Ok, picked up this fairly scratched up Samsung LE32B450 featuring a BN44-00260A power/inverter combo when I was visiting my parents. Fault was, it had a standby light but no picture, sound or backlight. I tried the usual trick of unplugging the power supply connector but was rewarded with no backlight. I suspected the power board to be at fault (possibly the common shorted diode issue, or a bad startup capacitor.) I had to leave it there as I had no multimeter with me then.
I brought it back to Leeds and started probing. With the main board power connector disconnected I had 5.3V standby, but no 13V run and the power on signal was dead about 0.01V. The PFC bus voltage was 330V and the PFC diode tested OK. Connecting the main board made no difference to the power supply voltages. I was confused by the low power on voltage. These boards require an active HIGH power signal to turn on (this is different from most Samsungs using an active LOW signal.) Built into the board is a pull up resistor, RM882, a 10kohm resistor. Even in the worst case the power on signal should be high without the main board connected but it's still low. A short could pull it low, but when I check PSON it's got over 40kohm to ground so that's not the cause.
So I decided to remove the power board and to my surprise RM882 is missing. It's completely gone; the pads are ripped too. WTF? The board is easy enough to trace out, so I soldered a spare 18k resistor across a nearby area. Now when the power board is connected with the main board cable disconnected I get backlights.
Now imagine my shock when I connect the main board cable and I'm rewarded with a fully functioning TV. Literally, it works perfectly. I can't figure this out. Obviously, the main board can only pull the line low or float it high - it can't drive it directly - so the resistor is critical to the operation of the TV. But HOW did it just disappear?! It's surface mount, on the bottom of the board, and virtually inaccessible. I'm glad it's working but I have no idea how this fault occurred in the first place. Perhaps, if the previous owner, maybe he knows a trick or two, tried to disable it they just removed this component (maybe warranty claim), but surely if they are knowledgeable enough to remove that, they would just put it back on if their warranty claim failed ... rather than just sell it to me for £20 ...
Eh? Am I going crazy here?
Schematic:
I brought it back to Leeds and started probing. With the main board power connector disconnected I had 5.3V standby, but no 13V run and the power on signal was dead about 0.01V. The PFC bus voltage was 330V and the PFC diode tested OK. Connecting the main board made no difference to the power supply voltages. I was confused by the low power on voltage. These boards require an active HIGH power signal to turn on (this is different from most Samsungs using an active LOW signal.) Built into the board is a pull up resistor, RM882, a 10kohm resistor. Even in the worst case the power on signal should be high without the main board connected but it's still low. A short could pull it low, but when I check PSON it's got over 40kohm to ground so that's not the cause.
So I decided to remove the power board and to my surprise RM882 is missing. It's completely gone; the pads are ripped too. WTF? The board is easy enough to trace out, so I soldered a spare 18k resistor across a nearby area. Now when the power board is connected with the main board cable disconnected I get backlights.
Now imagine my shock when I connect the main board cable and I'm rewarded with a fully functioning TV. Literally, it works perfectly. I can't figure this out. Obviously, the main board can only pull the line low or float it high - it can't drive it directly - so the resistor is critical to the operation of the TV. But HOW did it just disappear?! It's surface mount, on the bottom of the board, and virtually inaccessible. I'm glad it's working but I have no idea how this fault occurred in the first place. Perhaps, if the previous owner, maybe he knows a trick or two, tried to disable it they just removed this component (maybe warranty claim), but surely if they are knowledgeable enough to remove that, they would just put it back on if their warranty claim failed ... rather than just sell it to me for £20 ...
Eh? Am I going crazy here?
Schematic:
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