Flyback Transformer-How to Locate the ABL Line
Whenever there is a complaint about monitorcontrast problem I will check on the flybacktransformer ABL (automatic blanking limiter)circuit.Sometimes a weak picture tube might causedcontrast problem. 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 pre-amplifier ic, definitely you can find the contrast pin. For LM1203N video pre-ampic 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 pre-amplifiercan 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 remainthe same (dim contrast) replace the videopre-amp will usually solve the problem.
The contrast signal came from one of the pin(leg) 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,X-RAY PROTECT, VCC, to HOT, HEATER, horizontalcentering and etc. But there is one and simpleway that i'm gone to show you how to locate theabl pin. Use two analog multimeters set to x10kohmrange. Connect it in series (+ probe to - probe)and measure the flyback transformer from anode(the 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. If you have a schematic diagram or any monitor or television troubleshooting book, you will see that theinternal flyback diode are connected in series.By connecting two meters the ouput from the probeis around 24 VOLTS which is enough power to checkthe flyback internal diodes. After you have determined the abl pin, you can proceed to followthe line and find the bad components that cause the contrast problem.
Television or tv flyback transformer generallyhave the same design compare to monitor flyback.Whether it is a montblanc flyback transformer,rca, mitsubishi or sony flyback the checkingusually 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
