Dell lcd E173Fpf no power

[Davi.p]

Hi, much time ago i've already written about this but no solution, it is many years that i own this with the problem and it is driving me crazy, i don't want to give up even if i'm very near to do it, for now i have shot the incredible amount of 5 expensive power mosfet and 2 pwm ic's.
The problem is that i've found it with mosfet shorted, 1 smd resistor from gat pin of ic to gate pin of mosfet exploded, with the help of this forum and with the remaining of the writing on it i've found the value in 5,1 ohm, 1 smd ceramic cap damaged due to explosion of near resistor, maybe 1 pwm ic (i'm unable to test it), 1 2W resistor with value of 0,39 ohm from the source of mosfet to hot ground, 1 2w resistor of of low value about 0,22 ohm if i remember, in the path from + leg of main cap to 1 pin of the transformer, later i've destroyed 2 or 3 mosfet after replacement of optocoupler, mosfet, 0,39ohm 2w resistor, pwm ic and smd resistor without noticing that the duble big diode on the secondary was going short apparently only under the full load, in fact if i power only the combo board (psu+inverter+main/video bd) with a light load like a pc 12v fan (it has 1 only output at 12v) the board went ok, but when i've connected the panel it goes short on the primary!
the ceramic cap that was damaged, with the help of a newsgroup i have substituted with a 100nf thruhole plastic type since them have told me is a noise reductor maybe, is in parallel with the electrolitic cap that is on the vcc line of the pwm ic. I don't have found schematic for this monitor. I've always replaced AP2761I (650v 10A 1 ohm Rds threshold 4v vgs+-30v ton 16ns) with STP10NK60Z (N channel 600V 10A 0,7ohm Rds Vgs threshold 4,5v Vgs +-30v Ton 20ns).

[Davi.p]

Re: Dell lcd E173Fpf no power

Hi, i've read now my post and is quite full of errors, so sorry for my very poor english..
BUT.. is there any pious soul that can help me in this difficult challange? I will appreciate much 'cause i'm always very hard to trash away things like this..

[Rtech]

Re: Dell lcd E173Fpf no power

After trying to repair quite a few of these, where the Power supply Moseft, cap, resistors etc had blown up...I gave up !!.If the Power supply PWM IC has failed, which it does often, that that is easily replaced.

[Davi.p]

Re: Dell lcd E173Fpf no power

Quote:".If the Power supply PWM IC has failed, which it does often, that that is easily replaced."

What do you want to say with this? Sorry..
To understand well, from your posts i see you are an expert but how much do you know about the switching psu's, at least of flyback types like this? You are able to do advanced tests? So i can regulate myself that i'm not so expert if i have to give up too. Bye and thanks.
Now i add one or two details that i've forgot, the story is long about this, maybe i've forgot another... the first 4 times i have made attempts i've substituted the 0,39 ohm resistor with a 0,33 'cause shops had only that, even a big distributor.. this is been probably a bad error 'cause it regulates the threshold of the intervention of protecion to overcurrent in the mosfet, if the resistor is smaller it reaches later the level, but the last time i've put 3 1/4w 1,2ohm in parallel 'cause the components order was already made, this give me R/n=1,2/3=0,4 so i don't know if it's here the problem. The smd resistor that come in Sense pin in mine is a 51ohm, i remember that first times it was good, i'm sure, but maybe the next times i've changed it since 1 time the pwm ic is exploded damaging it maybe, i think it was the same value but in all other threads i've seen is 47 ohm.. the 2nd or 3rd time i've changed also the do41 type FR10-10 fast diode on the +320v rail with a UF4007 (UF=ultra fast) even if it signs no fault at the dmm's diode tester.

[Rtech]

Re: Dell lcd E173Fpf no power

I was just giving you my experience with this Board,I DID NOT ask you to stop trying,suggest you read things a number of times,before making uncalled for comments.

[budm]

Re: Dell lcd E173Fpf no power

Can we see the pictures of your board? Does the board has BenQ name on it?

[Davi.p]

Re: Dell lcd E173Fpf no power

Rtech: i think you have understood my text as polemical, nothing of all this.. only i don't know your complete skill about theese types of circuits, what is the bad in this? Yuo can be an amateur, a pro general purpose repairer, a high skilled pro repairer..
Budm: sorry i have no good camera, this is the exact type of board:
https://www.badcaps.net/forum/attach...8&d=1336598427

[lexwalker]

Re: Dell lcd E173Fpf no power

Fixed many of those Dells with catastrophic failures before (ie. MOSFETs, diodes, resistors, capacitors, etc). Always trace the "path of destruction" and replace those components in the "path of destruction". Especially have to beware of SMD components which fails easily whenever such catastrophic failures occur. Very often the diode and resistors failed, sometimes the capacitor may measure fine but breakdown already at above certain voltage. Hint, in this type of failures recommend replacing the primary side main filter/smoothing capacitor! Yes, that big one (rated at 400V to 450V).

If you're having trouble finding 0.39 Ohm resistors, simply get 2 pieces of either 0.75 Ohms or 0.78 Ohms and use them parallel (should equate roughly 0.375 Ohms to 0.39 Ohms). Get resistors with at least 5% tolerance (lower the better). If the original resistor was 2W, then can use 2x 1W in parallel. If can't find same wattage, then get higher wattage ones. Basic electronics 101...

Anyway, here's the monitor service manual (attachment)...

[budm]

Re: Dell lcd E173Fpf no power

Sorry to say, the service manual is not for this E173FPF which uses Power supply/Inverter/main board combo. The service you posted has separated main board, see picture of the board on the link as posted in post 7.

[lexwalker]

Re: Dell lcd E173Fpf no power

Sorry to say, the service manual is not for this E173FPF which uses Power supply/Inverter/main board combo. The service you posted has separated main board, see picture of the board on the link as posted in post 7.

Whoops! Missed the "..f" at the end. Yeah, wrong service manual. For "...fpf" only have the 1707fpf for 17-inch monitors. From the description of components above, could be using similar power supply circuit as E156Fpf which used a combo board also (see attachment)...

[Davi.p]

Re: Dell lcd E173Fpf no power

Thanks Lexw. for the schematics (i don't know how much will come useful now but.. anyway seems very similar) and for hints.. i don't know why to change big 400v cap. 'cause 320v was there.. as i've told i have used 3x 1/4w 1,2ohms that gives .4 ohms in par. I will test all "never changed" components with real voltage if i can..

[lexwalker]

Re: Dell lcd E173Fpf no power

Thanks Lexw. for the schematics (i don't know how much will come useful now but.. anyway seems very similar) and for hints.. i don't know why to change big 400v cap. 'cause 320v was there..

From my experience, most of the time when catastrophic failure occurs (something like 8 out of 10), the main filter/smoothing capacitor is always damaged/faulty. See attachment. That's one of the monitors I've repaired. The ones marked are all faulty, which includes the main filter capacitor. And yes, that PWM I.C blew quite spectacularly (can see the hole exposing the innards)..

as i've told i have used 3x 1/4w 1,2ohms that gives .4 ohms in par. I will test all "never changed" components with real voltage if i can..

1/4W resistors?! Also what was the tolerance on them? Stacking more than 2 resistors will increase the tolerance by quite a bit (which is a bad thing thus may not even measure 0.4 Ohms, perhaps more).

For example (always take worst case scenarios as safety precaution): If all 3 resistors are +5% off tolerance (each 1.26 Ohms), then in parallel it comes up to 0.42 Ohms which is +7.7% more than 0.39 Ohms! Those are way off the usual 5%. Imagine if those resistors are of 10% tolerance (will come up to +12.8% more for worst case scenario)...

The best is to use 2x 0.78 Ohms, of at least 1W each and at least 5% tolerance. Worst case would be 0.4095 Ohm which is exactly +5%, still within the tolerance range. Example, look for this part "RS01AR7800HE12" from Vishay...

Should use at least 2x 1W. Always use higher wattage resistors for Rsense (that low Ohmic resistor) because it can get very hot. If can get wirewound type then that would be better than carbon film types.

[wajid89]

Dell lcd E173Fpf no power

Please help me out, i have lost the Q850 transistor of my lcd can anyone tell me the name of Q850 transistor of dell lcd e173Fpf Thanks.

[Rtech]

Re: Dell lcd E173Fpf no power

Sss7n60b

[Davi.p]

Re: Dell lcd E173Fpf no power

I'm so sorry for replying so late, i had also health problems theese months...
Lexwalker thankyou newly, you're right to hit me the hammer on my head
those 1,2 ohms was right 10% tolerance, i'm a very poor repairer i know.. so now that i've also recovered from a job client another equal monitor with near the same problems (he said it had been damaged by a lightning) i have some info more to work on. Bye and see you later..

P.S.: I don't know why but here in Italy even from very prof. components resellers is very hard to find big 400v 'litic caps for equipement like this.. this is a problem now..

[Davi.p]

Re: Dell lcd E173Fpf no power

wajid, part is AP2761I (mosfet 650v 10A 1 Rds threshold 4v vgs+-30v ton 16ns)

[lexwalker]

Re: Dell lcd E173Fpf no power

I'm so sorry for replying so late, i had also health problems theese months...
Lexwalker thankyou newly, you're right to hit me the hammer on my head
those 1,2 ohms was right 10% tolerance, i'm a very poor repairer i know.. so now that i've also recovered from a job client another equal monitor with near the same problems (he said it had been damaged by a lightning) i have some info more to work on. Bye and see you later..

P.S.: I don't know why but here in Italy even from very prof. components resellers is very hard to find big 400v 'litic caps for equipement like this.. this is a problem now..

You can salvage these very low resistance high wattage current sense resistors, as well as 400V and 450V capacitors from switching mode power adapters such as those used for laptops/notebooks, netbooks, nettops, LED/LCD monitors, mobile phone chargers, low power mini-ITX machines, etc. Many of them use similar topology as found in LCD monitor power supplies. However its not easy finding/salvaging 400V/450V capacitors of 82uF, 100uF, 120uF and 150uF capacities with 18mm diameter (which are typical main filter/smoothing capacitors used in LCD monitors). I would suggest using online stores such as RS Components, Farnell/Newark/element14, Mouser, Digikey, etc...

[Davi.p]

Re: Dell lcd E173Fpf no power

thankyou too lex, unfortunately here in my town it's not possible to grab things in the ecologic platform, for the rest i will check them.. i think i will do the next try whenever i will find a free main switch mosfet 'cause i have burnt too much of them so it will pass some time, bye..