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Old 07-03-2006, 06:59 AM   #41
jumba
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Default Re: Dell E172fpb

Just fixed the Dell again, this time one the original transistors failed; a dead short between all pins. Noticed that even with pulsating the circuit gets hot quickly. If one leaves the monitor on this could over a period of time cook the pcb and even a tranny. So as a precaution turn off the power to the monitor when not in use, this this should reduce the secondary damage from occurring. After sketching out a circuit there are two caps with a value of 0.15j x 100v they are connected to the collectors push pull inverter cct. This causes the switching waveform from being square wave to be more sinusoidal one, with a consequence of increasing power dissipation.
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Old 07-03-2006, 11:39 PM   #42
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Default Re: Dell E172fpb

Hi Everyone - I too had this problem with my monitor and (thanks for all your help) it was the old 'bad transistor' issue... I'm actually an electronics engineer, so what follows might make a good read for those of you learning.

Hopefully, the insights I had during the repair process should save you all some $$$... Make sure you read right to the end of this post before you take action.

Firstly you don't need to use the exact same transistor to replace the faulty ones - Here in Australia, I went through the specs of the original transistor and found a few that do the job perfectly well - from our local electronics components store (Jaycar) what you need to do is get a NPN complimentary silicon power transistor - it doesn't really matter what sort it is, as long as it is capable of handling a collector emitter voltage of 60-100 Vdc, an emitter of about 5Vdc and can handle a constant collector current of around 10 amps dc - don't go to far over spec though. The 'package type' should be a TO-220 or similar.

For the person that asked 'how' to select a replacement transistor, the following link explains the basics of how to select a substitute transistor in any application when one has failed - it's simplified, but a great intro.

For those who asked how to test a transistor, the following link explains how to use a multimeter to test a transistor.. The collector, base and emitter pins are actually marked on the front side of the board just below the pins of at least one of the suspect transistors (in my case, Q740) depending upon board. the board shown in this link provided by another member shows the letters E, C, and B printed next to Q739, Q740, Q759 and Q760 - E stands for emitter, B stands for base and C stands for collector.

I was able to successfully directly substitute MJE3055 NPN transistors for those on the original board (see transistor specs here ) at a total cost of $US1 per piece from my local electronics store (jaycar here in Australia)- I only needed to substitute two as only two were faulty. You can also use a D44H8 (available from RS - an international component supply company) if you wish, but you'll find it's a bit more expensive (approx $US7 per piece).

Apart from having to hacksaw out a small piece of metal from the backing plate to accomodate the larger replacement transistor, nothing major was required other than desoldering the 'broken' transistors and resoldering the new ones.

Right - now the clincher. After replacing the 'broken' transistor, out of interest I chose to retest the 'broken' transistor with my multimeter - it was no longer broken - LADIES AND GENTLEMEN, MOUSIE IS RIGHT - BEFORE YOU GO TO THE TIME AND EXPENSE OF REPLACING THE ORIGINAL TRANSISTORS, SIMPLY DESOLDER THEM AND RESOLDER THEM IN - the problem is not a dead tranny at all (at least in my case), rather it is a dry solder joint on one or more of the six legs in the 'broken' pair of transistors, which shows up as a 'short' when tested using a multimeter whilst still installed - might save yourself a few bucks and a lot of effort. Your choice, $US20 for parts and a trip to the electronics store, or a cent for solder and a few minutes.

Also - A MUST - unplug the monitor completely before testing the transistors. It doesn't need to be on to successfully test them, and I don't want to see anybody electrocuted.
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Old 07-04-2006, 05:56 AM   #43
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Default Re: Dell E172fpb

Since when can bad solder joint test as short?
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Old 07-04-2006, 08:23 AM   #44
dockarl
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if you have a matrix of diodes, essentially two transistors in parallel, as per this circuit, it is extremely easy for a bad solder joint to result in a short. Every time you start your computer, the board heats up, every time you shut it down, the board cools down... let's call it a 'cycle'. Different metals expand at different rates, hence, over time, the leads of the transistors will seperate from the solder that holds them. Before poo-poo ing my recommendations, try it. I am not saying I am correct 100% of the time, but I am certain the people that read my post will agree I am right at least 70% of the time.. let's see..
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Old 07-04-2006, 08:25 AM   #45
dockarl
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Default Re: Dell E172fpb

short answer is, we are testing 'in circuit'..
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Old 07-04-2006, 08:36 AM   #46
dockarl
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I agree with rainbow, to an extent, but if you remove the 'shorted' transistors, you will find they test fine (unless it is another problem - rare from what I hear) transistors can easily test as 'shorted' if they are blown, simply because they then no longer act as valves - they simply let current through regardless of their settings. If you disagree with this rainbow, go right ahead. Rainbow contends this is not the case - reading back through rainbow's posts, I only see criticism - honestly, I am an instrumentation engineer and lecturer, trying to provide good solutions and learning materials - you can choose who you believe - cheers, dockarl - very best of luck
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Old 07-04-2006, 09:50 AM   #47
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Default Re: Dell E172fpb

Quote:
Originally Posted by dockarl
If you disagree with this rainbow, go right ahead. Rainbow contends this is not the case - reading back through rainbow's posts, I only see criticism - honestly, I am an instrumentation engineer and lecturer, trying to provide good solutions and learning materials - you can choose who you believe - cheers, dockarl - very best of luck
Hmm? What am I missing? From what I can see, Rainbow only posted one line in the whole thread, asking how a poor solder joint can result in a short.

I'm no expert, so I may well be overlooking something, but surely a poorly soldered transistor pin isn't going to result in a "false positive" as far as measuring continuity between the pins of said transistor is concerned. The only scenario in which this might occur is if we're dealing with a poor joint in the sense that the solder results in bridging of two of the contacts.

Regarding the other issue, obviously unless one knows the properties of the circuit to which it is connected, the only foolproof way to test the transistor is to desolder/disconnect it first.
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Old 07-04-2006, 06:01 PM   #48
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Hmm, I must humbly apologise - after these criticisms I have gone back and retested the two 'broken' transistors, as I only tested one yesterday. Out of circuit, one tests broken, the other tests ok - guessing that when testing in circuit, as these two transistors are tracked together, both appear broken.

Long and the short - I could have replaced only 1 - so a great new monitor for $1 - could be money for (perhaps I should stop having so much of that before I hastily reply)
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Old 07-04-2006, 06:08 PM   #49
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I guess a positive of this finding is that (if this is replicated in other cases, which I assume it will be) removing and re-testing the 'dud' transistor pair out of circuit will allow people to replace the outer transistor (nearest to edge of board) with the 'false dud', and the inner transistor with the new (larger) replacement transistor.

Given the lower height of the original transistors, this will negate the necessity for hacksaw surgery to the casing to allow the transistor to fit as the outer one (where the case curves in) is the only one that doesn't have enough clearance for a standard T0-220 package.

Ciao!
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Old 07-06-2006, 11:03 PM   #50
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Default Re: Dell E172fpb

Hey Dockarl,

Is the monitor still working? Do you happen to know the transformer replacement part number and where I can buy them?

THanks
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Old 07-07-2006, 09:17 AM   #51
enkay78
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Default Re: Dell E172fpb

Folks,
first of all thanks to Dockarl for his tips on using the "MJE3055 NPN " transistor as replacement. it did the replacement and the monitor worked for a while. When i was putting everything back in place the system stopped working all together. not even the breakdown flicker. i checked the 2 amp fuse and it looks ok. but ccfl backlight outputs are also reading only 1~2 v only and not the expected 90V or so. i inspected to verify if there is any damage but there is no damage on the board.
any guesses on what may the issue???
thanks and regards
enkay78
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Old 07-08-2006, 04:36 PM   #52
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Default Re: Dell E172fpb

Hey Guys,

I found a bad transformer in mine causing my transistors to go bad similar to prototech. This is what he said:
Please forward this response to the forum to help someone else.

To recap the symptoms of MINE, a Dell E172FPb by BenQ, the display would come on for about 1 second, then go off. The green power light stayed on just fine. Upon physical examination, I found one inverter output transformer (T752) that had a charred spot between the output pins. I chased down the part, replaced it, and it works.

I ended up persuading Norris Parts in Texas (norris_parts@norris1.com) to sell me a transformer. They charged $15 + $5 shipping (yeah, steep, but worth it). That fixed it.

Seems all four transformers in the inverter are the same. They measure the same anyway.

On the labels on mine, all of them had the same first number but different second numbers. The xfmr Norris sent had the same first number.

When you look at the board and the way the traces are connected, the wiring becomes a bit more obvious, but I measured 211 ohms between pins 8 & 10, about 1 ohm between 3 & 5, about 1 ohm bewteen 2 & 4 and apparently zero between pins 1 & 6. I did not use a low-ohms meter. I used an old cheapie DMM.

They all measured this way, for what it's worth.

Hope this helps.

Good luck everyone!
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Old 07-09-2006, 06:13 AM   #53
dockarl
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Default Re: Dell E172fpb

Hi Drexal!

Yep - over a week now and mine still working fine with MJE3055 NPN transistors - I will let you people know if any further problems - interested to hear your comments re: transformer - there always had to be a reason they were blowing.

Enkay - I have no idea what your issues would be - obvious one to check would be that polarity is right - the transistors should be replaced with the flat metal back facing the same way as the original if using the MJE3055 - pin for pin drop in replacement. Also check for solder bridges before turning on, using the continuity tester on your multimeter. That would be my bet as to what is wrong.

All the best,

Doc
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Old 07-10-2006, 06:22 PM   #54
dockarl
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Default Re: Dell E172fpb

Drexal,

How is your fix with the transformer going? For the benefit of the other members having the same problem as you, what was the part number on the transformer and from your supplier?

Cheers,

Doc
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Old 07-11-2006, 12:43 PM   #55
enkay78
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Default Re: Dell E172fpb

folks,
i did verify the orientation of my newly installed MJE3055. They look OK. for all practical purposes the board doesn't seem to turn On at all. i am waiting for a replacement board to arrive in a couple of days.
in the mean time can someone suggest how to approach this problem systematically, that way we can isolate the issue to the specific component.
thanks and regards
enkay78
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Old 07-11-2006, 04:41 PM   #56
dockarl
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Default Re: Dell E172fpb

Enkay78 -

I would firstly check the voltage at emitter and collector on all the transistors to check for any differences. If they look ok, I'd start to check the output of the four transformers (look back at about page 1 of this thread for the pictures to see the transformer Drexal is talking about) and check the output on all four.

Be VERY VERY careful when working with the live board.. there is mains potential on the board and I don't want to see you getting fried.

All the best,

Doc
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Old 07-12-2006, 11:30 AM   #57
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Default Re: Dell E172fpb

I had similar problem after replacing shorted Q759 & Q760. No power in backlight circuit. I traced the problem to an open Q757. Q757 turns on Q761 which powers on the IC751 and that controls the backlight circuit. Q757 gets its turn on signal from the video board. After replacing Q757 my "other" transistors blew again so I may still have replace transformers or try something else.
Hope this helps!

Last edited by Transend; 07-12-2006 at 11:35 AM..
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Old 07-12-2006, 12:35 PM   #58
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Default Re: Dell E172fpb

Just to throw in here... Don't forget the brightness pin tells this circuit how hard to drive the lamps.
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Old 07-12-2006, 01:30 PM   #59
enkay78
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Default Re: Dell E172fpb

Transcend
thanks for ur tip. in my case Q761 is cooked. can you pls let me know which transistor model should i use to replace that?
btw folks i had one new inverter board shipped from norris parts, and guess what that board also failed to work. i dont know what is happenin. maybe norris parts just took me for a ride and their website is not working.
thanks n regards
enkay
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Old 07-17-2006, 12:39 PM   #60
enkay78
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Default Re: Dell E172fpb

folks,
i observed that in Q761 both E/B are at 24V and the C which feeds to the IC751 supply pin9 stays at 0V. as far a q757 goes its e/b are at 0v and its c is at 24v(c drives the b of q761). does this mean both 761 and 757 are blown?
this is the case with both my boards (old n new alike). norris part's website is still down
regards
enkay
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