How to easily find out the abl line using an analogue multimeter

Whenever there is a fault on monitor about contrast problem i will test on the flyback transformer ABL (automatic blanking limiter) circuit line. Sometimes a fade cathode ray tube you can mistaken to be contrast problem. If the picture tube is good after testing it with crt tester, i will go look at the contrast circuit in crt board and flyback transformer. The question is how do we know where the contrast circuit is located especially there is so many models of  monitor in the market? Simple,if you have the datasheet for the video pre-amplifier ic which you can download free from the internet , definitely you can find the contrast pin. For LM1203N video pre-amp ic the contrast pin is located at pin 12 of the ic. You can direct replace this ic or trace from this circuit to find the cause of the contrast problem. From my experienced, usually capacitors shorted and resistor turned into high values were the caused of the problem. Sometimes a video pre-amplifier can also be faulty. To confirm whether is the circuit or ic problem, you have to take out the contrast pin and power on the monitor. If the monitor have a normal picture (contrast ok) then suspect the abl line circuit. If it remain the same (dim contrast or weak picture) replace the color video pre-amp ic will usually solve the problem.

The contrast signal are taken from one of the pin (leg) of a flyback transformer which we called it as abl pin. Many engineers and technicians have the difficulty to trace the abl pin because usually a flyback transformer have 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 easy way that i'm going to show you how to locate the abl pin effectively. Using two analogue multimeters set to the range of time x10kohm and connect it in series (+ probe to - probe) and test the flyback transformer from anode (the suction cap) to all of the pins under the flyback transformer. You have to desolder all the pin or just remove the flyback transformer from the mainboard in order to get a precise reading. If either one of the pin shows a diode reading that is the abl pin. The reason we use two analogue multimeters is because the flyback internally have couple of high voltage diodes connected in series. If you have a schematic diagram or any monitor or television repairing book, you will see that the internal flyback diode are connected in series. By connecting two analogue multimeters the ouput from the probe is around 24 VOLTS which is enough power to test the flyback internal diodes. After you have determined the abl pin, you are now can proceed to trace the line and find the faulty components that cause the contrast problem. Television or tv flyback transformer generally have the same design compare to monitor flyback except that the monitor flyback have an extra high voltage capacitor. Whether it is a montblanc flyback transformer, rca, mitsubishi or sony flyback transformer the checking usually is the same. Don't use a multimeter that have the time 100kohm range because the internal battery is not enough power to check the diodes in the flyback transformer.