Re: Dell ST2320LF Monitor: No power?
Just wanted to say thanks for the help and guidance on here. I replaced that diode (when the spare finally arrived) and the monitor fired up straight away! So happy. :-)
It's been a refresher for my soldering skills (very limited!), something interesting to research, and it's finally paid off and I have a working monitor again for just a few dollars. Wouldn't have done it without the guidance on here, so thanks again for that.
yls
Member
Last Activity: 07-17-2015, 04:31 AM
Joined: 06-16-2015
Location: UK
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Re: Dell ST2320LF Monitor: No power?
I removed one leg of each diode from the board and tested the impedance through each of them in both directions. They all behaved the same except one, which has zero resistance in either direction. I guess it's shorted internally.
Then I used the diode test function on the mumtimeter and the same thing, all were the same except that same one.
I don't fully understand how to test diodes, but I could easily see one was different to the rest.
The writing on the diode is:
MBR5200
...Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
I replaced the cap and I've still got the same problem. The voltage across that cap still fluctuates between 10 and 20VDC, and I only get 2V on one of the output pins and nothing on the others.
budm, you mentioned the cold side rectifier. With a standard multimeter, can I take each diode out one at a time and test them to see if they're ok? I have this multimeter if it matters:
[url]http://www.dontscrapit.com/Kansas-/Commercial-/Mastech-19-range-all-purpose-digital-multimeter-vom-amm-partpic.jpg[/url]Re: Dell ST2320LF...Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
Thanks. New cap in the post - fingers crossed!Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
Thanks for the tip. I've removed the small capacitor and need to find a replacement now. I'll try that before fiddling with the diodes.
The 'bad' cap says 47uF 50V 105C. Would a 47uF 63V 105C replacement be ok or should I find a better match?Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
Capacitor C855 gave varying readings between 10 and 20VDC. It seems to measure 20VDC initially and then drop through values until 10 or 11V (in a second or so) before jumping straight back up to 20 and then decreasing to 10 again and again.
The low resistance resistor that budm mentioned (R869) gave a reading of 1.1 Ohm.Last edited by yls; 06-18-2015, 11:12 AM.Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
Thanks, I'll try that this evening. Presumably I'm looking for a DC voltage across C855 whilst the mains supply is connected?
Thanks, I'll check the resistance.
The startup/running cap, is that C855 mentioned above?
You've lost me with the 'rectifier diodes on the cold side' - can you elaborate a little please? Is that the rectangular black thing next to the coil in the lower right corner of this picture?
[url]http://www.jestineyong.com/wp-content/uploads/2014/11/lcd-monitor-repairs.jpg[/url]...Last edited by yls; 06-18-2015, 04:35 AM.Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
So, the results of the tests.
1. 3.15A Fuse is ok. That would've been too easy.
2. The big capacitor had 331VDC when the mains was connected. Guess that's ok then?
3. According to the test in that video, my MOSFET is ok. Ideally it should be removed from the circuit, but it passed the test while in-situ.
Not sure if it's useful info, but when testing the 335VDC cap I also tried to measure the DC voltage across the other 7 large caps. They all read 0.
Also, I tried measuring the voltage...Last edited by yls; 06-17-2015, 03:25 PM.Leave a comment:
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Re: Dell ST2320LF Monitor: No power?
Thanks a lot, I'll let you know how I get on.
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Re: Dell ST2320LF Monitor: No power?
Thanks for the tip, I'll give it a try. :-)
Regarding checking the voltage of the big blue capacitor, is this the correct procedure? Assuming it's fully discharged at the moment.
1. Plug mains supply into board and leave it a minute or two.
2. Remove mains supply.
3. Measure PD across the capacitor legs.
4. Discharge capacitor before touching the board.
And as for testing the transistor, would this procedure be safe/correct? Assuming the big blue capacitor is discharged...Leave a comment:
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Dell ST2320LF Monitor: No power?
Hi all, first time poster here. I'm a metallurgist not an electrical engineer so I could do with some help!
My Dell ST23020LF monitor was working fine yesterday, then halfway through writing a document it 'turned off'. The power light went out, no standby light, nothing. Nothing on the screen, whether backlit or not. Checked/replaced the plug fuse (it was fine). I assume it's a fault on the power board, but now I need a little guidance of what to look for.
The monitor is in bits, both boards removed. No obvious signs of scorching/burning etc anywhere. On the power... -
Re: New Members - please post your introductions here
Small world, I used to live in Nottingham!
Came across this site after my Dell monitor failed, and it seemed like the best place to ask some questions as I'm a metallurgist by trade, not an electrical engineer!Leave a comment:
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