Dell E248WFP power cycling off/on

[dumpystig]

This has got me puzzled with the following problem -

The set runs fine for 30-40 mins and then the display turns off and the green Power LED starts flashing. The self test screen then tries to start but almost immediately turns off. It then tries to start again but again almost immediately turns off. And so it goes on, in a continuous start/stop loop.
This happens on both inputs with different interconnect leads.
If it's left unplugged for a few hours once plugged in and switched on it will run again for about 30-40 mins before returning to the same off/on cycling.

Any ideas before I delve inside?

[jetadm123]

Re: Dell E248WFP power cycling off/on

Best bet would be to take it apart and post some good focused photos of the power and signal boards.

[dumpystig]

Re: Dell E248WFP power cycling off/on

The fault is on the main Power board - with the other boards disconnected it still faults.
Voltage across main cap (C605) is 330VDC STBY/402V RUN.
Primary DC output to other boards is 18.9V STBY/RUN.
Once the fault kicks in both the above voltages fluctuate on STBY and RUN.

Power board is marked 4H.0DG02.A01

[PlainBill]

Re: Dell E248WFP power cycling off/on

The fault is on the main Power board - with the other boards disconnected it still faults.
Voltage across main cap (C605) is 330VDC STBY/402V RUN.
Primary DC output to other boards is 18.9V STBY/RUN.
Once the fault kicks in both the above voltages fluctuate on STBY and RUN.

Power board is marked 4H.0DG02.A01

This is a slightly unusual power supply. It has a single output; switching to Run mode apparently only turns on the PFC front end. What is interesting is that when the fault occurs both PFC 'boost' and output voltage fluctuate.

There are a number of possible ways to troubleshoot this. I'd suggest disabling the STBY/RUN input (as shown in the bottom picture, second pin from the right on CN701). The supply should still work at full load.

Another option would be a complete recap, certainly this has the earmarks of a defective cap destabilizing the circuitry. Before that, I would suggest checking carefully for bad solder joints.

PlainBill

[dumpystig]

Re: Dell E248WFP power cycling off/on

Cheers PB.
The 19V output goes to a 'secondary power board' that then converts to 3.3/12 etc. which then supplies the SSB.

Electrolytic caps just this minute ordered, will be picking them up this afternoon. All heavier joints reflowed already, didn't make any difference.
Let's see what happens after the re-cap...

[dumpystig]

Re: Dell E248WFP power cycling off/on

Replaced the first electrolytic [C622 - 1uF/100V] and waiting to see if it will run longer than the previous 30-40 mins before faulting. I'm going to do the replacements on a one-by-one basis, hopefully this will help pinpoint the main cause of this particular problem and be an indicator for future reference.

[dumpystig]

Re: Dell E248WFP power cycling off/on

Forgot to mention that after each replacement I'm connecting all 3 boards but not the screen - it's a bastard of a job to set up the removed boards for measurements and also have the screen connected too. I'm simply using the Power LED as my fault indicator. And as I type this - BUGGER! - LED started flashing. Just on the 40 mins mark.
So on to the next cap [C623 - 2.2uF/50V].

[retiredcaps]

Re: Dell E248WFP power cycling off/on

I'm going to do the replacements on a one-by-one basis,

That would be the way I would do it just so I know what is root cause.

And thank you for attaching the pictures. My little netbook went crazy loading inline pictures (CPU at 70%) resizing the Gateway thread even though I told the OP in two different places not to post inline and offsite.

[dumpystig]

Re: Dell E248WFP power cycling off/on

No prob RC.

Right, this is the state of play with replacements and the time it took to fault:

C622 - 1uF/100V - 17 mins.
C606 - 47uF/50V - 14 mins.
C611 - 47uF/50V - 5 mins.
C605 - 150uF/450V 32 mins. [nearest substitute for original 120uF]

Three more elects to do - C711/2/3 - all 470uF/35v - but they are across the 19v output so I can't see these making any difference.

[dumpystig]

Re: Dell E248WFP power cycling off/on

None of the above replacements made any difference but at least I now have more spares...

Onwards with the testing

[dumpystig]

Re: Dell E248WFP power cycling off/on

It's been nigh impossible testing this board due to it's design and the fault and it's really had me stumped. I've spent the last few hours literally reflowing every tiny little SMD/pad with my 25W soldering iron. I'm knackered - and my eyes are shot - but so far the board has been running with the Power LED permanently ON for 5 hours now. Reinstalled the boards into the back of the panel and connected the screen. Been playing a movie for another hour and so far it's all looking good - fingers x'd.

[PlainBill]

Re: Dell E248WFP power cycling off/on

Glad to hear you have it working. Fixes where you cannot identify a specific component are always stressful - did you fix it, or is it waiting to fail again.

PlainBill

[dumpystig]

Re: Dell E248WFP power cycling off/on

Been running for about 7 hours total now. Switched it off for an hour and then started fine when switched back on. I'm quite confident that the reflow has sorted the initial fault, a dry joint on one of the SMD's, although I can't tell which specific device. But I can say with certainty that it was within the area marked by the yellow box on the pic below. I had already reflowed all the heavier pads/terminals/jumpers on the board so I divided the board into smaller areas and reflowed every SMD in that particular area and then hooked everything back up to test - it wasn't until I did this area (the last!) that I had success.

So I'll just see how it goes from here, hopefully it's a permanent fix. If it's still running at the end of the week without any further probs then I will mark this monitor as definitely fixed.

[dumpystig]

Re: Dell E248WFP power cycling off/on

Glad to hear you have it working. Fixes where you cannot identify a specific component are always stressful - did you fix it, or is it waiting to fail again.

PlainBill

Cheers PB.
I'm hoping it's not the latter - and stressful is a good way of describing the fault finding(?) process.

I'll let you know in a few days if it holds up. But I'm still confident...

[dumpystig]

Re: Dell E248WFP power cycling off/on

Does anyone know what devices D701 and D702 are? They're outputting just under 19VDC. The only markings - E220B - give no results on a search.

[trebo]

Re: Dell E248WFP power cycling off/on

the D tells you its a diode
no help on the e220b sorry

[dumpystig]

Re: Dell E248WFP power cycling off/on

Thanks. I know it's a diode, but I want to know the type/spec etc.. Can't find anything at all, searches only lead me to a few caps.

Monitor is still running great, been on solid all day and no hiccups, no power-offs or shutdowns, no flashing Power LED....

[dumpystig]

Re: Dell E248WFP power cycling off/on

Well, this monitor has now had almost 50 hours runtime without a single glitch or hiccup, no faults whatsoever. I'm now confidently adding it to my 'definitely fixed' list...

What do you think PlainBill?

[PCBONEZ]

Re: Dell E248WFP power cycling off/on

I think it's a TVS Diode.
Should be more numbers though.

[dumpystig]

Re: Dell E248WFP power cycling off/on

Cheers PC, but I will have to google that one...

And they are definitely the only markings on the devices.

Edit: I may be wrong but I doubt that's what they are TC. The 2 devices have commoned outputs that supply just under 19VDC to a secondary power board where it's then converted to the different voltages for the logic board. Thing is, I can't remember the input voltage to them both.