Dell ST2320LF Monitor: No power?

[yls]

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 board there are no signs of bulging/leaking capacitors, cracked solder or damaged traces. I know capacitors don't always look bad even though they have failed...

Can anyone suggest some things to look for? Ideally without removing components from the board at this stage. My current plan is to go over it all again with a magnifying glass. After that, it's to replace all the caps on the power board, try to check the bridge rectifier, and to check the 3.15A fuse by the mains power connector, as per this site:

http://www.jestineyong.com/dell-led-monitor-repair/

This is the exact same monitor as mine.

Any tips would be hugely appreciated! I have a multimeter and soldering iron at my disposal, but that's about it!

[ben7]

Re: Dell ST2320LF Monitor: No power?

Check the red fuse near the power connector. It's marked as 3.15A on the PCB.

if the fuse is good, check the voltage on that big blue capacitor. It should be around 335VDC.

Next, go ahead and check the transistor on that heatsink (just above the "JestineYong"). I can't see what the part ID is, the picture is too small.
Be sure to make sure the big blue capacitor is discharged, or else you could possibly damage your meter, and/or yourself!

http://www.jestineyong.com/wp-conten...or-repairs.jpg

-Ben

[yls]

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 like you said.

http://www.wikihow.com/Test-a-Transistor

Thanks.

[ben7]

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.

Just plug the power supply in, and measure the voltage. Only thing to watch out for is if there is voltage, be careful not to short out the capacitor or anything else with the probes. You'll get a big flash and a loud bang, that will surely wake you up!

And as for testing the transistor, would this procedure be safe/correct? Assuming the big blue capacitor is discharged like you said.

http://www.wikihow.com/Test-a-Transistor

Thanks.

Actually the transistor on this power supply is called a MOSFET (metal oxide semiconductor, field effect transistor... also called a FET for easier spelling). It's more efficient than a regular transistor, which are called Bipolar Junction Transistors, or BJTs.

The PCB looks to have the FET pins marked already. They are Gate, Drain, and Source.
This looks to be a good tutorial on checking FET's:
https://www.youtube.com/watch?v=RBJGOOTEwfU
(He doesn't have the FET soldered into any circuit when testing)

If the power supply doesn't drain away the remaining voltage on the big capacitor, you can get a 10K resistor and put it across the capacitor's terminals. That should drain it in a few seconds or less.

-Ben

[yls]

Re: Dell ST2320LF Monitor: No power?

Thanks a lot, I'll let you know how I get on.

[yls]

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 at the pins for the ribbon to the other board. Going by this picture (not my board, but an identical one):


http://s9.postimg.org/gdqq5b3tb/Reverse_Copy.jpg


I meaured the voltage from each of the 9 pins in the strip on the top left of the board, against the earth symbol on the bottom edge of the board by the screw. The top 3 pins in that row are joined (in the blue box), and gave 2VDC. No voltage on any of the other pins in the orange box.

And lastly, over on the right side of the board in the picture you'll see a red arrow. In the picture the solder looks fine on that top pin. On my board there is a hole right exactly where the arrow tip is. Looking into the hole, I can see into the hole into the PCB but can't see anything in there. Is it meant to be like that? I could fill it with solder, but if I can't see anything inside the hole to put the solder on is there any point?

[budm]

Re: Dell ST2320LF Monitor: No power?

Check the resistance of the low Ohm (<1 Ohm) resistor that is connected between the Source pin of the power MOSFET and the negative leg of the main filter cap to see if it shows <1 Ohm. The problem can also be in the SIP module. You already check the rectifier diodes on the cold side? The start-up/running cap in the primary side?

[ben7]

Re: Dell ST2320LF Monitor: No power?

The problem can also be in the SIP module. You already check the rectifier diodes on the cold side? The start-up/running cap in the primary side?

Agreed. Check the capacitor (looks like it might be 47uF) next to the big blue one. That's the start-up/running cap. It's for the control circuitry on the SIP module. It looks to be capacitor C855. See what voltage is across that cap, if any.

-Ben

[yls]

Re: Dell ST2320LF Monitor: No power?

It looks to be capacitor C855. See what voltage is across that cap, if any.
-Ben

Thanks, I'll try that this evening. Presumably I'm looking for a DC voltage across C855 whilst the mains supply is connected?

Check the resistance of the low Ohm (<1 Ohm) resistor that is connected between the Source pin of the power MOSFET and the negative leg of the main filter cap to see if it shows <1 Ohm. The problem can also be in the SIP module. You already check the rectifier diodes on the cold side? The start-up/running cap in the primary side?

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?

http://www.jestineyong.com/wp-conten...or-repairs.jpg

If so, I assumed it was working as the large blue filter cap was getting charged to 335VDC. Was that assumption incorrect?

[yls]

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.

[budm]

Re: Dell ST2320LF Monitor: No power?

It sounds like it is in the startup shut down mode, either due to bad startup/running cap (the small electrolytics cap next to the big main filter cap, I cannot read the C number).
Or the rectifier in the cold (secondary side), the four big diodes with silver band.

[yls]

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?

[Porto]

Re: Dell ST2320LF Monitor: No power?

That 63v cap will do just fine, just always go similar or up in voltage, NEVER lower than original or you'll certainly blow the cap. Also, 105C is always the way to go with power supplies.

[yls]

Re: Dell ST2320LF Monitor: No power?

Thanks. New cap in the post - fingers crossed!

[yls]

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:

http://www.dontscrapit.com/Kansas-/C...mm-partpic.jpg

[yls]

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
pec 0209

Ordered a couple off ebay, fingers crossed that does it!

[Porto]

Re: Dell ST2320LF Monitor: No power?

You did good on the testing on the diode mode, it's nothing more to it really.

If one diode won't give a reading in either direction, it's blown.
If one diode gives a reading in both directions, it's blown (don't know for sure if this also counts for zeners).

A diode may only give a reading with the minus probe on the side of the white mark or line and the positive probe on the other side. With the multimeter on diode mode, mostly you get the forward voltage drop of the diode, 0.5v and higher for most diodes.

Watch this video about diodes, they're simple to use but very crucial in every circuit.

https://www.youtube.com/watch?v=MVy_MG0X2h4

[yls]

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.

[Porto]

Re: Dell ST2320LF Monitor: No power?

Always great to hear another monitor has been revived from the dead!

[budm]

Re: Dell ST2320LF Monitor: No power?

Thanks for the follow-up.