Can someone give me advice on repairing a monitor, most likely bad caps. - Page 1
Forum section: electronics / repair

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 18, 2017, 09:41:10 pm
content: I bought a 27 inch monitor from circuit city back around 2007 when they were having there going out of business sale for the bargain price of $400 dollars, crazy I could buy a small 4k TV or a 50+ inch 1080 lcd for that price these days! Anyways its a pretty solid monitor and I am not ready to let go of it yet, I want to use it as a display for my Radio Astronomy and SatCom SDR setup.About two years ago the thing started randomly shutting off after running for a while, eventually it started shutting off within 15 minutes of turning it on and quickly shot down to turning off within the first minute while using it as the only unused display with an HDMI port to configure a RasPI. It sat for a while and then I took it apart six months ago to fix it and just sort of forgot about it until now.Im not new to repairing flat screen displays, every TV, and some monitors I own were all repair jobs. I have successfully fixed mine and others displays of all different sizes and types (plasam, lcd), with all sorts of problems from bad caps to blown t boards, etc, etc. This monitor isn't an easy spot and replace issue though. From the behavior of the issues, it is obviously a problem in the power supply. By the way the issue slowly went from intermittent to quickly predictable and unusable I am going to say this is just a standard bad electrolytic cap issue. If you think it sounds like something else then please chime in and let me know where you think I should be looking.Ok so if its just bad caps, why am I asking for advice?! Well this is the first time I have seen a device totally take a dump from bad PSU caps and none are leaking or even bulging enough that I can tell them from good caps. Here is the PSU boardhttps://www.dropbox.com/s/dvx1j51nckxtfx3/monPSU.jpg?dl=0https://www.dropbox.com/s/tr7shri69a0y220/monPSU2.jpg?dl=0So as you can see none of these caps look like they have issues and they are all 105c rated, but I guess they could still have dried out inside? They only tool I have to check ESR is one of theseover dressed cheap m328 based component tester. Im not even sure how many ohm's is actually a "bad" ESR, when it comes to electrolytic caps, I know I like to see under 2.5 ohms but if a cap measures 3 or 5 ohms is trash? Lastly this little tester does not do in circuit cap testing (I verified this before posting) so how the hell do I go about finding bad caps on this power supply? Is there a better method than soldering every single testing its ESR and then soldering it back in if it tests good?? Is there a quick ballpark esr pass/fail type of test I can do in circuit using a function gen and scope or something?I really need to get this guy working, I can not afford to replace it at the moment, so my best cheapest quickest bet is bad caps, but they all look so good and have a hi temp rating it really has me questioning my cap theory on this one.

author: wraper
author_rank: Unknown
timestamp: December 18, 2017, 10:27:12 pm
content: Those look like nippon chemi-con so not very likely that they are faulty, although pictures are not clear and there is no company logo visible. ESR should be below 0.1 ohm for large capacitors on secondary side. IMO problem could be on the primary side, probably small electrolytic cap, ESR should be below 5 ohm.

author: SMdude
author_rank: Frequent Contributor
timestamp: December 19, 2017, 12:15:12 am
content: They do look like Nichicon, however if they are old and have been exposed to heat(I notice some dark spots on the board near the heatsink) then they can dry out. I have seen this happen(with Nichicon) many times. Physically the caps look good, but internally they have lost capacitance and have parallel resistance(going short).Try using some freeze spray and see if keping the caps cool keeps it running for a bit longer. Also apply heat to each cap and see which one(s) cause it to shut off.It would almost just be easier to just replace the lot..Cheers

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 12:21:52 am
content: Quote from: SMdude on December 19, 2017, 12:15:12 amThose are not Nichicon 100%. Also nichicon had huge issues with HM and HN series.QuoteCooling capacitors causes ESR to go higher.

author: Azu
author_rank: Newbie
timestamp: December 19, 2017, 12:24:36 am
content: The caps "look" good to my eyes have you tested the inverters or cathode with a spare?

author: dacman
author_rank: Frequent Contributor
timestamp: December 19, 2017, 12:30:18 am
content: Most of those capacitors could be tested in-circuit with a decent LCR meter. About the least expensive one is the DER EE DE-5000, although I have no experience with it. When I test capacitors, I want at least two parameters (usually Cp and D or Cs and Rs). Also, most capacitors that I've replaced had no outward signs of a problem, although a lot of them did.

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2017, 12:32:08 am
content: You can't tell from the appearance, I've replaced a lot of bad capacitors that looked just fine visually. Of course that doesn't mean that the capacitors are the problem, I would look for cracked solder joints too.

author: fable
author_rank: Regular Contributor
timestamp: December 19, 2017, 12:35:52 am
content: The problem is not in psu. The problem is most probably bad ccfl lamps and easy way to fix it is to remove protection that shuts them off.

author: fable
author_rank: Regular Contributor
timestamp: December 19, 2017, 12:47:29 am
content: The problem can also be inverter transformer and you can check them by measuring the resistance on secundary side of each transformer and the difference must not be bigger than 20-50ohm.If some of them has resistance significantly bigger than others then he is the reason for protection to kicks in. Sorry for my bad English

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 12:52:32 am
content: Quote from: fable on December 19, 2017, 12:35:52 amIf you shut off protection while something wrong with CCFL, you'll likely destroy the inverter quiet quickly. Also CCFL or transformer fault shouldn't cause complete shut off, only backlight going off.

author: fable
author_rank: Regular Contributor
timestamp: December 19, 2017, 01:01:16 am
content: If ccfl are bad then there is no harm in removing protection. If the problem is transformer then he will start to heats up and eventually fail

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 01:06:09 am
content: Quote from: fable on December 19, 2017, 01:01:16 amYou will kill transformer and it's driver may go short as well.

author: fable
author_rank: Regular Contributor
timestamp: December 19, 2017, 01:20:29 am
content: My two main monitors work fine for 4 years after disabled protection they just get a bit dimmer as time passes . This does not strictly need to be repair solution but troubleshooting as well

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 19, 2017, 05:32:21 am
content: Well thanks for all the suggestions, its been so long since I had it on I totally forgot what it does. I cant quite remember if it shuts totally off or the backlights stay on and the picture goes bad.Im not sure why there would be problems with the transformers I will test them as directed above, but the monitor never smoked, has never been mistreated etc. Seems to me like normal wear and tear from heat and environmental factors. I mean a transformer is just a core with wire, they dont really fail unless something draws to much power, they all look good no burns etc, as I said the thing just peacefully lost picture or turned off cant remember which. Im pretty sure at the very end it would only intermittently turn on though and to me that says PSU

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 11:39:18 am
content: Quote from: rwgast_lowlevellogicdesin on December 19, 2017, 05:32:21 amCCFL inverter transformers do fail quiet often. Voltage on secondary usually is IIRC around 600V, and before lamp ignites, can be a few times higher. And this is with very compact secondary winding consisting of very thin wire. Therefore secondary winding failure is a common thing.

author: kalel
author_rank: Frequent Contributor
timestamp: December 19, 2017, 12:21:10 pm
content: I recently had an issue with an old LCD monitor, which would restart multiple times at turn on.I had no idea which capacitor was faulty (they all looked fine visually), I don't have an LCR meter, so I changed about 10-15 electrolytic capacitors on the power supply board (all of the through hole ones except the main large one). For couple of USD/Eur the monitor was fixed.From what I've read on changing capacitors on monitors, sometimes the bad ones appear visually obvious (e.g. pressurised/bulging on the vent part), but that is obviously not always the case.I know that an experienced person that understands the circuit would know which are more likely to go bad, but in my case, this was the fastest method, even though I did not get to learn much from the experience. It may or may not be the case that some other capacitors would go bad eventually if I only replaced the faulty ones.The only thing I can suggest is that it might be worth having an LCR meter (as well as probably study the schematics of standard power supplies and learn about common failure points) if doing multiple repairs. The investment in the meter for a single repair might not be worth it.

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2017, 06:47:17 pm
content: My anecdotal observation is that high value low-medium voltage capacitors like 800-1,000uF 16V are more likely to bulge or leak while smaller stuff like 47uF 35V tend to go bad without any physical indication. If one lacks an ESR meter it may be worth replacing at least the smaller electrolytics in the power supply and see what happens. Often there is one low value cap in the startup circuit that will prevent the PSU from coming up properly if it fails.

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 06:54:02 pm
content: Quote from: james_s on December 19, 2017, 06:47:17 pmCompletely valid observation. Small capacitors often do not have vent and even if they do, they usually fail without bulging.

author: EHT
author_rank: Frequent Contributor
timestamp: December 19, 2017, 09:47:00 pm
content: Some advice for the OP:Please be aware the circuit is not isolated and is at dangerous high voltages. If you are just (blindly) replacing the caps it would be much safer to reassemble the case before powering it on. If you are testing it, start with the isolated side, which is the larger section of the board away from the mains socket, separated by cross-hatching. Do not connect a scope to any of the non-isolated part ever. If you are not sure about any of this, in particular how to tell what is high voltage and/or un-isolated and what is not, stay away from it.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 21, 2017, 12:17:48 am
content: So I tested all the transformers and they all read ok, I also pulled half of the larger filter caps out and tested there ESR. They are all reading at .04ohms, is that to low? I tried testing continuity and regular resistance across them to make sure they weren't dead short, the meter freaked out and kept trying to range on resistance (pretty normal when you test any cap on the resistance setting) and the continuity buzzer did not sound, so I am assuming that there is no physicall connection from one plate to another BUT .04% is way lower than any similar physically sized electrolytic caps I have on hand that have never been used. they all measure between 1.6 and .1 esr. A google search for "can equivalent series resistance be to low" didn't turn up anything useful.

author: wraper
author_rank: Unknown
timestamp: December 21, 2017, 12:36:43 am
content: Quote from: rwgast_lowlevellogicdesin on December 21, 2017, 12:17:48 amIt's normal ESR for them, please read my first suggestion in this tread.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 21, 2017, 05:01:14 am
content: thanks for the help, issue solved found 4 bad caps near to 220 dual rectifier diode package right after the SMPS transformer. I know you had recommended .1ohm and under, .04 just seemed really low compared to measurements on new unused electrolytics I have around.

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2018, 01:57:58 am
content: Were the capacitors shorted? If the ESR seems suspiciously low you should check the DC resistance with an ohm meter. When capacitors fail the ESR normally rises, but a short will fool the ESR meter.