Dell 17" LCD monitor shuts down.

Bad caps?

-- Top-posting not supported.

not

solder

The caps tested ok.

Thanks for your reply.

--
David Farber
David Farber's Service Center
L.A., CA

Did you check the power supply voltage(s)? Low power supply voltage and high ripple/noise can create a myriad of symptoms.

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Some days you're the dog, some days the hydrant.

"David Farber" wrote in news:enmio3$pso$ snipped-for-privacy@aioe.org:

Why did you reflow solder? Were there bad joints or intermittents?

tested how? ESR meter? capacitance meter? ohmmeter?

--
Jim Yanik
jyanik
at
kua.net

ESR with my Bob Parker, ESR meter and an ohmeter for shorts.

Thanks for your reply.

--
David Farber
David Farber's Service Center
L.A., CA

ESR with my Bob Parker, ESR meter and an ohmeter for shorts.

Thanks for your reply.

--
David Farber
David Farber's Service Center
L.A., CA

Judging from the described defects in operation there seems to be a problem in the electronic circuitry. I advise a basic troubleshooting routine be performed on this device to localize the defective circuitry.

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Hi David

I bought aDell E172fp off Ebay recently which was sold as faulty, the Power Light would flash on/off with no display.

Basically, what i have found is that the output transistors go s/c, these are the small 2sc5707 transistors that sit near the 4 yellow transformers. i had 2 go s/c in one of the channels, the other channel was ok. I replaced these and now the green light stays on , the display comes on for about 3 seconds like you's does and then goes off.

I put a scope on the o/p of the transformers and can see them switching ok for 3 secs then the o/p goes off. What is happening is that the Pulse Width Modulator chip that drives the lamps is shutting down after 3 seconds, this is the 16 pin chip marked TL1451A. I have the datasheet on this chip and i am currently trying to work out why it shuts down. I have found that whe you first switch on the monitor, the supply to this chip is 18vdc, when the display comes on for 3 seconds this drops to 14 volts (because this supply is unregulated) and then goes back to 18vdc.

I would be interested to know if you can confirm the dc voltage you are seeing on pin 9 the 16 pin TL1451A chip during this startup period and see if it compares to my results.

Theres a common problem here with these monitors, it may be overcurrent due to s/c transformers or another transistor s/c somewhere, i am sure we will track it down shortly.

Graham

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Hi Graham,

I checked those 2SC5707 transistors again. This time I found two of them, Q759 and Q760, were shorted b-e. I'll order those parts, replace them, and let you know what happens.

Thanks for your reply.

--
David Farber
David Farber's Service Center
L.A., CA

David, ok, i havent been able to get any of them over here but i have used BD911 as test devices.

If you get a minute would be interested to know what voltage you have on pin9 of the PWM chip as i mentioned in my last post.

Graham

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Graham,

Is it ok that the bad transistors are removed when I check the voltages? I haven't received the replacements yet.

--
David Farber
David Farber's Service Center
L.A., CA

Hi David, i believe it should be ok with the trannies out, it just means that one channel will not be driven.

Would be interested to know if you still have the same symptom (on for 2-3 secs and then off) with the trannies out.

I think what is happening is that the PWM chip is detecting overcurrent and shutting down. I found that with BD911 transistors in in place of the

2sc5707 i am still getting the same problem but i cant find any more s/c devices. As i understand it , with these monitors this is a real common problem, even if you manage to get the thing working its likely to break down again, with the same devices s/c a day/week/or even months later. It seems its always the same channel (the top lamp) that goes but no-one really knows why. I also understand that the transformers are suspect and go s/c or arc internally. Dry solder joints are also a major problem as are poor quality electrolytic caps.

I am coming to the conclusion that this monitor suffers from a major design fault from new, they seem to just make it out of warranty and then fail :-)

Apparently many service shops now refuse to handle them, they are a warranty claim waiting to happen.

I am going to continue working on it though as i hate to give up on things like this, its my own monitor so cost isnt an issue but for repair shops it well cold be.

Anyway, let me know what you find with the voltages, should be interesting.

Regards

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interesting.

Hi Graham,

I checked the voltage at pin 9 of the TL1451A chip and have come up with the same results that you found in your monitor. The voltage swings from 14 to

18 volts after shutdown.

--
David Farber
David Farber's Service Center
L.A., CA

Thanks David, will be interesting to see what happens after you get the replacement transistors in. Perhaps my continuing problems are due to having tried BD911 transistors in place of the 2sc5707.

Let me know how you get on please.

Graham

Just made another interesting observation on this board.

When the board has 2 BD911's fitted in place of the 2sc5707 in 1of the channels and has 2sc5707's in the other channel then plugging in a pair of ccfl tubes gives the same symtoms (works for 3 seconds or so then goes off). The 2sc5707 transistors have been removed and metered and appear to be ok.

If i put BD911's in both channels then the unit works (stays on). If i unplug one of the CCFL tubes then i get the original fault and the unit goes off after 3 secs or so.

I am wondering if the Pulse Width Drive chip that drives the ccfl tubes is detecting an im-bablance in the 2 channels and is shutting down.either that or the 2 2sc5707 transistors are still defective despite reading ok on cold test.

Tricky one this !!

Graham

So are you saying that it works fine if you replace all four of the transistors with the BD911's?

-- David Farber David Farber's Service Center L.A., CA

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Hi David, sorry if the last post was a but confusing. What i was trying to say was that the circuitry seems to be very sensitve to an im-balance in any

1 channel.

When i was doing some comparitive resistance checks on the 2 channels the other day i noticed about 700k difference in resistance around the area of the 2 transistors, this was traced to having different transistor types in 1 channel. I think the PWM detects this difference in current flow in 1 channel and shuts down.

All i would say is replace all 4 2sc5707 transistors (2 per channel) plug in the ccfl's and see what happens. If it works , then, as a test unplug 1 of the ccfl tubes and you will see it shuts down again.

Also, if you replace the trannies and you get the same symptoms try plugging in a different pair of ccfl's , you might have a duff tube which is causing the imbalance.

Let me know how you get on

Graham

Graham,

I substituted the four 2SC5707's with the BD911's and now the monitor works fine.

Thanks for all your help.

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Hi David & Graham,

i'm spanish, and my english is regular sorry :(

so... i have find solutions for my problem at my Dell e172FP in google and i find this post that you recently reppairs it.

My monitor bought at eBay too like Graham but i both new more than 2 years ago. It was new in his original box and it works fine even today.

It turn on and the display show anything during 1 second (or less) and then display off and the led green flash on/off quickly. I think its like Graham occours.

Well, i don't undestand well yours and i think the solution is change the C5707's transistors (Q759, Q739, Q760, Q740) with 4 BD911??

i see that David works fine, but i don't undestand if Graham too works already with do it???

I try repair in the shops here, at Spain, and they not want repair it, and Dell say that they sold me any repaired for me less expensive than new, but a bit. i don't interest this option, if i can repair it better than better.

Thank for your reply. McRight2k

--
McRight2k