Sharp. tv 25" model#25rs100s. 4 years old . first thing find vertical transistor #la7840. burn up . replace back same # la7840, tv come on but shut down . unplug& plug in back tv still shut off itself right way. regulator in power supply , horizontal transistor ok when i test, anyone could solve my problem please? thanks!
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Re: shut off itself
Sounds like you might have an auto overvoltage shutdown circuit kicking in. The hivoltage goes to high and the monitoring circuit auto shuts down. More than likely trouble is in hivoltage circuitry. If you have a transformer that will allow you to bring up the set slowly without shutting down and you can look to see what's happening.
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Horizontal line on screen denotes the absensce of vertical sweep itself. Check the vertical output section circuit to LA7840. Check for any loose solder terminals. It will be best to measure the input voltage to that IC from the LOT secondary supply. Try by replace the IC. If these do not help you, measure the frequency from the main IC. It should be at 50Hz. If this frequency is not present, the main IC is faulty. If you wish to get some more details about the circuit, check this link. http://electronicshelponline.blogspot.com/ Pull
up older posts there, either by its left side bar, or see it in Timslide
option. Schematic diagrams, that show vertical IC LA7840 is given. You can compare it.
2blink-then 4 blink---------vertical circuit problem
This is a general answer based on error code--problem in vertical circuit then the first thing is to look for ANY signs of Glycol having leaked from one of the picture tubes onto that area of the board.
If you do not know how or what to look for let me know and I will explain.
Vertical circuit deflects picture from middle to top and middle to bottom of screen; if set detects that it shuts set off instantly to prevent a instant BURN MARK from happening on all three tubes this set uses--prevent permanent damage to the screens of the tubes.
There are several versions of this model. They all have the same problem. The problem I'm speaking of , the location numbers of bad capacitors. So I will give you values and you will need to search and replace these. Find an ESR meter to locate bad capacitors. In the vertical deflection look for 50 volts at 4.7Mf, 35 volts at 100Mfs,50 volts and 25 volts at 1 Mfs. Be sure to resolder the vertical IC and don't solder the pins together. Find the smallest electrolytic caps valued at 50 volts 1 Mfs and replace these. These are scattered through out the main board. Other small caps include the 25 volt 10 Mfs, several of these as well. You also may want to inspect the capacitors near the flyback transformer and horizontal transistor and the horizontal output transistor. You will have to seek and find the weak or bad caps here. I will tell you, the shut down is mainly a capacitor issue. Caution is required when you remove these. The circuit traces in these TVs are very delicate and you can destroy this TV in seconds.Thanks for asking me and show all hands of support!
Same no sound issue happened to me; except I stopped at adjusting vertical size (unfortunately it seems horizontal cannot be set thru service mode; which was my original intent). Complete lost of sound signal processing; oddly enough speaker volume leveling did worked. (Eg, audio-in woks; so a big WTF?!) I suspect something to do with not recovering properly the data bus from muxing the volume input keys; ie, some kind of weird service/back-to-normal switch bug. I've waited for a couple of days for the damn thing to unlock itself out it (like some stubborn transistors seem to do); but to no avail. (BTW, only dif between TVs is that mine's a slightly smaller and cheaper model, a 19U-M100S; a minor branch out of the main line.) I'm afraid something did "burned out" and that the only real fix is to actually change the "defective" part. Had I known silly sh1t like that could happen, I obv would've not messed with it to begin with. It's just ridiculous, and lame. Besides being obsolete. In any case; I also think it's nowadays better to get a (full) digital one (wisely cost effective).
There are a few things that can cause that. Pretty much the TV is shutting itself down because it recognizes and issue in one of a few circuits. If you're getting a flash and then it cuts off my first guess would be a vertical issue. May be best to take it in to a repair center and have them take a look at it. Shouldn't be any more than $20 for a diagnostic check on the unit.