How To Locate Flyback Transformer ABL Pin Using Analog Multimeter
The flyback transformer abl pin play a very important part in a monitor and tv circuit. If there is a problem in monitor about contrast not clear or no contrast, the first thing i will do is to test on the flyback transformerABL pinout (automatic blanking limiter) circuit line. Sometimes a weak picture tube you can be mistaken to be contrast problem. If the cathode ray tube is good after checking it with a crt tester, i will head straight to look at the contrast circuit in crt board and flyback transformer. The question is how do we know where is the contrast circuit located especially there are so many models of monitor in the market? Is easy, if you have the information or specification for the video pre-amplifier ic which you can download free from the internet , definitely you can locate the contrast pin.
LM1203N video pre-amp ic is very common used in computer monitor circuit and the contrast pin is located at pin 12 of the ic and marked as contrast. You have the choice to replace the 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 contrast problem. Sometimes the video pre-amplifier ic can also be the caused of the problem.
To confirm whether is the circuit or ic problem, you have to remove the contrast pin and switch on the monitor. If the monitor have a good 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 most of the flyback transformer have 10 pins and you do not know which is the abl pin. 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 going to show you how to locate the abl pin effectively. Combine two analogue meters set to the range of time x10kohm and connect it in series (+ probe to - probe) and measure 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 monitor board in order to get a precise reading. If either one of the pin shows a diode reading that is the abl pin. I guessed now you should know about testing diode, if you don't know you can always refer back to my website to find the diode testing page. The reason we use two analogue meters is because the flyback transformer internally have couple of high voltage diodes connected in series.
If you have a schematic diagram or any monitor or television electronic servicing magazine, book or circuit, you will see that the internal flyback diode are connected in series. By connecting two analogue multimeters the output from the probe is about 24 VOLTS which is enough power to measure 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 designed compare to monitor flyback except that the monitor flyback have an extra high voltage capacitor which we called it the internal capacitor or just internal caps.
Whether it is a montblanc flyback transformer, rca, mitsubishi or sony flyback transformer the checking usually is the same. Please do not use an analog multimeter that have the time 100kohm range because the internal battery (only 3 volts) is not enough power to check the high voltage diodes in the flyback transformer. With more practice i hope that you can find flyback transformer abl pin easily and be able to tackle the contrast problem.
source:Electronicrepairguide.com