You should try to remove the burned area of the circuit board with sandpaper or a rotary tool, deep enough to expose clean circuit board material. The burned area is likely to be somewhat conductive, but hopefully it doesn't go all the way through the circuit board. When done, spray a thin coat of clear acrylic over the repaired area to keep moisture out. Or use clear fingernail polish.
Its burnt and damage to the other side also. If I scrub and clean it will break into hole. Can we repair this damaged circuit board? Hope I will learn and become good in electronics and its terms. Thank you Hevans 1944
It is possible to cut out the damaged area of the board (assuming it is NOT a multi-layer board... in which case you are SOL) and then epoxy in place a clean section of bare FR4 board material. That will at least give you support for wire-lead components if you drill appropriate holes through the patch.
Now I see that you are up against more urgent REAL problems, as is now affecting your television.
In reading the initial part, I saw little info until I realized that the thread had extended on into a SECOND page.
Now, let me fill in with what further usefull info that I can supply to help you in a solution of that sets repair.
Your photos and drawings were MOST helpful, and I reply in kind, with the further taking of them and placing of them beside
a photo of a virgin power supply board.(Your's are now YELLOW bordered.)
Initially, your little drawing add on of the schematic flow of the power supply is being correct. UNTIL you show a varistor and and a
resistor dropping off to frame ground.
NOT correct . . . . as the present "varistor" is SUPPOSED to be one of a set of 2 like "CY" safety rated HV ceramic capacitors
(Marked as CY102 on the PCB). Your other unit that looks unburnt is the BLUE one ( Marked CY101 ) being wired between the grounding tab
and the line EMI choke (Line Filter 101 transformer), with it having some white RTV bonded to it.
You should be able to read its capacitive value on that good unit. A "middle of the road" value of those units would be 4700 pf.
Or stamped as 472 plus whatever its voltage rating is and is usually being up to 1KV or even upwards.
The capacitors are used as "old school" capacitor spike voltage supressors, in addition to the A.C. line circuits earlier dedicated
"lollipop" BLUE varistor.
That, being located up above, just below the BLUE block poly "CX" safety rated capacitor.
The other wrong item installed is your resistor beside it, which you say is now being a very-very burnt up 47 ohms of resistance.
Well, that resistor needs to be of 10 megohms value instead of that present 47 ohms.
The resistor is making a weak "electrostatic " grounding between the chassis frame and the AC line cold side.
OVERALL, it looks like you have been trying to use one INCORRECT part type (varistor) and another part of a fiercely LOWER resistance value.
PLUSSSSS that carbonized portion of PCB . . . . I have no idea of how low of a resistance that has been gradually decreasing to.
I might even expect it to sit there and glow, just short of popping the AC line fuse, since the RED line original foil path, that you have drawn in,
is having that carbonized path between it and the other foil path on its left, that comes down to the top lead of CY101 cap.
My friend, that is then placing raw 220 VAC being across the carbonized path between them !
I see that the carbonized area has not encroached upon the Full Wave Bridge rectifier unit and only the bottom winding of the AC line EMI filter choke.
That means that the top terminals of the choke will still provide solid mechanical mounting for the unit whie allowing quite a substantial paring away of the
carbonized bottom PCB area.
Take ohmmeter in hand and confirm if that foil path still is intact from top left choke coil down to the shared TOP two foil paths for the "CY" cap
and 10 meg resistor.
In seeing what board area is burt, I would initially suggest that you try to save the triad foil path that the ground terminal lug and the BOTTOM two
foil paths for the "CY" cap and 10 meg resistor were soldered to.
Grind away the TOP "CY" and resistor terminals and the carbonized area that I have added to your drawing at the very top left corner.
(It is being outlined in YELLOW.)
Unsolder and pull the good CY101 cap, the AC line choke and the Full Wave Bridge rectifier unit, so that you will have working room and then start
removing all of the carbonized PCB.
This brown urea-phenolic type of board certainly will work easier than if it had been a FR fiberglass board ! Less itchy too !
A tool of primary choice would be a Dremel motor grinder and burrs/stones, but you may have to use rat tail and or Swiss pattern files, along with
some strong solvent to leave the board cleaned of the last vestiges of any carbon.
Use a new 10 meg resistor with ~ 1/2 inch lead lengths and a like length on the new CY ceramic cap *** and mount the bottom leads of both in the
saved 2 pads that the other were in.
Make nice right angle bends at the parts top to join them together , sind they will be mounted vertically to the board and support themselves.
Place right angle bends on a wire that you carry across from the top left corner of the AC line EMI filter choke (that you drew in RED) to the
floating, already connected and joined CY cap and resistor.
These ceramic "CY" safety caps are EASY to be "procured/cannabilized" from any TV or computer from as far back as the nineties, plus there is a
wide variance in the capacitance that the particular designers choose to use.I have seen ANYWHERE from 1000 pf (102) on upwards of 5600 pf(562) being used.
Wel l l l l l l, Im going to stop and see if you need any procedural points explained or further refined upon.
I believe that you will have just lucked out, with the borderline amount of board that has carbonized, and will be able to get the unit functional again.
BUT I don't know how much abuse your set might be subjected to due to power line conditions and AC line voltage regulation.
Or if you are out in areas that depend upon motor generators.
Public Television had one program on some deplorable power thievery in India some time back.
In PRIME population areas like New Delhi, one might expect the first shown picture below, BUT, what if the second picture is then being experienced in neighborhoods ?
Your relevant technical referencing aid is just below: