Dell 2405FPW - Power Supply Issue

[derka]

Hello everyone,
I'm writing here hoping to fix my Dell 2405FPW monitor.

I was able to unbuilt the assembly and discover that the main power supply (48.L1E02.A03) had a few bulged capacitors (C702, C711, C712, C713, C714, C715), while the remaining ones (including the massive 450V one seem to be intact).

After replacing them, I was able to measure that the top connector (CN702) that goes into the backlight TDK board correctly receives 19V on pins 4/5/6, and that the connector to the other board with the display inputs (CN701) receives 19V on pin 4 but nothing on pin 7 or pin 9 (this last is supposed to be 12V), as seen on a schematic found on this forum.

When fully plugged in, and in standby mode, there is an evident coil whine noise coming from this PSU board or the backlight board (cannot identify it precisely), which disappears once one attempts to boot the monitor. At this point, the backlight briefly flashes for one instant and occasionally displays a very faint green screen; then goes immediately to black.
By pressing the front display buttons, the source LEDs rotate correctly so that logic seems functional even when there is no image / backlight.

To summarize, apart from those bulged caps I did not notice any other burned/damaged parts on the rest of the screen and would like to know how you would proceed.

Thank you in advance for your help !

[momaka]

Re: Dell 2405FPW - Power Supply Issue

Hi derka, welcome to the forums.

Check the PSU voltage outputs again with the PSU board disconnected from the main/logic board and also the inverter. Since you have 19V, that means the 12V rail must be working too, because feedback on this power supply seems to be derived from both the 19V and the 12V rails.

As for pin 9 on CN701... looks like that one is only going to supply a voltage when you have an external power adapter connected to the PSU board through CN703.

If you still get no voltage on pin 7 after double-checking your voltages with the PSU board disconnected from all other boards, then check continuity (with power supply board disconnected from the wall / power, of course) between positive (+) lead of electrolytic cap C702(?) and pin 7. You should get a low resistance / beep, since they should be connected together. If not, it's possible that the via on the filter cap for that rail could have been damaged during the recap.

Also, nice work on the recap!
Once you do get the monitor to work, though, you may also actually want to replace that big 450V cap. I see "KM" written on the side of it, suggesting it is made by CapXon. Generally, we don't usually recommend to replace those big high-voltage caps. But CapXon is one of the brands that is quite unreliable. And furthermore, this power board appears to have an APFC circuit, which puts even more stress on the big input cap. So very likely that CapXon will fail down the road, if it hasn't started to already.

[derka]

Re: Dell 2405FPW - Power Supply Issue

Hi momaka, thank you for your interest

I was able to measure around 19.2V on the positive lead of C711, C712, C713, C714, C715 so that line seems to be fine.

Next, I made sure that the positive lead of C702 is well connected to pin 7 of CN701.

I also improved my solder joints further and was now able to measure 7.12V on pin 7 (and on the positive lead of C702) instead of 19V. I followed this line back to the two following resistances:
- R712 gives 7.12V on the side closest to the connector and 5.65V on its other side
- R716 gives 7.12V on the side closest to the connector and 2.5V on its other side

Not sure how to proceed from here since the schematic seems to differ slightly on this section of the board.

[derka]

Re: Dell 2405FPW - Power Supply Issue

Update:
As this video confirms at 1:58:15, https://youtu.be/rQwCqatbLr4
this psu actually differs from the one in the schematic and it seems that 19V on pin 4 and 7V on pin 7 should be correct.

However, after replugging everything together I still get a very brief flash onto a green screen followed by darkness.
Do you think I should start looking at the tdk backlight board next?

[momaka]

Re: Dell 2405FPW - Power Supply Issue

this psu actually differs from the one in the schematic and it seems that 19V on pin 4 and 7V on pin 7 should be correct.

Gotcha.
Yeah, looks like your PSU voltages are correct now, as per the video.
So looks like your PSU is fixed. Of course, do consider replacing the big main CapXon KM cap eventually, if you do get the monitor fixed. The damage on Agent24's PSU in the above video was from a faulty mains cap.

However, after replugging everything together I still get a very brief flash onto a green screen followed by darkness.
Do you think I should start looking at the tdk backlight board next?

Well, the fact that you seem to get a green screen instead of a more proper backlight flash makes the issue a little broad.

I suggest to first make sure the PSU is still running OK. To confirm that, after trying to power on the monitor and you get a green flash on the screen, can you repeat this symptom if you press the power button to turn Off the monitor then back On again? Also, does the power LED seem to indicate the power state of the monitor properly (i.e. On when the monitor is On and Off when you cycled it Off?)

If yes, please post some pictures of your logic/main board. It's possible that there may be more bad caps there or perhaps a bad regulator somewhere.

[derka]

Re: Dell 2405FPW - Power Supply Issue

Gotcha.
Of course, do consider replacing the big main CapXon KM cap eventually, if you do get the monitor fixed. The damage on Agent24's PSU in the above video was from a faulty mains cap.

Yes, I'll definitely won't forget to do that once I get some other signs of life out of the monitor!

Gotcha.
I suggest to first make sure the PSU is still running OK. To confirm that, after trying to power on the monitor and you get a green flash on the screen, can you repeat this symptom if you press the power button to turn Off the monitor then back On again? Also, does the power LED seem to indicate the power state of the monitor properly (i.e. On when the monitor is On and Off when you cycled it Off?)

Yes, the behaviour of this logic is functioning correctly:
- Power button works as expected.
- The brief green screen with backlight flash is repeated at each iteration
- The monitor also has 4 front LEDs which should indicate the input source and they rotate accordingly with their own dedicated button.

As requested, I've attached pictures of the all the other boards. They seem intact at first sight: no bulged caps or other evident signs of damage.

[momaka]

Re: Dell 2405FPW - Power Supply Issue

Sorry I didn't see your reply in this thread earlier and thanks for providing good quality pictures.

With that said, I do see 4 toroid inductors (in black heatshrink tubes) on your mainboard (show in this photo. That means there could be (and likely are) up to 4 buck voltage regulating circuits. The green caps appear to be Lelon (or Ltec?) general purpose caps, which isn't really idea for buck regulators (not to mention the damage a buck regulator can cause if its caps go bad.) So those may be due for a replacement as well.

Of course, I do like to try to find the problem first before replacing parts blindly, if possible. With that said, could you measure the voltages you get on each pin of the inverter board connector, shown on the right here in this picture? In particular, the bottom two pins close to the screw hole appear to be signal lines. One is likely control for the On/Off function, and the other should be the brightness/dimming level. Basically, measure the voltages when the monitor is plugged in but turned Off/in standby and when the monitor is turned On. Then report back what voltages you get. With this, we want to see not only if the inverter board is getting power, but also if the main/logic board is sending a signal to the inverter board to turn it On.

Another thing I would like you to try: send video signal to the monitor and beam a flashlight close to the monitor's screen. When you do this, can you see a faint image of the video signal you are sending to the monitor? If yes, that means we might be looking at an inverter issue. If no, then the problem could be somewhere else still.

[dick_barton]

Re: Dell 2405FPW - Power Supply Issue

In your first photo the mosfet looks damaged or is it just the photo.