Have a Proscan 2 yearsold. when i turn on a light flashes across the screen but no picture

Do you have sound? If you have sound and no picture most likely you are dealing with either a back light fault or power supply problem. Dim the lights in the room, take a bright flash light and shine it on the screen with the set on. If you see a faint picture, it's back light/power supply related. If I have been of assistance to you please sign my gues book

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Does the audio still work normally? If so uits most likely the inverter board which has failed and you can get a replacement inverter from www.tvcircuitboards.com

dry soldering re solder vertical IC section

Pic flash is a symptom of back light inverter fault. The unit will need to be bench tested to clarify. Usually the transformers, mosfets or a pf has gone and will need to be replaced.

I JUST GOT HOME AN TRIED TO TURN MY JVC PLASMA TV ON. THE TV WILL NOT TURN ON. EVERYTHING ELSE HAS POWER TO INCLUDE THE TV BUT THERE IS NO PICTURE OR SOUND. BLANK SCREEN. THE BLUE POWER LIGHT IS FLASHING ON AND OFF. THE MODEL NUMBER LT-42P789

There is no power coming from the step down transformer

From elsewhere on the web:

"Proscan used to be the RCA high-end trademark; RCA (at least the TVs) was purchased by Thomson; they leased the proscan name to a Korean outfit that has the TVs made in China.
I've never owned a Proscan (post RCA)... but the internet buzz is that they are of inferior quality and that it's virtually impossible to get support in the USA. I would qualify that: Costco is selling Proscan TVs; they extend the warranty by one year (to two years); and if you have a problem, ya got Costco behind you. So I guess unless you buy one at Costco, you should pass"

Perhaps you need to contact the entity at which you purchased the television.

There is some electronic circuit problem with your TV. Best if you want to repair it you find a reputable TV repair shop. Sorry.

need electrolytic capacitor replaced in vertical circuit

How old is this TV?