Acer AL1916 monitor PSU repair (DM0565R blown)

[807]

I'm repairing an Acer AL1916 monitor PSU at the moment. The 400v high voltage smoothing capacitor C114 is measuring completely O/C. The case of U101 has blown apart. Fuse F101 has blown. ZD121 reads 6 Ohms in both directions & ZD120 reads 99 Ohms in both directions. While looking at the data sheet of U101 (DM0565R), I noticed that the absolute maximum voltage for Vcc is 20v. However, the zener diode across the Vcc pin to GND is a 1N4748A (22v).

Now, whether that zener is used to constantly regulate the voltage, or whether its's used to protect the Vcc voltage from rising too high, I can't understand why 22v was chosen, given that the absolute maximum voltage for that pin is 20v? Wouldn't a 20v or 18v zener have been more appropriate?

Until the replacement components arrive, I dont know what voltage the auxiliary transformer winding is supplying to the Vcc pin of the DM0565R.

[fzabkar]

I would consider replacing C123 as a precautionary measure, even though it is probably fine.

As for the choice of Zener voltage, that puzzles me, too.

[807]

I would consider replacing C123 as a precautionary measure, even though it is probably fine.

As for the choice of Zener voltage, that puzzles me, too.

Yep. I'll replace C123 as well as C120 and the optocoupler. Even though they all check out OK, it looks like U101 broke down & put a high DC voltage on the Vfb & Vcc pins.