RCA RTRU5028 No Backlight

[neilc6]

Owner reported he had it diagnosed as bad backlights. US model number may be RTRU5027. Checking the backlight connectors with an LED tester, the two wire pairs had the same voltage. After opening up the tv, all the leds tested fine. Then I noticed the burnt FET on the power supply board. Source pin was blown out. There are two on the hot side (not heatsinked) and they look mirrored but most of the components on the burnt side seem damaged. I can match the resistors but the SOT23 transistor is suspect and the diode is bad and can't read the markings on both. Power supply board is HSL55D-2S9 and a bit expensive for a used one.

LED driver IC is TI UCC25710.

[neilc6]

Re: RCA RTRU5028 No Backlight

I've identified all the blown parts.

Q506 TF5N50 (NCH 500V 5A)
C523 220pf 1kv
Q504 (2T marking) MMBT4403 PNP
D509 (T4 marking) 1N4148 100V 250ma
R506 240 ohm 1206
R533 300 ohm 1206
R529 10K 0805

It looks like Q507 shorted blowing up Q506.

[budm]

Re: RCA RTRU5028 No Backlight

https://www.badcaps.net/forum/attach...2&d=1569376944
It looks like one of the upper MOSFET Gate pin has bad solder joint so may not have turned off so when the bottom one turned then it caused the damages.
Q504/505 are for Gate's discharge.

[neilc6]

Re: RCA RTRU5028 No Backlight

Q504 was blown but Q505 I replaced anyway. Assuming both are PNP since they appeared wired the same. Going to increase the FETs to 8A from 5A but may not be necessary if problem was the cold joint.

R533 is 30 ohm. The second one R528 was open as well

[neilc6]

Re: RCA RTRU5028 No Backlight

I was pretty close to giving up and buying a new ps board. Installed all my parts, had a standby light and the fuse popped when I turned on the power. Checking for shorts again, I found D401 shorted (SF26, 400V/2A). I replaced temporarily with a MUR460 and replaced fuse with 2.5A instead of 4A since that was all I had. Fuse blew again and no standby light. Checking again, I realized Q506 and Q507 had shorted again so replaced those a second time. TV finally turned on. Guess the original problem was D401 and not the cold joint and the FET blew before the fuse originally. Made Digikey cutoff so should get the proper fuse and diode tomorrow.