If the external controls on an old CRT monitor won't make it bright enough to
see, then you may be able to get some more life out of it by using the internal
controls.
WARNING! High voltages are present inside a CRT monitor. Take care not to touch
anything directly with your hands and do not use any metal tools.
The controls to look for are on the flyback transformer. That's a large,
plastic part with a big wire connected.
Most flyback transformers have two controls. One is usually labeled focus, the
other is brightness or screen.
Either obtain some plastic TV and electronics adjusting tools or you can
whittle some from Popsicle sticks. (For flybacks that have knobs that stick out
and plenty of clearance, I just use my fingers... and keep the other hand
behind my back.)
It's easiest to do the adjustments by placing the monitor in front of a mirror
so you can watch the reflected screen image.
Set the external brightness control all the way up and the contrast control to
the middle of its range. Turn the brightness/screen flyback control up until
you can just see the angled retrace lines and some glow outside of the graphics
area. Now turn it back until that just becomes invisible. The screen should
still be quite bright, too bright for a room with dim lighting or darkness.
That should give the external control enough range to handle a wide variety of
lighting conditions.
Now tweak the focus control to get what looks to you like the sharpest image.
It's best to check by looking directly at the monitor to confirm its focused.
On some monitors, adjusting the focus alters the brightness so you have to go
back and forth a few times to optimize them both.
Unless the phosphors are next thing to completely worn out, this adjustment can
get a bit more life out of an old monitor. If the image is still so dim you
have to be in a completely dark room to see it with the external and flyback
controls turned up, the CRT is shot and only a replacement tube can fix it,
which ain't worth the effort.
There is one other procedure, which isn't one a typical home knob tweaker can
do. Old time TV repair shops had a device to carefully run excess voltage
through the electron gun heaters and electrodes of a CRT. Metal burned off the
shadow mask and stuff from the phosphors could get deposited on them, reducing
their output.
It's a fine line between burning off the deposits and frying the electrodes and
heaters. Newer CRTs were designed to minimize those deposits. If the phosphors
are shot then this treatment won't help.
Yet another trick from back in the day when cabinet model TVs sitting on the
floor were in the majority (and hideously expensive) was a device that boosted
the voltage to the electron gun. It was a permanent install and much cheaper
than a new TV, but only a temporary measure until it'd either burn out the gun
or the phosphors deteriorated further. It was also a favorite of unscrupulous
electronics shops to sell used televisions with CRTs about to die.
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