Mon, 15 Nov 2021

samsung 245T monitor power supply repair

I've been using a pair of Samsung 245T monitors since 2008 (13 years!) and I really quite like them, something about the 16:10 aspect ratio that just feels right. They were very expensive monitors at the time, but they have really lasted quite well since I use them pretty much every day. Recently I had an issue where one of them would not power on very reliably. This started happening a few months ago, and my solution at the time was to plug it into a UPS so that it would not ever power down which worked for a few months. Plugging it in and powering it on with the power button on the front would just not respond. Eventually if you left it for a while and got lucky, as you hit the button it would power up and work just fine until loosing mains power again. It would go to sleep just fine and wake up from that, but if it lost power it would not power back on reliably. Anyways, this finally happened where it got powered off and I just couldn't turn it back on again. I took a little time and disassembled it and tried a few things. I gave it the heat treatment hopeing to reflow the boards to maybe fix the issue but no luck there. I didn't see any visible issue on the power supply board, but decided to try replacing a few caps to see. Initially I replaced the larger caps on the DC power supply side but that also didn't help. After a bit reading into power supply design, I came to the conclusion it might be the small standby power supply that is used to sort of bootstrap the power supply. So I replaced the caps around the little IC that handles this and controls the switching for the main AC to DC conversion. This seemed to work. In the end I replaced the 6 capacitors from the board highlighted below. These wre 50V 22uf and 50V 2.2uf caps, which I swapped out with some slightly larger caps that I had in and around those ratings. The monitor power powers up perfectly now and is back up and going.

/blogimages/20211115/power_supply.jpg

posted at: 22:41 | path: /electronics | permanent link to this entry


Sun, 06 Nov 2016

magicjack go device troubleshooting

We've been using a magicjack go device for the past couple of years, and it's worked quite well. It's a small device that is powered via usb, plugs into your LAN, and then you plug your regular old landline phone into it and it provides your phone service. Recently the device seems to have stopped working, and after wasting a bunch of time with the online chat tech support to indeed confirm that the device is dead, I decided to tear it apart just to see. Getting the device open was not easy, it seems to be glued together (no screws), and I ended up just cutting along the blue line with an exacto knife until it broke open. After getting it open I noticed it has two pads for tx/rx on the PCB and hooking them up to an oscilloscope it looked like it was transmitting some data. So I hooked up my buspirate to it and was able to read the following data (at 38400 baud I think):

2nd
eCOS
MZ decompression
                MZ decompression error trap
                                           spi[2]:000000EF ,00000016
err header: FFFFFFFF

I'm not entirely sure what that is telling me, but it looks like it's probably having some trouble loading the firmware on the microcontroller and that explains why the device is dead (the blue power light comes on, but it won't connect via the lan port, and plugged into a computer it doesn't show up as a usb device at all).

Anyways, the warranty only covers one year on these little devices, so I can either buy a new one ($30 USD), or for the time being I might just use the android app as that seems to work well enough.

/blogimages/20161106/magicjack_inside-tn.jpg

posted at: 01:26 | path: /electronics | permanent link to this entry


Sat, 10 Dec 2011

fixing a broken power supply

A couple of days ago the lights on my fishtank came on in the morning, but when they did they were flashing on and off like some kind of underwater disco show. I don't think my fish appreciated it much, and so I disconnected the light and started looking into the problem. The lights on the tank are a track of LED lighting that are powered from a 15.2V 1600mA power brick. I figured that was where the problem was and sure enough putting my mutlimeter on it I noticed it fluctuating around a bit. I decided to put my scope on it to see what was going on and got this:

power supply broken

Here we can see the supply is jumping up to 12.5 volts, then dropping down 2 volts every 200 miliseconds or so, which explains why the lights were flashing on and off, but this certainly isn't the desired output. Next up was to open it up and see if anything looked wrong. This proved harder than expected since there wasn't a single screw in the brick design and it was just glued together. I ended up breaking the seal with a screwdriver and finally pried it apart. After some poking about I noticed this one capacitor looks like it had shot its load onto the heatsink.

power supply insides

And here is a not that great picture of the bad cap after it was out, you can see the brown electrolyte that leaked out of the cap

bad cap

And after replacing it we get a steady 15.2V from the supply as it was designed to do (it's not exactly a clean supply, but it does the job for lighting leds)

power supply fixed

Ended up epoxying the plastic case back together and almost as good as new :)

posted at: 00:47 | path: /electronics | permanent link to this entry


Wed, 18 May 2011

Fixing bad capacitors on lcd monitors

I had monitor with a problem where it would not power on correctly after it was off. It would just flash over and over again until it got warm, and then would start working and be fine until it was turned off for some time, and then back on again. These types of problems usually end up being something wonky with the power supply and often end up being caused by a capacitor or two that have gone bad. If you crack it open you can usually find them and simply replace them to get the monitor working again. This has happened to me a few times over the years and it's a relatively easy fix, so I thought I'd go over the process here with this one.

After cracking the case open and digging out the power supply, you can identify the bad capacitors by the bulgy bent up ones. Sometimes they will have some gunk on the top. In this case you can see the two caps here that are bad (the tall brown ones):

bad caps

Using a solder pump, or wick, suck off the solder from the joints (note, I de-soldered the wrong lead in the bottom right, had to go back and de-solder the proper lead after noticing the cap wasn't budging)


And finally install new caps to replace the ones you removed. Make sure you use a similar spec cap. The ones I removed were 1000uF 16V caps, and I replaced them with 1000uF 25V caps. The same or higher voltage will be fine. Also polarity matters, so make sure you line up the negative lead on the cap with what is marked on the board, or the same orentation as the caps you took out.

good caps

Put it all back together and if everything went alright it should be working like new.

posted at: 18:56 | path: /electronics | permanent link to this entry


Tue, 06 Jan 2009

joule theif

jouletheif

http://www.evilmadscientist.com/article.php/joulethief
http://www.talkingelectronics.com/projects/LEDTorchCircuits/LEDTorchCircuits-P1.html

posted at: 05:14 | path: /electronics | permanent link to this entry


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