How To Easily Restore 14 And 15"
Monitor CRT With This Little Trick
Picture become weak or dim usually happen after the monitor have been used for
many years. Again it depends on the quality of the picture tube or cathode ray tube
(CRT). Some grade A CRT can last for many years, but for the lower grade one it can
only last for couples of year. Don’t be surprise that I do come across picture tube that
had gone dim in less than a year! If you switch on your monitor for 24 hours without
turning it off, then expect the tube to become dim fast. Besides dim (weak emission) ,
CRT may also develop many problems such as open filament, short between cathode
and heater, G1 and G2 short, poor gamma performance, stripped cathode and focus
and G2 short. In this article, I’m going to touch on the weak emission problem only
and show you a possible way to brighten up the picture tube.
The cathode ray determines the
brightness and sharpness of a picture
tube. Its electrons are emitted from the
cathode surface into the cathode ray
tube’s vacuum. The electrons then
strike the screen in the form of a ray
and cause it to shine. Weak or used up
cathodes produce few electrons and
emit weak electron rays.
This causes the picture to become dark
and fuzzy. Sometimes the cathode
emission ceased due to a layer of
contamination coating covers the
cathode surface. This contamination
prevents the electrons from leaving the
cathode’s surface thus you get a dim
picture.
In order to identify whether is the tube
that cause the dim or from bad
components, one can always use a CRT
tester/meter to check it. Some call it
CRT rejuvenator, CRT restorer, CRT
rebuilder and others named it as CRT regenerator. These testers are designed to test
and check CRT and fix it if the picture tubes have problems.
I personally owned two CRT testers- the BMR 2005 Muter regenerator and a less
feature one I bought from Thailand. With the help of CRT tester, I can know if a
picture tube is working or not. If you do not have one, don’t worry because by
checking the voltage at the CRT socket pin outs, you will have some ideas if the CRT
is good or bad.
From my experienced, I noticed that the older tube can be brightening up with the
help of CRT tester but not for tubes that was made in the year 1994 and above. I
suspect the getter (made of barium) inside the tube have been used up. The functions
of getter inside the tube are to absorb gas that was release when the cathodes are heat
up. If the getter fails to absorb the gas, then there is no point to restore a picture tube. I
have brightened up many picture tubes with the help of CRT tester and some looks
like a new tube. But somehow after one or two days the picture tube get dim again
because the getter cannot absorb the gas and the gas will travels back to the cathode
surface and turn it into carbonate. Electron emission will cease again. I even have
tried some home made CRT restore schematic downloaded from the internet and the
result is still the same-the picture tube will dim back after couples of days.
After done some research and
experiment, I found that the best way
to brighten up the tube is to increase
the heater voltage. Normal heater or
filament voltage is about 6.3 volt and
if you increase the voltage to about 8
to 9 volts, this means the cathode will
produce more electrons and thus the
screen will become brighter. Bare in
mind that do not apply any voltage
that is higher than 9 volts to the
heater otherwise the internal filament
will burn out and broken. Once it is
open then there will be no way to
rescue it.
If the picture tube is heading to
rubbish dump then there is no harm
to try to restore it. About how long it will last, I cannot guarantee it. Some will last
even more than two years and some only couples of weeks. Recently there was a
badge of Compaq and Hp 15” monitor that have the symptom of blur display with
bright picture. No matter how hard you try to adjust the focus adjustment at the back
of flyback transformer, the picture still look slightly blurs. Using the methods above I
had saved lots of picture tube (I don’t need to replace a second hand one to customer).
It is a simple modification and this
only applies to the 14 and 15”
monitor picture tube only. First you
need to find which secondary output
that have output from 12 to 15 volts.
Once you have locate it, then solder
a wire at the cathode pin of the
diode and join it to the input pin of
7808 (output 8Volts) voltage
regulator.
Please don’t tap from the B+ voltage
which is usually 45 volts and above
and this may kill the 7808 IC
because this IC can only withstand voltage up to 38 volts maximum.
The current drawn from the 14 and 15” picture tube is less than the 17” tube and if
you use this way to brighten up the 17” tube, the power supply may go into shutdown
mode and sometimes will even lead to power blink.
I do know some technicians like to use the method of turning few rounds of wire at
the flyback ferrite core to restore the brightness. This method can work in Television
but not in Monitor. Computer monitors runs in many resolution and the higher the
resolution the higher the B+ voltage and thus making the output voltage at the wire
increased. This will lead to a sudden brightness and may shutdown the monitor. Even
if it didn’t shutdown, the customer may get annoyed with the Monitor brightness
(raster) whenever the customer change the monitor resolution.
Now solder the pin 2 of 7808 IC to cold ground and the output of 7808 IC pin 3 to the
CRT heater point as shown in the picture. Remember to cut off the 6.3 volt supply
line from the power supply because you don’t need it anymore. What you need is the
8 Volt supply to the filament in the picture tube. You can also connect a low ohm
protection resistor in series to the heater point. Just in case if any short circuit happens
the resistor will open circuit. Make sure you attach the 7808 IC to the heatsink,
preferable the heatsink at the flyback transformer.
This method only work for tube that is slightly blur and not too dim picture. If a
picture is very dim even if you raise the heater voltage to 12 volt, you still would not
see a bright picture. As mentioned above there is no harm trying to modify it and who
knows the tube can be brought to life and serve the customer for many years to come.
Hope you enjoy the article.