Monitor power supply was seemingly fried by modified sine-wave inverter. Managed to mysteriously revive it just by poking around?

Thread Starter

LMF5000

Joined Oct 25, 2017
76
So, we had a power cut yesterday and after the UPS ran out, I tried running my desktop computer on a cheap modified-sine-wave inverter running off a surplus car battery. The computer booted up fine, but the monitor never turned on (no standby light, no operation).

After grid power came back online, the monitor still wouldn't show any signs of power (not even the standby light). I disassembled it a few times but couldn't find anything wrong. I tested with multiple known good power cables. I probed all the fuses - all good. All the diodes, transistors and rectifiers had the right voltage drop in only one direction. All the metal-oxide varistors were open-circuit as they should be. And yet nothing could get the monitor to switch on the standby light, much less turn on.

Finally I took a computer power supply and used test leads to supply GND to one of the pins with black wires and +5V to one of the pins with red wires. That got the power light to turn on. Encouraged, I connected +12V to one of the pins with yellow wires, and got the same thing. Then I plugged in the ribbon cable feeding the LED backlight, and the monitor came to life.

I still couldn't figure out what was wrong with the power supply, so out of desperation I unplugged it from the logic board, plugged it into 240VAC and started probing the outputs. The result was 17V on the yellow wire and 5V on the red one - in other words, it seemed to be working.

Started reassembling the monitor in steps, and still working. The monitor is now entirely working, and I haven't a clue what broke it in the first place and what could have fixed it (all I did was clean the dust off with a tissue, clean some solder flux residue with alcohol, power the outputs of the built-in power supply with an external power supply, and probe many of the components on the board).

Does anyone have any insight or ideas?
 

AlbertHall

Joined Jun 4, 2014
10,234
Maybe, just maybe, a spike when the mains failed tripped a protection circuit in the supply which didn't reset until the capacitors had fully discharged. I have known this to take some minutes but I don't know the time scales of your problem. As long as the supply has input power it will never reset.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
76
Maybe, just maybe, a spike when the mains failed tripped a protection circuit in the supply which didn't reset until the capacitors had fully discharged. I have known this to take some minutes but I don't know the time scales of your problem. As long as the supply has input power it will never reset.
It failed yesterday night. It revived today afternoon. In between it was plugged in half the time and unplugged the other half of the time.
 

sagor

Joined Mar 10, 2019
231
It may be that F901 is a resettable fuse, and it may have tripped but took a long time to reset? Just a guess... There are others like F903 and F904.
 

Thread Starter

LMF5000

Joined Oct 25, 2017
76
Well, we had another power cut and I left the monitor running on a UPS until it ran out of battery, and monitor is dead again. This time I pinpointed the problem a bit more - the PSU is supplying +17V on the yellow wires and +5V on the red wire. So it seems to be a logic board problem not a PSU problem.

Last time I managed to revive it by powering up the PSU board alone then plugging it to the logic board and it started working (and has worked flawlessly until the latest power cut). Now it won't turn on again. Any ideas? Should I just leave it plugged in for a few days, maybe there's a faulty capacitor that needs to charge slowly or something?

Tl;dr: It's a problem on the logic board not the power board. Any ideas? Photo of logic board below -

1597699404484.png
 
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