Dell E172fpb

[UMD_EE]

Re: Dell E172fpb

Regarding my previous post... The attached schematic (from model DV172) does not completely reflect the E172fpb's circuitry. In comparison the F172fpb is similar to that of the DV model schematic referenced; however the short circuit protection, as well as the feedback loop, are VERY different (much more complex). I believe this is where the design flaw is at. When designing any switch mode power supply feedback and short circuit protection (SCP) are critical. If any components in this region are not optimized, changes due to temp, fault... may cause failures (i.e. shorted components).

Switch mode power supplies are nothing like linear power supplies... they are very efficient, but dumb! They need constant feedback in order to maintain proper voltages on the secondary side. If a lamp shorts out, the primary needs to adjust for that, otherwise poof transistors blow... I am in the process of creating a PSPICE circuit identical to that in the dell monitor.

However, I may just alter my circuit board to reflect the posted circuit (DV model) eliminating such a complex SCP and feedback circuit. As far as I can tell, there haven't been too many complaints on that model. I'll keep you posted.

[jmain]

Re: Dell E172fpb

UMD_EE, Thanks for the schematics and the info. Although the sch is not for this model, but it's close enough.
I myself am Electrical engineer, but have been dealing with digital electronics for last 15 years. Don't even remeber much about the analog side.

dockarl, there is absolutely no way for a cold solder joint to cause the short. No matter how complex matrix of diodes or anything you have, COLD JOINT WILL NOT CASUE SHORT. Cold joint is just a no connect (usually at above room temp), it can cause open but not the short.

The reason you found two bad transistors while testing in-circuit and found that one of them is good when you removed it from the circuit is because the transistors in question are in pair. There is a very good reason why you should remove the components from the circuit before testing them. These trannies can be tested for short circuit pair while remain in-circuit, but you won't be able to test for single short transistor and/or open and/or for proper diode drops in reverse polarity.

[Gwentiv]

Re: Dell E172fpb

Hi !
After months fighting with that stupid dell monitor i still have the same pb: it works fine 5 mins and swithces off !
I ve noticed that both Q759 and Q760 get very hot: much hotter than their counterparts. Someone knows what the reason could be ? and what to do ?

[dilahk0]

Re: Dell E172fpb

Is the FU9024N same the IRFU9024N?
thanks

[dilahk0]

Re: Dell E172fpb

is the FU9024N same the IRFU9024N?
thanks

[UMD_EE]

Re: Dell E172fpb

Yes, FU9024N is the same as IRFU9024N... The "IR" merely indicates the company name (International Rectifier). I have found that Digikey (www.digikey.com) carries this component... there you will find IRFU9024NPbF (the PbF indicates that this is the Lead free version).

[UMD_EE]

Re: Dell E172fpb

If Q759 and Q760 are getting very Hot; chances are that one of the switching transformer coils are shorted (T751 or T753). Also check (C754 C755) which lies across the collectors of Q759 and Q760. These caps dampen the voltage and current across the tranny's (V*I=P (Watts=heat)). I am under the assumption that Q759 and Q760 are not failing (have you removed, and metered them individually?).

Resistance from Emitter to Collector (Ohm'd out in both directions) should be infinite. Also, to test the switching transformers you will need to perform a "Ringer" test using a special tester (which a TV repair shop or University would have). Be sure to check the primary and ALL (3) of the secondary terminal pairs.

See the attached schematic which I made based on my inverter board. Keep note that this schematic is NOT complete. There are many other subsidiary circuits indicated by ">>". It does however show the critical aspects required for proper operation. Best of Luck!!!

[NICKW]

Re: Dell E172fpb

Having been given one of these monitors by a fiend (sic) I first had a look on the net and found this wonderful site full of info on it. Having read everybody`s
input, I pulled mine apart and found Q759 and Q760 US, Also board a bit cooked
in that area, examined solder joints under a microscope and found circular fractures in solder joints round component leads, not dry joints but fractures in
in the surrounding solder, I found about 20 joints like this all in the cooked area.
none anywhere else on the board. Resoldered these joints, stuffed in 2 tip41c`s.
On powering up, monitor worked, but tip41`s ran hot, did a temp check on them, and they soon got to 75c. Remounted tips via flying leads on the inside of
the casing which covers and supports the main board. Powered up, monitor working ok, tip casing stabilised at 30c due to heat sink of casing. Left monitor
on for 10 hrs, working fine, still working.
Conclusion, madness to replace transistors with expensive replacements not
mounted on heatsink, I think they`ll go sooner or later as cause of the fault is elsewhere on the board, and not the transistors themselves, they are simply
being overdriven and cooking. Note to newbie electronic people who copy my
mod, the TIP41c`s or similar must be mounted via mica insulating washers and
plastic washer in the tip body hole to electrically isolate the metal body of the
transistor from the metal casing being used as a heat sink. My thanks to the
time you have all put into this which has saved me so much, also to "mousie"
and "dockarl" whoe`s work I have used, Hope this helps someone out there!
Will post if and when it blows up!

[jdaproduca]

Re: Dell E172fpb

I just picked one of the monitors up broken, has same symptoms tries to power on then over current in the smps kicks in. Turns out to be one of the transistors (2sc5707) which drives the high voltage transformer for one of the lamps was shorted. I also noticed some small cracks in the solder around the switching MOSFET FU9024N. This may have caused the problem that once the monitor was on for a while and the solder around the MOSFET began to come loose and make a poor connection killing the switching transistor. Will let you know what happens as soon as I get my replacement transistor, and run the monitor for a long period of time.

[printone2]

Re: Dell E172fpb

I have my E172fpb apart to check the transistors. One pair is shorted, so I went to Radio Shack to see about parts. Their part # TIP3055 seems to be what I wanted. The label on the transistor is actually MJE3055, like what Dockarl has used. They are priced at $1.79 each. I think that I will replace all four instead of just the pair that are shorted.

I also have an E171fpb with issues. It takes 5 minutes to turn on and then works fine, until it goes into sleep mode. If in sleep mode or if turned off, it will need 5 minutes to warm up. After it warms up it will turn off and turn on just fine, but have it off for 10 minutes or more and it needs some warm up time. Before it warms up, unlike the E172fpb which often has the green power light on, it does not have any lights showing. It does seem to flash like the E172fpb, but the power light does not shine until it warms up. Any clues on what causes this, or what are the first things to try?

[printone2]

Re: Dell E172fpb

I finally took the E171fpb apart and two very buldging caps are located at positions C703 and C707. They are Elite brand. This board is very different from the E172fpb. It does not have any of the problem transistors and is much easier to work on. This may be why very few of them have failed.

[9Shenandoah]

Re: Dell E172fpb

Is this monitor really fixable? I am a pretty good DIYer. Howver after reading all this stuff from people much morecapable than me, i wonder if my minitor, which as ll the symptoms as those that you ahve described, is even worth spending all that time and money on. Should I just go get another one?

[9Shenandoah]

Re: Dell E172fpb

1. Where is the Link to the alleged Repair Kit on Ebay or whatever?

2. How do you get the case gracefully open? Theer are just no screws which means you have to pry gracefully. I see the round symbol with the Unlocked Lock next to it with an open slot inside it. Does this play any role to some trick in opening the case?

3. Is this monitor fixable? I see some posts which say "bad solder", others which say "bad transistors", but no common theme.

4. When mine started to fail, I noticed by flexing the cord on the power supply connection at the back brought it back to Life. Once I stuffed a plastic shim back there which saved it for about two weeks. Then it finally died and now it has the infamous flashing green light problem. This leads me to believe that it indeed was solder problem

[printone2]

Re: Dell E172fpb

I sure would like some help on retrofitting the transistors on the shield as a heat sink or just mounting some heat sinks from another board. The ideas of flying leads to me would mean some solid core copper wire like maybe some 24AWG unshielded twisted pair (utp/regular ethernet premises cable, not a patch cable). Can I just solder that to the holes where the trannies were and bring them to an open area inside the protective metal case and solder them to the leads of the trannies. Then screw the trannies to standard heat sinks like the ones located elsewhere on the board. Like at D701 and D702? If I did this what issues do I need to overcome to isolate the heat sinks if any?

It would be great to see some pictures of the mods that Mousie and NickW have made. I am just not quite sure how to finish this up.

[printone2]

Re: Dell E172fpb

I put my E172fpb back together with the transistors off the board. Now when I turn on the power it gives me a flashing power light and it makes a pulsing squeak. It sounded like a bird to me. But sure enough when I turn the power off the bird stops squeaking. When I look at the transistor reading from my VOM, they are very different once the transistors on soldered on from how the transistors read out of the circuit. I do not know if this is normal or not.

My soldering is not that good, but I suspect some other issues are going on that I just cannot detect yet. Are the transformers shot? I do not have any way to test them.

Can anyone tell me about the squeaking sounds? What monster have I created?

My other LCD the E171fpb has been running just fine for over a day now. It just had some bad caps. I wish I had two of them instead of one of these E171fpb.