Dell E172fpb

[pedro]

Re: Dell E172fpb

Anyway... Mine worked one day, and wouldn't the next time it was powered up. There were no warning signs in between which was odd. When it gets plugged in, I see a faint flicker of the monitor. The power button also flashes on/off about every 1 second.

That behaviour is characteristic of a SMPS trying to start, shutting down on a fault, then starting over. Typical burp frequency is about 1 per second.

First two things I'd be doing, after a visual and ESR check of the usual suspects:

Check the rectified (ex line) DC on the large cap - should be around 1.4x line voltage. Check the small electros near the SMPS controller.

[rokkaholik]

Re: Dell E172fpb

I know nothing of what you just said. I'm not an electronics guy. I can pick out crappy or broken solders and domed capacitors and replace them, but not much else. I'm lost beyond that unless someone pointed to what to replace

[retiredcaps]

Re: Dell E172fpb

An ESR check is using a specialized piece of test equipment to a capacitor. This equipment costs between $40 to $80. If you are just fixing the one monitor, it is cheaper to replace all the (big and small) capacitors on the board regardless if they are domed or not. Capacitors do not have to be domed in order to be bad.

To check the rectified DC on the large cap, you will need a multimeter. This DC voltage should be around 165V DC if you have 120AC voltage. Caution is required in measuring main voltage. If you are unsure how to do this, get someone who knows how.

I know nothing of what you just said. I'm not an electronics guy. I can pick out crappy or broken solders and domed capacitors and replace them, but not much else. I'm lost beyond that unless someone pointed to what to replace

[rokkaholik]

Re: Dell E172fpb

I have plenty of different capacitors to mess with, so I may just replace the main ones then just to see. I always thought they had to be domed or corroded

[retiredcaps]

Re: Dell E172fpb

A non bloated/domed cap can be bad because it is out of tolerance or have high resistance.

For example, a 1000uF 16V cap may measure 500uF (out of tolerance) and have a resistance reading of 10ohm (normal might be less than 1 ohm depending on the cap).

[QUOTE=rokkaholikI always thought they had to be domed or corroded[/QUOTE]

[Mikeley]

Re: Dell E172fpb

Hello,
Some great information and solid advice here. I have a Dell E172FBp that comes on for a short time, and then the backlight shuts down. Green power button remains illuminated. It’s a classic problem for this monitor judging by these threads.
This one had been opened by the owner as they noticed something rattling inside the monitor when it stopped working. Something fell out when the covers were removed, but they did not believe it was part of the monitor! The 'thing' that fell out was actually
Q739, a 2C5706... dead no doubt. Extreem example of dry joints! Evidence of heating on component side of PCB and the solder side has a number of dry joints on transformer pins and other transistors. Q739’s first cousin, Q740 was certainly dead short. Q759 and Q760 tested fine (diode checked out of circuit).

So two 2SC5706 transistors on order! Fuse PF751 was good, and no problems with remaining transistors to my surprise. I have re-soldered all transformer connections. Checked capacitors around the board and all are good (used my trusty ESR meter).

Wondering if there is anything else to be done while I wait for parts? What checks can be done on the CCFL?

Mike

[Mohonri]

Re: Dell E172fpb

Wondering if there is anything else to be done while I wait for parts? What checks can be done on the CCFL?

Swap the CCFL cables, to ensure that both CCFL's indeed work.

[Mikeley]

Re: Dell E172fpb

I tried that suggestion. Confirmed that the other CCFL comes on. Goes into shutdown after 2 seconds.

[Mohonri]

Re: Dell E172fpb

I tried that suggestion. Confirmed that the other CCFL comes on. Goes into shutdown after 2 seconds.

Well, that rules out bad CCFLs, then. I've repaired a couple dozen of them now, and it's quite common for one pair of transistors to go bad while the other pair is fine (for the time being). In any case, you'll want to make sure to resolder all those solder joints, to make sure they don't fail in the future.

[Mikeley]

Re: Dell E172fpb

One pair of transistors (2SC's) did the trick. Dry joints a major problem for this PCB. There were other components with dry joints not just the traffos. Well worth resoldering.

Good advice! Thanks

[jlatenight]

Re: Dell E172fpb

I've already replaced the fuse at PF751, the 4 2SC5707 Transistors, and the 2 FU9024 Transistors, which fixed the problem, but it failed again soon after. I did NOT resolder the transformers. After reading this entire thread, I looked at all 4 transformer connections, and the side of the T752 transformer with the 6 pins looks really bad (see the pic). I'm SURE that's the problem that keeps burning out my transistors, wouldn't you say? Now, should I assume the transformer's bad, or should I just resolder it?

[retiredcaps]

Re: Dell E172fpb

I did NOT resolder the transformers.

Now, should I assume the transformer's bad, or should I just resolder it?

The "standard" advice for Benq boards is to resolder all the inverter transformer pins. It is rare the transformers are bad.

If you don't resolder, the board will fail again.

[jlatenight]

Re: Dell E172fpb

Thanks retiredcaps, I did that and now I'm waiting for a new set of transistirs. Now, I tested all four of the 2SC5707 transistors in-circuit with my DMM, and a pair of them gave me different reading than the other pair; I measured resistence between pins 1+2 and pins 2+3 one pair of transistors was ~560 for both tests, and the other pair was ~560 for pins 1+2, but pins2+3 were ~240 or something like that. So are those two bad? Or maybe the first two are bad?

[Mikeley]

Re: Dell E172fpb

I would recommend inspecting and resoldering all joints on the inverter side of this PCB. They generally are a mess based on my experience. Resistors, capacitors, inductors and large schottky diodes between the traffos and around the transistors are particularly prone to dry joints.

Recheck the fuse. After completing the resldering, try it again.

You may also have electrolytic caps dried out and I would also recommend checking with an ESR meter if available, or just replace these as you have it opened up.

[jlatenight]

Re: Dell E172fpb

I measured resistence between pins 1+2 and pins 2+3 one pair of transistors was ~560 for both tests, and the other pair was ~560 for pins 1+2, but pins2+3 were ~240 or something like that.

What I meant to say is I did the diode test, not resistance.

[Mikeley]

Re: Dell E172fpb

2SC is NPN
As you look at the front of the transistor, (writing side)
Pin 1 Base
Pin 2 Collector
Pin 3 Emitter.

Unsure how familiar you are with measurements. Make sure you include the decimal point when reading the meter. On diode check, the meter is displaying Volts. Be sure of probe and pin positions.

With Red Probe on Base (Pin 1) you should measure .6 Volts approx to pin 2, and again pin 1 to pin 3(.560 Volts is ok). The meter forward biases the junctions of the transistor. Any other combination should read 'OL' or 1 on the left side of the display, depending on your meter. If you have a short circuit it should read 000 or close to that.

In circuit, measuring 2-3 or 3-2 on diode check might be giving funny results. Collector to Emitter should be open circuit, OL or 1.

Set your meter to lowest resistance and measure. If you get readings post them here.
Still unsure, remove component from circuit and check.

Most modern meters output less than .6volts across the probes when measuring resistance. This means the meter will not turn on or bias the junctions of a transistor. If you measure a short circuit, or just a couple of tens of ohms, then the transistor is certainly dead, or something in circuit is dead! Should not matter in this case which way round the meter leads are. But confirm for yourself.

Let me know what you find.

[jlatenight]

Re: Dell E172fpb

Weill I tested the four transistors as you suggested and my results are attached. I also did the ohm test on the lowest setting and I didn't get a reading on any of them in any position. I assume polarity doesn't matter with an ohm test, right? Nothing seemed out of ordinary, so I tried to just resolder the four transformers as everyone suggested. I decided to try it again in the monitor and it worked!!! We'll see how long it works for, but I have a feeling the issue may be (gulp) resolved. So I wanted to thank all the geniuses on this forum for their help!!

[Mikeley]

Re: Dell E172fpb

Lesson in dry joints!

The Diode measurements illustrates the problem of interpreting with other components in circuit.

Best diagnostic method is lowest ohms on a digital meter when in circuit. Out of circuit, diode check is useful, identfication of the base, forward drop etc. Polarity can impact, good to always check both ways.

Finally, you might want to consider replacing capacitors as these can be the cause of running hot! I tend to replace caps when they fail specification and always check all of them with an ESR meter.

Great result! Well done!

[spider12]

Re: Dell E172fpb

First of all, a big thanks to all Concerned

i have a Dell E172fpb which was powering up OK, but had a black screen,
but had an image on screen when a light was shone at it,

removed Q759 and Q760 [one of which was short circuit]
but decided on changing all 4 transistors [cost me £3-60 for 12 ]

tested monitor, only to find power button and screen both flashed
about once a second.

checked all 4 transistors while in circuit, but where still OK
so continued to read past the first few pages of this thread

and read i should have re soldered the 4 transformer joints.

i actually soldered them, then de soldered them [to remove all the Crap solder]
then re soldered them with Proper Solder [none of this lead free crap ]

after which the monitor works as it should.

so again, a big thanks to all concerned for their hard work in this thread

[jetadm123]

Re: Dell E172fpb

First of all, a big thanks to all Concerned

i have a Dell E172fpb which was powering up OK, but had a black screen,
but had an image on screen when a light was shone at it,

removed Q759 and Q760 [one of which was short circuit]
but decided on changing all 4 transistors [cost me £3-60 for 12 ]

tested monitor, only to find power button and screen both flashed
about once a second.

checked all 4 transistors while in circuit, but where still OK
so continued to read past the first few pages of this thread

and read i should have re soldered the 4 transformer joints.

i actually soldered them, then de soldered them [to remove all the Crap solder]
then re soldered them with Proper Solder [none of this lead free crap ]

after which the monitor works as it should.

so again, a big thanks to all concerned for their hard work in this thread

Congrats on your successful repair!