In a two-transistor ckt, there is a resistor from Q12's emitter to the negative supply. D31, C32, and R58 make up the snubber/damper. R57 is a load for D28/C19 and a return path for the zener. That's how these can regulate without an opto. The ratio of the tertiary winding is such that just as the zener starts conducting, the AUX and preregulated 5Vsb supplies are of the proper voltage. When that happens, drive to Q12 is removed and energy is no longer transferred to 5Vsb, AUX and tertiary. Eventually, the voltage on C19 gets low enough to cause ZD2 to conduct. C19 is the 'critical cap' and its rectifier is D28. After a few cycles, C19, C23, and C24 begin to charge up. The tertiary winding provides feedback via C3. Starting bias is supplied through R55 and R56. That 'important' cap would go bad and 5V would run away. This diode configuration is NOT what caused the psu to be a mobo killer If you get one of these to fix (for yourself, not for customers), you can drill two new holes and relocate the capacitor.

Bestec needs to move that capacitor to a cooler location. On the down side, capacitor C36 is bulging, as usual. Maybe the old A6351 workhorse has gone obsolete. Looks like Bestec is moving away from the A6351 chip I have seen in so many of the Bestecs. This pwm chip runs at 132 khz and has lots of nice new features. Inside the psu, what’s new? The 5vsb uses a TNY276 chip. No LED indicator light on the back of the power supply case. No six pin or eight pin connectors for high speed graphics cards. Since it has a HP part number I am guessing that this is a OEM for an economy pc by HP. It has a 24 pin connector for the mother board and a four pin connector for cpu. What’s new here? First, it has two SATA connectors as well as five molex connectors and one floppy connector. More heavily oriented toward older 5 volt boards, this power supply has one 12 volt output at 19 amps and 5 volt output at 25 amps.

Bestec power supply atx0300d5wc