When there is a complaint about keep track of contrast issue I will check on the flyback transformer ABL (automated blanking limiter) circuit. Sometimes a weak image tube may caused contrast issue. If the image tube is good, I will go directly to the contrast circuit. The question is how do we find exactly where the contrast circuit situated? Easy, if you have a datasheet for the video pre- amplifier ic, certainly you can find the contrast pin. For LM1203N video pre-amp ic the contrast pin is situated at pin 12. Replace this ic or trace from this circuit to find the trigger of the contrast issue.

From skilled, usually capacitors shorted and resistor turned into high ohms were the caused of the issue. A video pre-amplifier can also be faulty. To confirm whether or not is circuit or ic issue, just solder out the contrast pin and switch on the keep track of. If the keep track of show regular image (contrast ok) then suspect the abl line circuit. If it stay the exact same (dim contrast) replace the video pre-amp will usually solve the issue.

The contrast signal came from 1 of the pin (leg) of a flyback transformer which we known as it as abl pin. Numerous technicians have the problems to trace the abl pin simply because usually a flyback transformer consist of about ten pins. Some pins are B+, Ground, AFC, ABL, X-RAY Safeguard, VCC, to Hot, HEATER, horizontal centering and etc. But there is 1 and easy way that i am gone to show you how to find the abl pin. Use two analog multimeters set to x10kohm assortment. Link it in sequence (+ 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 eliminate the flyback from the mainboard in order to get an accurate studying.

If either 1 of the pin shows a diode studying that is the abl pin. The reason we use two multimeters is simply because the flyback circuit internally have couple of high voltage diodes linked in sequence. If you have a schematic diagram or any keep track of or television troubleshooting guide, you will see that the internal flyback diode are linked in sequence. By connecting two meters the ouput from the probe is about 24 VOLTS which is enough power to check the flyback internal diodes. After you have determined the abl pin, you can move forward to adhere to the line and find the bad elements that trigger the contrast issue.

Television or television flyback transformer generally have the exact same design evaluate to keep track of flyback. Whether or not it is a montblanc flyback transformer, rca, mitsubishi or sony flyback the checking usually is the exact same.

Jestine Yong is a electronic repairer and a author. For more electronic fix information make sure you visit his website at http://www.noahtec.com/electronic-repair-articles.htm