PLASMA TV DAMAGED DURING REPAIRS

On the 28/04/2013 i bought a 39inch Sinotech flat screen plasma tv for R3799 at Game Stores in In Vincent Park. Within a year of purchasing the TV i took the tv in for repairs as it wasn't showing a clear picture but distorted spectrum of colors instead. Fine enough the tv was taken in for repairs and returned to me within 21days and was working. However within 4weeks while watching tv the screen just went black and there was no power to the tv. I took the tv in for repairs again for a second time within the space of 2years from purchase date. However after not receiving any report form the retailer after 21days i went to go check up on it. They gave me a report saying i was being quoted R3100 for repairs because the screen was broken from the inside and the warranty was voided. My question is how come the TV had no signs of being broken when booked in for repairs but once it reached the technicians they say it's broken from the inside yet the tv had never been open except when they opened it to repair the faulty screen PLEASE HELP! FRUSTRATED CUSTOMER!