This ASUS Motherboard H81M-A Rev.:1.03, 4th Generation was brought to the Workshop because it was fully dead.
First of all, the boardview of this Mobo was open and the repair began. An energized SMPS was connected and the +5VSB (Standby) were present. In this diagram these +5VSB was supplied for the SMPS.
Next step was to check the +3VSB, which was generated by LDO REG. AZ2085D, PU601 in the diagram.
This voltage was present, so far so good, but when the Mobo was switched on by jumping pins 6 and 8 of the F_Panel it started in a bucle way, it was a bad sign, of course.
The v-out of the different power supplies had to be tested by order. The first was the RAM, 1.5v were present. Next power supply that should be taken into consideration was the N-MOSFET PH7730DL (P701), and a strange behavior was occurring right here. Let´s analyze next photo.
In the Drain of PQ701 the +1.5VDUAL supplied by PU502 LIN REG. RT9045GSP was present, so far everything was fine. But this MOSFET had no v-out in the source pins (pins 1; 2 and 3). By analyzing the circuit, there was a fatal error, the gate had no polarization.
This voltage should be supplied by PU701, the OP AMP. GS358ASF, in the pin 3 of this IC there was 1.04v (as shown in next picture), but not voltage in the pin 2
This IC acts as a comparator, but it was not performing like this, so it was changed by a new one. Now the mobo started and the PCH was supplied with 1.05v, but the system turned off and no bucle. A new behavior appeared at this time so it was time to check the PWM CONTROLLER ASP1258QGJ-A (PU101)
In the pin 2 (P_VCORE_VCC5_20) of this IC there was +5v, but in pin 1 (P_VCORE_EN_10) there was no voltage. By analizing the circuit there was a transistor in bad conditions, switching on not in the right moment and conducting the +3.3v of enable to GND.
This transistor was PQ802 in the diagram: NPN METR3904-G. It was changed and everything got to normality as usual.
Thermal compound for the UP, preventive maintenance to the mobo and it was tested for 3 days in a row without any malfunction.
Double satisfaction: for the owner and for me. Mission accomplished.
This article was prepared for you by Humberto Rodriguez, one of our ‘Master Authors’ from Cuba.
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You may check on his previous article on UPS-SL650 Repaired
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