Whenever there is a complaint about monitor
contrast problem I will check on the flyback
transformer ABL (automatic blanking limiter)
circuit.
Sometimes a weak picture tube might caused
contrast problem. If the picture tube is
good, I will go straight to the contrast
circuit. The question is how do we find
where the contrast circuit located? Simple,
if you have a datasheet for the video pre-
amplifier ic, definitely you can find the
contrast pin. For LM1203N video pre-amp
ic the contrast pin is located at pin 12.
Replace this ic or trace from this circuit
to find the cause of the contrast problem.
From experienced, usually capacitors shorted
and resistor turned into high ohms were the
caused of the problem. A video pre-amplifier
can also be defective. To confirm whether is
circuit or ic problem, just solder out the
contrast pin and switch on the monitor. If
the monitor display normal picture (contrast ok)
then suspect the abl line circuit. If it remain
the same (dim contrast) replace the video
pre-amp will usually solve the problem.
The contrast signal came from one of the pin
(leg) of a flyback transformer which we called
it as abl pin. Many technicians have the
difficulty to trace the abl pin because
usually a flyback transformer consist of around
10 pins. Some pins are B+, GROUND, AFC, ABL,
X-RAY PROTECT, VCC, to HOT, HEATER, horizontal
centering and etc. But there is one and simple
way that i’m gone to show you how to locate the
abl pin. Use two analog multimeters set to x10kohm
range. Connect it in series (+ probe to – probe)
and measure the flyback transformer from anode
(the red cap) to all of the pins under the
flyback. You have to desolder all the pin or
just remove the flyback from the mainboard in
order to get an accurate reading.
If either one of the pin shows a diode
reading that is the abl pin. The reason we use
two multimeters is because the flyback circuit
internally have couple of high voltage diodes
connected in series. If you have a schematic
diagram or any monitor or television
troubleshooting book, you will see that the
internal flyback diode are connected in series.
By connecting two meters the ouput from the probe
is around 24 VOLTS which is enough power to check
the flyback internal diodes. After you have
determined the abl pin, you can proceed to follow
the line and find the bad components that cause
the contrast problem.
Television or tv flyback transformer generally
have the same design compare to monitor flyback.
Whether it is a montblanc flyback transformer,
rca, mitsubishi or sony flyback the checking
usually is the same.
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