TCL 55RP620K power board issue

[notallbad]

Originally the tv arrived with dimmed backlights (not failed leds) but everything else worked ok. Adjusting backlight in menu did nothing but voltage changed on dimming control pins,
0% p_dim at 1.65v and a_dim at 1v
50% p_dim at 3.3v and a_dim at 1.65v
100% p_dim at 3.3v and a_dim at 2.88v

Changed backlight leds anyway as always like to have complete peace of mind they wont fail shortly after selling the tv. No change as expected!

Leds are 2 rows of 15 6v leds.

On inspection of the power board found diode MBR3200 shorted so replaced with SR3200 but no change, so then changed backlight control i.c GB98TDN which then appeared to fix the issue but 2 or 3 minutes later the tv powered off completely with no led indicator, the only voltage on power to main board plug was around 2v on the 12v pin. Found mosfet PCO158D shorted so replaced and again seemed to fix the issue but 2 to 3 minutes later back to just 2v on the 12v pin.

When briefly working voltages on power-main board plug were
when in standby
p_dim 0.2v. bl_on 0v
gnd 0v. p_on 3.2v
gnd 0v. a_dim 0.34v
gnd 0v. gnd 0v
12v 11.94v. 12v 11.94v
12v 11.94v. 12v 11.94v
12v 11.94. 3d_en 0.34v

when powered On

p_dim 3.3v. bl_on 3.3v
gnd 0v. p_on 3.2v
gnd 0v. a_dim 2.25v
gnd 0v. gnd 0v
12v 11.94v. 12v 11.94v
12v 11.94v. 12v 11.94v
12v 11.94. 3d_en 0.34v

Doesnt seem to be a widely available board so hoping to repair.
Thanks in advance!

[nomoresonys]

I believe you need to change all 3 of those that you changed at the same time, so one doesn't keep blowing, maybe make sure the blue ceramic caps are all ok, have seen them cause odd problems.

[notallbad]

thanks, shall try replacing all 3 at once as I have more spares.

How do you test the blue ceramic caps?

[nomoresonys]

A multimeter with capacitance mode is a good tool, some testing here, can still test for shorts etc. with a basic multimeter tho: https://www.youtube.com/watch?v=OguFmhiPnZM

[notallbad]

ok so have now changed all 3 - diode, mosfet and led driver i.c, the diode and led driver i.c seem to be ok comparing readings out of circuit to new components so just the mosfet blown this time.

[notallbad]

Been running the tv with backlight at 45% for a good few minutes more than it lasted before so fingers crossed. Shall increase the backlight once happy its ok at 45%.

I did also check the blue ceramic caps beforehand and all seemed good.

[stj]

MBR3200 isnt really suitable for more than 20v.
i dont expect quality these days though.

before you ask, a 200v schottky diode is a bad idea on something probably running 50-80v
you need a rating atleast 10x the running voltage with schottky rectifiers because they are so sensitive to spikes.
it's a common problem with power-tool battery chargers.

[notallbad]

just curious but would a power board from a 43" or 50" of the same tv range work i.e 43rp620k, 50rp620k?

I did buy one in the hope it would but backlights just flash on and off as soon as power is applied. I guessed it only for 43 or 50" with fewer leds to run i.e 40" 21 leds, 50" 26 leds rather than 55" with 30 leds.
With the leds disconnected the tv turns on and operates fine.

Though it was sold and advertised for 43, 50 or 55".

Perhaps just needs repairing properly as the only replacement on the board is the led i.c driver, and not done very good at that as can be seen from the photo. Its possible it was replaced and only tested briefly rather than bench tested for any length of time and as I have seen it may work for a few minutes before something else fails.

The board was purchased on ebay and received no response from the seller so may name and shame!

As a note on the 43" power board the MBR3200 diode is a HER303G and the PCO15BD mosfet is a CS630A4H.

[nomoresonys]

Sure, that makes sense, might need a higher rated diode.