Old monitor Power supply - Page 1
Forum section: electronics / repair

author: 2ndhandspaceman
author_rank: Newbie
timestamp: December 21, 2015, 02:44:54 am
content: Hi,I've recently been watching way to much EEV and I've been inspired to work on my old monitor that's had flickering issues and refusal to power on I replaced the caps on the power supply board a while and boom it worked again. I thought It was good till a month later It started flickering and refusing to power up and here is good part it will power on if it gets warm enough such as blowing a hair dryer into the back. I went though and tested all of my caps again and even my replacements using a voltmeter in ohms mode (As a note I have no idea how valid that test is I just found that method on the internet). After testing the caps and coming back with null I decided to check the voltage output on the 12 and 5 volt rails and it was not pretty looking the 12v rail went between 9-12v and the 5 volt rail between 0.29-4 and again when it was warm the voltages were 12.1 and 4.9 respectively. I'm a complete noob here and only have a basic understanding of how a lot of this works and I'm just puzzled as to what the heck is wrong and why it works when warm I'll attach some pictures of the board down below. any help here is really appreciated and I'd love learning about this stuff so please excuse any seemingly silly questions I may have to your reply's

author: wblock
author_rank: Regular Contributor
timestamp: December 21, 2015, 03:37:05 am
content: As capacitors warm up, their ESR drops and sometimes the monitor works again. It's not the most common problem with monitors, but it's not rare, either.Testing caps with a voltmeter will not show their ESR. ESR is resistance to AC, usually tested at 100KHz, not DC.Using general-purpose caps is a waste of time, they will fail in a few months.Please show pictures of the power supply board.

author: wblock
author_rank: Regular Contributor
timestamp: December 21, 2015, 04:42:06 am
content: C114 (green, lower right) looks like it was not replaced. It's a bit large to be a startup cap, but the others look like they might be smaller than the originals.

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2015, 04:45:40 am
content: There are some low value ( 100uf 35V or so) electrolytics on the primary side, probably well hidden by the heatsink under the blob of white silicone, which should also be changed. If the secondary side ones are not low ESR they will also cause this till they heat up.

author: 2ndhandspaceman
author_rank: Newbie
timestamp: December 21, 2015, 05:07:26 am
content: c114 was not replaced passed the test I did but as you pointed out that's not very valid and I need get my self an ESR meter to test and honestly I wouldn't be surprised if its the caps I replaced with I asked the guy that runs the local electronics store and he just grabbed some down and said these will work upon research they are pretty large piles of crap soooo looks like I'm buying some new caps again thank god they are cheep even for the good ones

author: wblock
author_rank: Regular Contributor
timestamp: December 21, 2015, 03:22:57 pm
content: Try to match the size of the originals, because that also affects ESR value. Get recognizable brand names, watch out for fakes, and make sure they are really low ESR (correct series).

author: 2ndhandspaceman
author_rank: Newbie
timestamp: December 22, 2015, 03:50:07 am
content: Thanks Wblock and everyone else that commented I went ahead and ordered some new caps online went with the Panasonic brand and got the low ESR ones hopefully this fixes the issue

author: jh15
author_rank: Frequent Contributor
timestamp: December 22, 2015, 04:33:49 am
content: Where in USA is your 'local' electronics store? You are lucky.. I have one here in Maine, and they even know stuff.