Hi everyone
Attempting my first ever LCD/LED TV repair. The problem is that the TV has power but is stuck on standby. When the power button on the TV or the remote is pressed, there is no response. The LED at the front of the TV just stays red and doesn't blink or flash or do anything else.
I opened the back of it apart and saw no visible sign of the typical capacitor faults discussed in other threads; bulging, leaking etc. However, I decided to replace all the capacitors, except for 2 (The big one and the second biggest one) because I'm still waiting for it to be delivered, with panasonic, rubycon and nichicon ones (Low ESR where appropriate).
I checked the voltages at the terminal connector to the mainboard, before and after the capacitors change and the results are the same.
12 V ~ 12.4 V
12 V ~ 12.4 V
5 V ~ 5.1 V
5 V ~ 5.1 V
5 VS ~ 5.0 V
STB ~ 5.0 V
ON/OFF - 0V
ADJ - 0V
From these results I am assuming the issue is with the mainboard. I try finding another mainboard online but no luck. There's no schematic as far as I can tell, so I'm thinking of trying to fix the mainboard myself.
Can anyone please point me in the right direction on where to start my diagnostic of the mainboard? Which are the usual components that fail?
I try doing a bit of diagnostic myself and these are my findings:
1) All caps seems to be working fine under load (but I might replace them later just to be safe)
2) Tracing where the signal from the IR goes when I press the power button on the remote
Refer to the Attached picture
When I press the power button on the remote control the voltage at the IR terminal connector on the IR input pin changes (I think from ~5 V to 4.6 V from memory). I did a continuity test and found that the signal from that pin goes to the two SMD components in the picture I attached before it goings into the big Chip. However, the white looking component seems to fail the continuity test. The voltage change does happen on the left side of that component when I press the power button on the remote but there is no voltage change on the right side. I'm not sure what that component is to perform a proper test on it. Does anyone know? Thanks.
3) From visual inspection of the photo (I have attached this photo as well) I took of the mainboard, I also found that one of the Voltage Regulator the BM1117-ADJ looks discolored compare to the rest of the components on the board. Is this normal? or is it burnt?
4) I disconnected the On/Off pin and ADJ pin from the mainboard. Then connected the On/Off pin to 5VS with a 1k resistor and the ADJ pin to 5VS also with a 1k resistor.
The backlight and screen turned on (and stayed on). However, nothing came up on screen.
It seems I'm still stuck on standby because the red LED at the front of the TV did not change; it just stayed lit up the whole time even though I was pressing buttons on the remote and the controls on the side of the tv.
I also measured the voltages across that discoloured voltage regulator (BM1117 - ADJ) and these are my findings.
Assuming pins are numbered left to right:
Between GND - Pin 1 = 141.4 mV
GND - Pin 2 = 0.790 V
GND - Pin 3 = 5.18 V
I also measured the other voltage regulator (BM1117 - 3.3)
Assuming pins are numbered left to right:
Between GND - Pin 1 = 0V
GND - Pin 2 = 3.295 V
GND - Pin 3 = 5.17 V
Any help would be much appreciated. Thanks
Best Regards
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