Flyback Transformer-How to Locate the ABL Line

Whenever there is a complaint about monitor

contrast problem I will check on the flyback

transformer ABL (automatic blanking limiter)


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.