Yes, the problem was a defective power supply. Instead of replacing it, made some inspection on the 12V circuit. The 12V voltage created at the capacitors are splitted into two circuits: the standby voltage output and (which is unswitched and goes out straight forward) and the main 12V output which passes through two high-side MOSFETs in parallel that acts as an on/off switch. I found that one of this MOSFETs was defective (gate was short-circuited) which was keeping this switch always ON but with a fairly high output impedance. This is why i have measured 12V on the main output even with the PSU turned off but then the voltage drops when computer turned on and starts to drain some current. Apparently, the motherboard circuits were able to work fine with 10,5V, but the backlight driver board didnt.
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Yes, the problem was a defective power supply. Instead of replacing it, made some inspection on the 12V circuit. The 12V voltage created at the capacitors are splitted into two circuits: the standby voltage output (which is unswitched and goes out straight forward) and the main 12V output which passes through two high-side MOSFETs in parallel that acts as an on/off switch. I found that one of this MOSFETs was defective (gate was short-circuited) which was keeping this switch always ON but with a fairly high output impedance. This is why i have measured 12V on the main output even with the PSU turned off but then the voltage drops when computer turned on and starts to drain some current. Apparently, the motherboard circuits were able to work fine with 10,5V, but the backlight driver board didnt.
So, i have simply replaced the defective MOSFET and voilà... everthing worked as expected. Of course, replacing the PSU would also solve the problem.
Hello Michael,
Yes, the problem was a defective power supply. Instead of replacing it, made some inspection on the 12V circuit. The 12V voltage created at the capacitors are splitted into two circuits: the standby voltage output and (which is unswitched and goes out straight forward) and the main 12V output which passes through two high-side MOSFETs in parallel that acts as an on/off switch. I found that one of this MOSFETs was defective (gate was short-circuited) which was keeping this switch always ON but with a fairly high output impedance. This is why i have measured 12V on the main output even with the PSU turned off but then the voltage drops when computer turned on and starts to drain some current. Apparently, the motherboard circuits were able to work fine with 10,5V, but the backlight driver board didnt.
So, i have simply replaced the defective MOSFET and voilà... everthing worked as expected. Of course, replacing the PSU would also solve the problem.
Good luck!