My monitor won't turn on unless I remove power completely first - Page 1
Forum section: electronics / repair

author: agehall
author_rank: Frequent Contributor
timestamp: February 27, 2017, 11:02:05 am
content: So my Dell U2711 (circa 2011 vintage) decided to die on me a couple of weeks ago. I cursed a bit, got myself a new screen and left the Dell on the bench. Basically, it didn't react to me pressing the power button.When I got a bit of time, I decided to attempt a repair as I really liked the screen. The obvious problem would of course be bad caps so there was little surprise when I opened it up and found a whole bunch of bad caps inside. There were a few bad ones on the transformer boards and I also saw a couple of obviously bad ones on the logic board.I proceeded to order all the caps from Digikey and started replacing them. Unfortunately I managed to mess up my order and got a whole bunch of caps with the wrong values. I proceeded to replace the ones I did order correctly and decided to test the monitor out. Turns out it now switched on without any issues so I reinstalled it on my desk and used it.However, the next day, it again refused to turn on.However, what I did notice, was that if I remove the power cord and reinstall it, it works again.My plan is to replace the caps I didn't get to the first time around and hope that fixes the problem, but is there anything else I should be looking for that is typically a problem as well?

author: CJay
author_rank: Super Contributor
timestamp: February 27, 2017, 11:07:20 am
content: Replace the caps with correct values (higher voltage is acceptable) and equivalent or better ESR values, check your prior work.Also check high value resistors, it's possible something has gone open circuit or risen in value.

author: nctnico
author_rank: Super Contributor
timestamp: February 27, 2017, 11:10:47 am
content: Quote from: CJay on February 27, 2017, 11:07:20 amI've seen this happen before (high value resistor broken) which caused similar symptoms.

author: wraper
author_rank: Unknown
timestamp: February 27, 2017, 11:23:24 am
content: Don't bother with measuring any resistors before replacing small capacitor(s) on the primary side.

author: agehall
author_rank: Frequent Contributor
timestamp: February 27, 2017, 11:32:29 am
content: Thanks for the replies!All the caps on the primary side were replaced with proper new ones.There are about 35x 47uF ones on the logic board that I didn't touch as I managed to order 470uF ones.I'll take a look at the resistors as well. It will be a bit tedious, but hey, that's what spare time is for, isn't it.

author: CJay
author_rank: Super Contributor
timestamp: February 27, 2017, 12:00:07 pm
content: Quote from: wraper on February 27, 2017, 11:23:24 amDefinitely, my rule of thumb is 'if it's got a 4 and a 7 in the value, it's probably faulty' when it comes to capacitors in power supplies but they tried to fool me and started fitting 22, 10 and even 100uF capacitors so your advice is betterI should probably have said if the fault persists then check resistors but I would be checking any resistors that are intended to provide a 'startup' or 'kickstart' supply if I had those symptoms anyway.

author: Armadillo
author_rank: Super Contributor
timestamp: February 27, 2017, 12:49:34 pm
content: The description fits a OVP or thermal latch up protection that need power supply recycling to reset. However, I supposed some failed capacitors may camouflaged and imitate the same symptoms as well. Until then....

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2017, 08:02:26 pm
content: Quote from: agehall on February 27, 2017, 11:32:29 amI hate that. I have the same problem when I go looking through my parts stock for something. If I need a 33k resistor I'll find 3.3M, 330k, 3.3k, 330 ohm, maybe even 33 ohm or 3.3 ohm, but no 33k. Seems like it happens every time.