Dell E172fpb

[LLLlllou]

Re: Dell E172fpb

Let The Truth Be Known!! My Wife Took My Screen To A Repair Shop Whilst I Was Away On Business!! That Explains It All!!!!!!! It Didn't Make Sense Why Some Of The Components Are Different But The Circuits Are The Same! Now I Need To Find A Pair Of Fu9024n's Somewhere Online!! Q805 And Q812 Are Both Fed After The Diodes D801 And D803.

Damn women!

[LLLlllou]

Re: Dell E172fpb

I must add - the transistors in question get BLOODY HOT - hotter than I would have thought would be usual, even given the large number of watts they are designed to handle. I've thought about adding a small fan etc - but that really begs the question - WHY ON EARTH should you need to go to these lengths to keep it cool? I reckon Benq has simply done a pretty shoddy job when it comes to design and reliability testing on this model, and I hope they've ironed the problems out now for their newer models.

Doc

The reason why the transistors run so hot is because the MJE3055 is not even close to an acceptable replacement for the 2SC5707 in this application.

Although the MJE3055 meets or exceeds Voltage, collector current, and power dissipation ratings, it is sorely lacking in gain bandwidth product, and in gain (hFE).

The 2SC5707 has a gain of 200 to 560, while the the MJE3055 only has a gain of 20 to 100.

The 2SC5707 has a gain bandwidth product of 330Mhz, while the MJE3055 is only 2MHz.

In short, this means the MJE3055 would need more current driven into the base to turn on fully, and would take much longer to turn on even if it was getting enough current. Either would mean the transistor would spend much more time in a higher resistance range somewhere between on and off, not where you want to be if you're the switch in a SMPS. Unlike a "close to zero ohm" state, which would cause very little power dissipation, they're spending WAY too much time "getting on" and "getting off" and cooking themselves in the process.

The 2SC5707 is a VERY specialized beast, not many transistors can match it for gain at high current, and when you add a 330MHz GBP, it pretty much stands alone.

[GLORK]

Re: Dell E172fpb

Final summation: FIXED

i too had this DELL E172fpb with BenQ board inside.

triple THANKS to all who contributed to the 24 pages of input, i read most of it, i think that took longer than the actual repair. Surprisingly not a "bad-caps" issue ! (they all looked and tested fine)

i found the usual suspects 2SC5707 transistors near the output (backlight - driving) transformers, replaced the pair, and reflow'd all the solder in these 2 sections. Its been on for an hour and YES... (cue music) I FEEL GOOD, like i know that i should !

GLORK

[retiredcaps]

Re: Dell E172fpb

When I visit my used electronics parts store I see numerous 2003-2005 circa Flat Screens in the "as is" $10 pile, I know now to pass on them,

It depends. Monitors can die from a number of things including bad caps, dodgy soldering, overheated transistors, bad fuse, etc. If the monitor dies "early", then the ccfls could be in very good condition.

I don't know if you can buy raw CCFL tubes - installing used ones from an equally old monitor is a crap shoot.

ccflwarehouse.com, ccfldirect.com, and ebay have sell bare ccfl tubes.

[PlainBill]

Re: Dell E172fpb

Here's a guy who replaced his CCFL backlights with LED's. Don't know what language he's speaking...

http://www.instructables.com/id/LCD-...ED-Back-Light/

Indonesian.

This is one modification that backlightnya LCD monitor died and I replaced it with the LED on the bottom of the LCD as many as 12 pieces of LED and relay install + Bc109 as LED driver.

Google is your friend.

PlainBill

[eric6023]

Re: Dell E172fpb

Here's a guy who replaced his CCFL backlights with LED's. Don't know what language he's speaking...

http://www.instructables.com/id/LCD-...ED-Back-Light/

[eric6023]

Re: Dell E172fpb

I have the same symptoms with the E173FPb, did the transistor swapouts, and it worked a little while, but then it went to the white flashing screen with the green blinking power light, however I noticed it did this when the monitor was cold. I remember when growing up in Idaho, when it was 30 degrees, the shop lights in the garage (which were flourescent) struggled to light up when it was that cold. I think that's what's going on with this monitor. It doesn't ionize and light up and that throws the inverter board into a tailspin.

I tore down the LCD and found the top CCFL Tube had a very black end, reminiscent of a worn out flourescent lighting tube.

I had an extra LCD from a Mag 17" that had the same symptoms, and it had the same Optronics LCD and CCFL leads. One of those CCFL tubes was cracked at the end and useless, however the other was "ok", I installed this one and so far so good. However in the end, I believe CCFL's are wearing out because these monitors are very old. When I visit my used electronics parts store I see numerous 2003-2005 circa Flat Screens in the "as is" $10 pile, I know now to pass on them, I don't know if you can buy raw CCFL tubes - installing used ones from an equally old monitor is a crap shoot. The whole reason they're there in the junk pile is probably because they've led a long hard life. Time to upgrade to a LED Flat Screen.

[PlainBill]

Re: Dell E172fpb

So I found the service guide for the QT74 which suggests Q805 is an FU9024N and Q804 is a 2N3906.

Q805 on my QT73 is a D10PF which I believe is similar to the FU9024N so maybe I can use a 2N3906?

I would think so, or any SMD equivalent part. Voltage and current handling capabilities aren't particularly demanding here.

PlainBill

[seanc]

Re: Dell E172fpb

So I found the service guide for the QT74 which suggests Q805 is an FU9024N and Q804 is a 2N3906.

Q805 on my QT73 is a D10PF which I believe is similar to the FU9024N so maybe I can use a 2N3906?

[seanc]

Re: Dell E172fpb

Have a BenQ QT73 which is very similar to these.

Q804 failed when Q805 shorted.
Q804 seems to be the same as Q802 as far as I can make out the markings and reads
"s4 907"

It seems to be a sot-23 package.
RS has a few hits for "907" such as this http://uk.rs-online.com/web/p/genera...istor/1126226/

Has anyone identified what Q804/802 are and if what I've found might be a suitable replacement?

Edit: Added a photo as a stopgap....
iPhone is the only thing to hand right now.

[PlainBill]

Re: Dell E172fpb

I have a Dell 173fpb and a Dell 153fpb that both have the same problem, I have replaced the 6 transistors and the fuse on both, and now when I power them up, the secondary pin on one of the inverter transformers short to the ground pin, on both monitors. I have tried disconnecting the leads to the lamps of the display and they still smoke. Is this indicative of shorted transformers? Here is a picture to help explain the issue, but it isn't the best quality...

What do you mean ' the secondary pin on one of the inverter transformers short to the ground pin'? Are you saying an arc forms between the secondary pin and the ground pin?

You DO realize that the inverter is a constant current AC source. If the CCFL is disconnected (or a wire to the CCFL is broken, or the CCFL is broken), the inverter will ramp the voltage up for a few seconds until the OVP (Over Voltage Protection) shuts the inverter down. Once an arc forms, the epoxy resin in the PC board breaks down and is carbonized. The carbonized material is conductive, and you have a self-repairing flaming resistor formed. It's not a great accomplishment, it's been done by many people, some of them brighter than you.

Considering the demand for 15" LCDs, it's barely worth repairing them. HOWEVER, if you want to give it a try, scrape away ALL of the charred material between the two pins. If you have a Dremel rotary tool (or equivalent), I would cut a slot in the PC board between the two pins. Do this CAREFULLY. On slip and you may destroy the transformer.

PlainBill

[s_carter]

Re: Dell E172fpb

I have a Dell 173fpb and a Dell 153fpb that both have the same problem, I have replaced the 6 transistors and the fuse on both, and now when I power them up, the secondary pin on one of the inverter transformers short to the ground pin, on both monitors. I have tried disconnecting the leads to the lamps of the display and they still smoke. Is this indicative of shorted transformers? Here is a picture to help explain the issue, but it isn't the best quality...

[retiredcaps]

Re: Dell E172fpb

1. I leave them on the board, or unsolder?

2. test with a meter on diode mode

3. none of the legs should read full contact (0000) or open (1) on meter

See post #13. There is a multimeter and a transistor section.

https://www.badcaps.net/forum/showpo...0&postcount=13

[lessthanchris1]

Re: Dell E172fpb

Having a little trouble understanding the transistor testing. Would anyone care to help?

1. I leave them on the board, or unsolder?

2. test with a meter on diode mode

3. none of the legs should read full contact (0000) or open (1) on meter

Correct?

[PlainBill]

Re: Dell E172fpb

I have two different model BenQ monitors. Both have blown 3Amp fuses and both were full of cold solder joints so the manufacturer was having much more that one bad day. Both are now working again. What I want to know is why there is an inverter circuit in the monitors in the first place. These monitors are designed to plug into the mains. They have a ready supply of AC voltage just waiting to be transformed into whatever voltage the backlights require. Why rectify the AC to DC and then invert the DC back to AC? Maybe there is a reason but I can't think what it would be. Any suggestions?

Yeah, the engineers who build LCDs are brighter than you.

The inverter does much more than boost the voltage. It is a constant current AC source with current limiting and over voltage protection.

The efficiency of a transformer depends on it's frequency. A transformer to step 120 VAC, 60 Hz up to 1200 Volts would weigh several pounds. Also, the AC input voltage can vary from as low as 90 volts to about 250 volts. That complicates the design significantly.

PlainBill

[JFA]

Re: Dell E172fpb

I have two different model BenQ monitors. Both have blown 3Amp fuses and both were full of cold solder joints so the manufacturer was having much more that one bad day. Both are now working again. What I want to know is why there is an inverter circuit in the monitors in the first place. These monitors are designed to plug into the mains. They have a ready supply of AC voltage just waiting to be transformed into whatever voltage the backlights require. Why rectify the AC to DC and then invert the DC back to AC? Maybe there is a reason but I can't think what it would be. Any suggestions?

[lessthanchris1]

Re: Dell E172fpb

Ok, got the new caps in, and tested the picofuses. Monitor still is dark, but with flashlight I can still see the images.

I'll read up and test for shorted transistors next. ugh!

[smason]

Re: Dell E172fpb

(up to the point of reattaching the rear cover) and test. Joy at last! My hopes are that BenQ was having a bad week of soldering and not the heat stress. Only time will tell.

Hope this helps

Congrats on the successful fix. Fun isn't it?

If you've read the many threads, the bad solder joints are the the root of many of the problems with this model.
My attack plan for this model and variants is, recap, re-solder the transformers and any questionable joints test the 5707 transistors the FETs and pico fuse.
If I find a shorted transistor, I replace 'em in pairs, then power up and test.
I don't shotgun parts in ('cept caps, I ALWAYS recap) just replace failed parts, and their partners.

There's a good technical description in one of the threads somewhere about how the bad joints cause the 5707s to fail, which sometimes blows the pico fuse.

[Digital Technophile]

Re: Dell E172fpb

Hi all,

I've a Dell E173FPc (dunno why all of the diffrnt variants), with the dreaded "power on" for 2 sec. then screen goes blank, but the power button stays lit at a steady green. Scouring the 'net for possible solutions, I found many. I did buy the four trannies and replaced them (2C5707 originals). All kinds of excited, I put this monitor back together soooo fast there was, I say, there was smoke a comin' off my fingers! Thinking to myself, "Wow, that was so easy can't wait to see this baby fire up!" Then, just as I was preparing to snap the back cover onto the front bezel, a voice from above echoed down (only I could hear), "Whoa fella, don't you think you ought to plug this bad chicken back in and test it before you go through the trouble of the plastic clip BS?". To which I replied (to myself), "Maybe I ought to plug in....". Well don't you know...same shiite as before. Hmmm, what the blue blazes could the problem be?

Sitting there bewildered, no joy, I went back to the 'net. Start searching. Finally found this forum (Interesting star of a discussion forum, the humble capacitor), and finally this thread. I read through the first few posts, all very informative. "Bad Caps!", they shouted. "No try the four transistors prone to failure!". Still yet, "What about the transformer, mine's was busted!". More over "Heat! Heat is the real culprit!". That got me thinking.

So I break out my jeweler's loupe, shift down into inspection gear, and really start scrutinizing the area between the two backlight transformers where the four transistors live. First observation, the board does have some discoloration in the immediate area. Flip board over (because I know full well where the problem is), and begin to look at EACH and every solder joint in the vicinity. Second observation, a few solder joints at both backlight transformers seem to have the telltale cold solder joint, looking closely you can see a minute "ring" around the posts. Could this be the result of extreme heat cycling? I'll leave that for discussion, although my theory at this point is a combination of bad soldering and heat cycling (which isn't a flame on two wheels).

What worked for me was to reflow the solder at any point that looked suspect (especially the pins for the transformers, two on the one side, and about 6 or 7 on the opposite side). After I was satisfied that everything was kosher, I began to reassemble the monitor (up to the point of reattaching the rear cover) and test. Joy at last! My hopes are that BenQ was having a bad week of soldering and not the heat stress. Only time will tell.

Hope this helps

[AlwaysImpedes]

Re: Dell E172fpb

Most here would recommend replacing them now. I have no strong opinion either way. If this were a bigger monitor or LCD/plamsa TV, I would suggest replacing the caps now.

THanks alot for your help, bro. This forum and its people rock.