When we turned the tv on it only the top backlight lit, but we could see the picture very faintly at the bottom. It also had audio. We were hoping for advice as to how to repair the bottom backlight
Most LCD flatscreen tvs use LEDs in series as backlights, so if one LED fails open circuit the whole string stops working. Sounds like this model uses 2 strings and 1 of the LEDs in the bottom string has failed.
If this is the reason then the repair requires the open circuit LED (or the whole string) to be replaced. Is it still under warranty?
Most repair shops won't do this because the time involved and potential for breaking the LCD screen make it "uneconomical to repair", but it's already not usable so there's not much to lose if you are careful, have reasonable knowledge, and feel like a challenge.
If so here's how:
I have done this successfully with a 55" LCD, although it was a bit of a learning curve for me because there is almost no information available about what is required...
Not a simple operation but with care and a bit of knowledge of electronics it can be done.
UNPLUG from power unless testing, don't touch internals if plugged in!!
You can find the open circuit LED with a suitable LOW VOLTAGE LOW CURRENT (e.g. 4 to 6v battery IN SERIES WITH a 1k resistor), connect across each led in turn (set unplugged!), should light up a little if polarity is correct (low enough voltage to not damage a led if reverse biased).
1. The replacement LED must be very similar to the others (colour temperature, on voltage and power capability). It's not likely that you can get the exact replacement LED but there are many available and it's not critical as long as it can cope with the current flowing through the string (can measure the current in the good string, add a safety factor -e.g. if 500mA is flowing use a LED that is rated at 800 or 1000mA continuous at least). You may even be able to get a complete string from the manufacturer. Otherwise a 2 or 3W white LED will probably work ok.
2. Make sure that the LED polarity is correct before powering up or it will fail instantly due to high reverse bias voltage!! Also make sure that there is sufficient heat sinking to keep the LED cool.
3. The TV needs to be carefully dismantled to get to the backlighting array. The screen is easily cracked and needs to be supported / kept flat during this, no point loads, bending or twisting.
4. There are fairly high voltages present (up to 300v in some sets) so don't touch exposed wires or cases if powering up to test. Don't slip with meter probes.
RCA Televison &...
on Feb 06, 2019