I have this 17" viewsonic and finally it fails, i check inside and found the horizontal output transistor BU4522AX fails, i've replaced it three times but keeps failing. dc voltages seems to be normal. please help me.
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Re: Horizontal output transistor keeps failing
The horizontal output transitor is always connected to the primary winding of the flyback transformer (via collector) and also to the horizontal deflection coil via an electrolytic capacitor.
The most likely suspect for damaging the transistor is the fylback transformer. In some countries there are electronic stores that test flyback transformer by winding a few turns on the ferrite core and applying a test voltage. This test will only test the secondary.
The primary winding is difficult to test for turn to turn short because the resistance is quite low and you do not know the 'good' resistance.
What do u mean by LOT ( EHT ), in this problem, the main 200v supply intended for the horizontal output transistor wud passed thru a MOSFET, The gate was connected to the output of an IC that determines the presence of signal from a computer. The MOSFET would reduce this voltage to 165v for the hor transistor. now the presence of signal from computer would change the LED from orange to green, but the 200V did not passed to the hor transistor, the MOSFET was not turned on. One thing i've noticed was if i remove the hor out transistor, the MOSFET would turn on. This MOSFET will reduce the 200v to 165v so i could not try shorting it when the hor out transistor is soldered. i've already change shorted 0.22mf capacitor in the flyback transformer that i thought was the cause and the 2.2uf/50v capacitor blown before. please explain to me what u mean LOT.
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Check and replace damaged component/s at its horizontal scan output section circuit. Check the LOT (Line Output Transformer) too for damage. Replace the horizontal scan output transistor. Check the input voltage to LOT. If the voltage is absent check the feeder resistor; if any. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" or "Magazine" will make surf easy. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/ Do a search by type in the brand name of your device at its search box.
Hello Check its horizontal output transistor for short circuit. It is a common fault to all type of CRT based TVs, and show this type of fault. If it is found shorted [collector to emitter], the LOT should also be checkd for winding short [internal]. Normally; LOT cannot be checked with normal measuring instruments. If LOT has internal winding short, the horizontal output transistor will get overheated and finally shorted. Replace the horizontal output transistor first, and check whether it heats up ubnormally within seconds after switch ON the Tv. If it heats up so, make sure that the LOT is defective, and it should also be replaced. OK.
Six blinks denotes the absense of horizontal sweep. Check the horizontal output transistor, whcih can be located very near to the LOT [Line Output Transformer]; for short. If it is shorted, LOT must be suspected. Replace the line output transistor with a new one, and switch ON the tv for a short period of time, say 6 seconds or so, and check the heat of the replaced transistor. If it overheats within this time, be sure that LOT is damaged; shorted inside; and should be replaced with same type and number. If a digital multimeter which can measure frequency is available, take the frequency at the collector of the horizontal driver transistor. It should come around 15525Hz. If not replace the jungle IC. OK.
Before replacing an other one, replace the freewheeling capacitor, the capcitor fitted between the collector of the 2222 and earth; usually of a value of 3500pf 2000VDC, some times this value may differ from set to set. If this capacitor has opened of developed in value, the result will be the failure of the horizontal output transistor, at switch ON itself. The second reason is the short in horizontal yoke windings. Even one winding get shorted, it will cause to a heavy load flyback transormer which in turn to 2222 failure. The last reason for this fault is defective Line Output Transformer itsef. Check these three.
Before proceeding, desolder out the horizontal output transistor 2222 fro circuit, and measure the frequency at the collector point of horizontal driver transistor. It should be around 15625HZ [may differ a little], with no signal. If this frequency is found higher than normal, The main IC jungle should be suspected.
Anyway, go forward after checking this frequency. OK.
You just replace the freewheeling capacitor, normally come up to a value of 5600pf 2000V or so, connected to the collector of the horizontal output transistor, fitted very close to the collector lead. Replace the yoke return capacitor, 0.33uf or so also. Usually samsung sets use no heatsink for the horizontal output transistor. You just put a small heatsink to it and check. If it doesn't overheat, you can remove it later.
You can suspect the deflection yoke also for shorted winding. To check this. pull out the CRT base board out of the neck of the picture tube, place it where it doesn't get short circuited, and unplug the deflection yoke connection from the main board, and switch on the set. See whether the transistor overheats now. If it doesn't overheat, suspect the deflection yoke for shorted turns of winding inside it. Till the transistor overheats, suspect the LOT. NOTE: Never switch on the set while unpluging the deflection yoke connection plug from the circuit board, without dismandling the CRT base board. CRT may get damaged, with a black burned spot inside the center of the screen. OK.
If horizontal output gets shorted always, something may be wrong with Line Output Transformer [LOT], or deflection yoke. If LOT windings inside is short, hrozontal output transistor will get overheated, and will be get shorted soon. If there is winding short in the Deflection yoke, even a sinlgle winding gets shorted, it puts a great load on LOT, which in turn effect horizontal output transistor. The third main reason for horizontal output transistor failure is due to an open or dry solder at the capacitor connected to its collector to negative It will be usually 2500pf to 3500pf, according to the TVs screen size. You just check these three and be sure about the fault.
If you mean 'flyback' or 'horizontal output transformer' and have the part number available, try one of the sources here:
If you do not have the number you may have to search Viewsonic's website to loacte it.
Warning; the number of suppliers for CRT monitor parts is shrinking rapidly, my best supplier, ICM in California is long gone.
Resolder the power transistors on inverter board and also check them.If display is not there at all some of this 4 transistors C5707 should be defective.Check associated fuse and Fet which drives the transistor.
Also check with Benq ,Normally offers three year warranty.
When the horizontal output (2SC4927) is shorted it means one of 2 things is wrong - the horizontal drive transformer has cracked solder joints or the flyback transformer is shorted inside. If the solder joints on the horizontal drive transformer (the small grey transformer connected to the base of the horizontal output transistor) are cracked then the set will play for a little while and then the parts you just replaced will again be destroyed. ALWAYS resolder the driver transformer when replacing the parts you mentioned! When I replace the parts you mention and I do not see any obvious cracks on the driver transformer then I unsolder the B+ pin of the flyback (labeled 135, for 135 volts) and put a 60 to 100 watt lightbulb (drop-light) in series with the flyback. This means that the 135 volts B+ goes through the lightbulb and then into the flyback, so the lightbulb acts as a current limiter to protect the horizontal output. When you turn the set on the bulb should glow no more than about half bright and you should hear the high voltage come up, but you should not see a picture. If you turn the set on and the bulb glows as bright as if it was plugged into the wall then you can bet that flyback is shorted and must be replaced.