable going to the side of the CRT (suction cup cover). Almost
all capacitors discharge in less than a minute, but the CRT can retain
a high voltage charge indefinitely. Actually the most common risk
from shock is that it causes you to jerk your hand, either voluntarily
or involuntarily, and hit something, possibly cutting your hand or,
much worse, hitting the glass CRT and causing it to explode. Glass
breakage is an overlooked hazard and can also occur when the monitor
tips forward (its center of gravity is at the front).
Do-it-yourself repair is cheap unless an expensive component, like the
CRT, flyback, or an odd chip, is defective, but fortunately 90% of
monitor problems are either:
1. Bad solder joints -- a magnifying glass and strong light is needed
to find hairline cracks, but don't ignore joints that look odd,
especially spherical (not sticking). If you don't see any defects,
solder the joints of the large, heavy components, connectors,
potentiometers, and the hot devices (circuit board is often darkened
around them).
2. Main power supply. This is a switching mode supply that takes AC
and powers everything inside the monitor. You can find it by
following the AC connections.
3. Horizontal output circuit. This not only helps draw the picture on
the screen but also generates several voltages, including the 25,000V
used by the picture tube. It uses a large transistor (HOT -
horizontal output transistor, perhaps the largest in the monitor and
on the largest heatsink) to drive the flyback transformer (large
plastic device with that thick CRT cable mentioned earlier).
4. Electrolytic capacitors (cylinders covered in plastic with writing
on it).]
Since the monitor shows some activity, the main power supply may be OK
-- or not. First look for burn marks and electrolytic capacitors that
are bulging or leaking brown or white goo or powder (don't confuse goo
with glue used to hold bigger capacitors in place). But capacitors
can be bad yet look perfectly normal, and the only fairly sure ways to
find defective ones are by either using an ESR meter (this is not an
ordinary capacitance meter) or substituting new capacitors. Since the
latter costs about as much as an ESR meter, it's reasonable to simply
replace every capacitor surrounding the flyback transformer or in the
main power supply circuit, especially those rated 100V or higher.
Replacement capacitors must be rated for at least as much voltage and
temperature as the originals and should be within 20% of the original
capacitance. The capacitors should also be rated for high frequency
operation, except the very large one near the AC input, which operates
at low frequency and is usually rated for just 85 degrees Celcius.
You should also check the HOT and any other discrete transistors for
shorts. These components often operate at high frequency and may be
insulated from their heatsinks to prevent shorts, and replacement
parts must be similarly and adequately insulated (packaging differs
and can require different insulation, such as an extra nylon shoulder
washer). Try to get exact replacements and avoid NTE general-purpose
parts because some are completely wrong (counterfeit?), and they cost
more than the better parts anyway. But their web site,
www.nteinc.com, can be useful for finding information about
components, including pin-outs.
The best single source of parts may be
www.bdent.com because they
carry not only a large number of transistors and chips but also
high-frequency capacitors.
www.mcmelectronics.com (
www.mcminone.com)
carries transistors and chips, but their capacitor selection is
lacking.
www.mouser.com and
www.digikey.com have good capacitors.
When you remove a circuit board to gain access, keep track of all the
cable connections that you may have to remove, and if you can't remove
it completely because of cables to the picture tube, put some padding
between it and the CRT to prevent scratching the glass. You may have
to unplug the small circuit board from the rear of the CRT, but it's
often glued in place, and cutting the glue can scratch the glass, so
be very, very careful. Don't force anything.
Don't turn any adjustment pots since some require taking measurements
with the power on, and some interact, meaning that turning just one
pot can require readjusting several others.
