While testing vdc at the base board for the cathode wires found that the red and blue were at 4 VDC and green was at 8. Picture has mainly blue and green tints.No loose soldier joints found.
Have you measured the voltages at the collectors of the three video output transistors?. There must be a voltage over 124 to140VDC at least. All voltages at the base of that transistors should be between 2.5 to 3.75VDC according to the level of chroma signal from the chroma Ic.
You must check these voltages with a good quality digital or analogue multimeters, otherwise you will get only false values instead of the real. You just check the main voltage that goes to the video output stage, that is the high voltage comes to the crt base, from the main board. It should be more than 260VDC. Normally in all tvs this voltage is derived from the LOT. After rectifying it and filtered with a 10Mfd 350V capacitor, and given to the baseboard as the main supply to video output stage. If the smoothing capacitor [10Mfd] gets value changed, that is if it gets a low value due to age, otherwise whether it has an internal short, you won't get the above said 260VDC at the CRT base boad. At first locate this capacitor and replace it with a new one with same capacity and voltage ratings. and check all video output [ 3 of them] transitors too. Check the transistors for collector to emitter leak, not on circuit, but after take it out desoldered. Then only you will get the proper measurement of leak. Set the multimeter in MegOhms rage, and don't held the transistor by hand too. If you do so you will see some deflection in the meter due to your body resistance.
Check the capacitor first. Replace it. It is best. Check the rectifying diode that rectifies this voltage from the LOT winding. If it has a leak, you will get a low voltage at this rail.
Remember, LOT works at high frequecy. So the diode used there as retifier is a special type, fast recovery diode, and should be repaced with the same number. Never use ordinary diodes that we uses for line rectification purpose.
Remember, colector voltages at the video output transistors must be above 140VDC. Again recomends to use a good quality multimeter, as cheaper one cannot measure this voltage accurately.
Ok. Wish you all sucess.
Oct 23, 2010 |
Proscan PS36125 36" TV