Summary
Asus 27” VP276H monitor had no power. Report of the fault was that it was working fine, and then suddenly without other symptoms the power turned off.
The fix involved replacing the 2.5A PCB mount fuse and KBP208G-C bridge rectifier, both on the AC input circuit of the power supply.
Safety Note
This is an experience of a repair process that I did for a monitor. It is not intended to be instructional for others. It involves being around 240VAC mains voltage which is very dangerous and can cause death. Repair work involving AC mains voltages should only be carried out by fully qualified personnel.
Background
I bought this monitor 2nd hand for $10 from a seller claiming it just failed and they were going to throw it in the bin, but reconsidered to see if they could get a bit of money first. This is not a bad idea, and I see lots of sellers try to do this.
After confirming the pickup and a 20 minute ride, it was loaded into my trailer.
Investigation
Monitor internals contain 3 common function PCBAs: menu buttons, power supply, video controller.
Button board was hard- wired to a flex cable, I didn’t fully test this since the fault seemed more in power supply than on/off button functions. It appeared to be a common style, multiple buttons pulling different resistor values to ground. When connected to the controller board, these form a voltage divider. The voltage created can be read by a processor to determine what buttons are being pressed.
Since there is no life to the monitor at all, I next started probing the AC mains input circuit and primary side of the power supply board. It looks like a pretty basic circuit – fuse, PTC, filter, bridge rectifier, DC storage cap.
All components looked visually OK. No suspicious burn marks, no burnt components, no swollen/leaking caps. Measuring the resistance across AC active and neutral showed values in the mega-ohm range. So no shorted tracks. No apparent shorts on the rectified DC either.
When AC voltage is applied, the main DC storage cap would normally be at around 334VDC, but here it was 0V. No AC voltage was getting passed the fuse. Blown!.
I desoldered this part and busted it open. Clearly blown.
Always ensure that safety glasses are being worn. I hacked in a fast-blow 2A glass fuse (I have no fuses in that package available) and it also blew when powered. Did it blow from the fault or the inrush current. Hack in a fast blow 4A fuse. It also blew violently, and as did the KBP208G-C rectifier.
I removed the bridge rectifier, soldered in a 1A glass fast blow fuse (soldered wires to the fuse, covered exposed electric parts in heatshrink, soldered the wires to the PCB), and reconnected power. Fuse remained intact and the rectifier input pads showed 240VAC.
A datasheet for the KBP208G-C bridge rectifier is here: https://www.diodes.com/assets/Datasheets/ds21205.pdf.
Specs show 2A average rectified current, 560V RMS reverse voltage.
I don’t have any if these rectifiers on hand, so I hacked together a full bridge rectifier from 4 x 1N4006 diodes and heatshrink and put this in as a temporary check. I don’t suggest this is done as a permanent fix, I did it just to allow investigation to continue.
After applying power, the monitor came to life as expected. The rectified DC voltage after the hacked rectifier was 334VDC – just what I expect.
Replacement parts can be bought locally from:
https://sg.element14.com/multicomp-pro/kbp208g/bridge-rectifier-800v-2a-kbp/dp/1861417
https://sg.element14.com/multicomp/mst-2-5a-250v/fuse-radial-slow-blow-2-5a/dp/1566104
After receiving the correct parts and soldering them in, the monitor powered up normally and showed the expected picture.
