Help in troubleshooting monitor - Page 1
Forum section: electronics / projects

author: darko31
author_rank: Regular Contributor
timestamp: May 24, 2013, 09:06:41 am
content: Hi everyone,a friend of mine gave me a monitor, which turns on for about a few seconds and then turns off. I've immediately suspected of bad caps, but after I've opened it, to my surprise, all caps looked fine at the first glance. Since I don't have an ESR meter, I've replaced them anyway, to be sure, but that didn't fixed the problem.I've tested the PSU board with a dummy load, so I'm sure that the PSU is at fault. It's rated to output 5V at 3A. With no load, the voltage is 5.2V, at 1A voltage drops to 4.8, and at the rated 3A the voltage is 4.45V, which, I presume, isn't enough for the electronics driving the monitor.Any ideas where to look for the problem?The monitor is Viewsonic VA1916W, I've attached the datasheet of the current mode PWM IC, it's STRW6252, and the service manual. Note that, in service manual, there is also added 13.8V rail, which isn't available in the monitor I have.

author: amyk
author_rank: Super Contributor
timestamp: May 24, 2013, 09:52:49 am
content: When you say "turns off" do you mean the entire monitor dies or just the backlight? Hold a torch to the screen and if you can still see the display then the fault is in the inverter section --- possibly old CCFLs worn out and triggering overcurrent protection. The 13.8V is for the inverters and not the main panel.I've had success with just powering the monitor off the computer its connected to when the PSU died -- in my case it needed 5V and 13V, but worked fine with 5/12V from the PC. So you could try giving it 5 and 13.8 from a bench supply and seeing if that gives the same symptoms.

author: Paul Price
author_rank: Super Contributor
timestamp: May 24, 2013, 02:27:28 pm
content: I had the same problem with the same model and it was a CCFL lamp failing to light and so the HV transformer arced over and shorted out and the monitor switched itself off under overload.I could not access the CCFL tube for replacement due to flexible printed circuit interrconnects were already becoming intermittent by my playing around. I managed to use the monitor for another four years by powering both CCFL's using the other transformer, until the other CCFL's starting potential rose to point that caused the other resonant transformer to arc over and fail. The monitor worked fine in this way, with a noticeable decrease in brightness near the topmost of the LCD screen.

author: darko31
author_rank: Regular Contributor
timestamp: May 24, 2013, 03:39:33 pm
content: Yup, you're right, it's the backlight. I've used torch to see if I can see the picture on the screen, and there it was, slightly visible.So, probably CCFL tubes are bad? Ok, I'll try to find replacement, but I probably won't try to repair it myself, I don't have a lot of experience, and since the monitor is still repairable, then I'll tell my friend to take it to someone else more skilled than me.Thanks amyk and Paul.

author: Strada916
author_rank: Frequent Contributor
timestamp: May 24, 2013, 05:06:40 pm
content: might cost you more than to buy a new monitor. ??

author: wraper
author_rank: Unknown
timestamp: May 26, 2013, 06:37:24 am
content: Check resistance of secondary windings of both HV transformers. They should be something about 1k. Resistance of both must not be different more than by 5%. The usually become open but shorts in windings also possible. If transformers are ok then usually CCFL tube is bad.

author: Psi
author_rank: Super Contributor
timestamp: May 26, 2013, 07:03:19 am
content: Does it have two CCFL tubes and two drivers? The driver is what normally failsMost monitor are still usable and bright enough with only one CCFL.First thing i'd do is figure out which of the two CFL drivers is faulty. You can do this by checking the CCFLs one at a time.(unplug one and see if you get any image)If you find you get an image for a few sec with only CCFL A plugged in but nothing at all with only B you can assume the fault is with B tube or driver.Next up, figure out where driver B gets it's power from and temporarily disconnect it. This will prevent it from shorting out the supply and hopefully will let the LCD run with only one CCFL tube and driver without switching off.Unfortunately some LCDs sense the output of both driver channels and refuse to run if only one driver is working.It can be a bit annoying tracking down the monitor circuit and bypassing, but it can be done.

author: amyk
author_rank: Super Contributor
timestamp: May 26, 2013, 11:03:51 am
content: In the theme of this forum (Projects, Designs, ...) maybe this might be a chance to try an LED conversion.

author: darko31
author_rank: Regular Contributor
timestamp: May 26, 2013, 01:11:45 pm
content: @StradaIt might be, I've found one supplier from my country, who sells various tubes, for 15$ to 40$, not the cheapest ever, but cheaper than new monitor.@wraperThanks for the tip.@PsiI was going to ask, is there any way of bodge repair, I've disconnected one tube, and the monitor turned on, and then turned off. I assume that there is protection when there is no load on one of the transformers or maybe the other tube is also bad. I've disconnected the other one, then the screen didn't light at all, I assume that that's the bad tube or transformer. Service manual should be helpful for bypassing the protection.@amykLED conversion would be very nice, but first, I have to ask my friend what he wants to do with it, after all it's his monitor.

author: wraper
author_rank: Unknown
timestamp: May 26, 2013, 02:13:38 pm
content: Have you checked resistance of transformer? Better do not disable protection or something can burn later. Transformers usually are reasonably cheap on ebay or aliexpress. And they go bad more often than tubes from my experience. You can also check tubes by connecting both to one output leaving another disconnected. If one lights up for a moment but another doesn't you will know that tube is bad. Exchanging tubes is not easy job, also you can get light bleeding or another backlight uniformity problems.

author: darko31
author_rank: Regular Contributor
timestamp: May 26, 2013, 04:40:33 pm
content: I didn't measure it earlier, but I'm measuring them now, yep both are 1K, to be more precise 1041R and 1047R. So one of the tubes is definitely bad.

author: wraper
author_rank: Unknown
timestamp: May 26, 2013, 04:46:58 pm
content: You can check them as I wrote in previous post. Just connect one at the time to known good output to be 100% sure. There still is small possibility that something wrong with inverter.

author: Rasz
author_rank: Super Contributor
timestamp: May 27, 2013, 05:28:16 am
content: now try the bulb that you know is ok with the second transformer

author: Psi
author_rank: Super Contributor
timestamp: May 27, 2013, 07:22:50 am
content: Quote from: Rasz on May 27, 2013, 05:28:16 amYeah, that is worth a try.But i would bet money it's the driver not the tube. Either a bad transformer or a blown driver Fet/IC