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.

Jestine Yong is a electronic repairer and a writer. For more electronic repair information please visit his website at http://www.noahtec.com/electronic-repair-articles.htm