Flyback Transformer How to Locate the ABL Line

Flyback Transformer How to Locate the ABL Line

If the picture tube isgood, I will go straight to the contrastcircuit. The question is how do we findwhere the contrast circuit located? Simple,if you have a datasheet for the video preamplifier ic, definitely you can find the contrast pin. For LM1203N video preampic the contrast pin is located at pin 12. Replace this ic or trace from this circuitto find the cause of the contrast problem.

From experienced, usually capacitors shortedand resistor turned into high ohms were the caused of the problem. A video preamplifiercan 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 okthen suspect the abl line circuit. If it remainthe same dim contrast replace the videopreamp will usually solve the problem.

The contrast signal came from one of the pinleg of a flyback transformer which we calledit as abl pin. Many technicians have the difficulty to trace the abl pin becauseusually a flyback transformer consist of around10 pins. Some pins are B+, GROUND, AFC, ABL,XRAY PROTECT, VCC, to HOT, HEATER, horizontalcentering and etc. But there is one and simpleway that i gone to show you how to locate theabl pin. Use two analog multimeters set to x10kohmrange. Connect it in series + probe to probeand measure the flyback transformer from anodethe red cap to all of the pins under theflyback. You have to desolder all the pin orjust remove the flyback from the mainboard in order to get an accurate reading.

If either one of the pin shows a diodereading that is the abl pin. The reason we usetwo multimeters is because the flyback circuitinternally have couple of high voltage diodesconnected in series. After you have determined the abl pin, you can proceed to followthe line and find the bad components that cause the contrast problem.