Dell E172fpb

[juane414]

Re: Dell E172fpb

Interesting, thanks for the information krankshaft! I will look into replacing those as well. Its annoying having to unplug the monitor when I'm not using it.

Here is a shot of my inverter board. Its actually a BenQ monitor, not a Dell, but the inverter board is similar to the Dell boards I've seen. I replaced SIX transistors, which are circled in red. I also replace two capacitors that were bulging.

Krankshaft, can you tell me which MOSFETs you are referring to in this picture?

[Krankshaft]

Re: Dell E172fpb

I think I know what the problem is here can you give me the part numbers of the 6 transistors you replaced?

[juane414]

Re: Dell E172fpb

yea they are C5707

I also just realized that the capacitors I used were 85C, while the originals were 105C... They might not last long that wouldn't cause the problem would it?

[Krankshaft]

Re: Dell E172fpb

Check these transistors for shorts that I circled in blue.

I believe without tracing out the board the upper 2 transistors you replaced were the ON / OFF power switching transistors.

However if they are new and you are certain that they were originally C5707s and not FETs with a different part number on them and the monitor still won't shut down.

That means that one of the ON / OFF transistor driver transistors is shorted this doesn't happen as frequently as blowing the FETs but does happen.

One of the pairs of transistors circled in blue.

No the caps wouldn't cause this they just won't last as long and may bulge sooner.

[juane414]

Re: Dell E172fpb

Well, I originally replaced the 4 lower transistors, but there seemed to be no change in the original problem of the monitor. It would light up briefly to display "BenQ" and then briefly again to display the image of the desktop before going off indefinitely. So, I figured remaining top 2 transistors were bad, and after replacing those the monitor lit up and stayed on.

I'm assuming its the smaller driver transistors that are bad.

[Krankshaft]

Re: Dell E172fpb

Yup process of elimination check the drivers.

[juane414]

Re: Dell E172fpb

hmmm... my monitor actually just fried itself i think. Image faded out, smells like something melted... I'll have to check it out.

[juane414]

Re: Dell E172fpb

Well I opened it up again and it looks like 2 of the transistors that I replaced burnt out already. Any reason why this would happen? What else could have caused the transistors to fail so suddenly?

The transistors that failed again are circled in red.

[Krankshaft]

Re: Dell E172fpb

The only thing that could have done that is shorted turns in the high voltage transformer that they drive.

The only cure is a new transformer.

[juane414]

Re: Dell E172fpb

how do i know which transformer it is, and where do i find one?

[juane414]

Re: Dell E172fpb

I just noticed something new... Check this out.



If you look at the blue thing circled in red, you'll see that its bulging badly compared to the others. The transistors that died are circled in blue, and are slightly browned from shorting. What do you make of the blue thing?

Would the transformer right next to that be the bad one? It seems that each transformer has its own blue capacitor thing.

[Krankshaft]

Re: Dell E172fpb

That blue thing is a high voltage capacitor good to 1,000 or more volts look at the other blue caps for the capacitance and voltage rating. It's to filter the high voltage going into the CCFL backlight lamps.

If that capacitor was bad to begin with it could have killed the new transistors.

But if those HV caps weren't bad then the a faulty HV transformer could have done this as well.

This monitor is just full of suprises isn't it?

[juane414]

Re: Dell E172fpb

I'd say! Well, physically, that blue capacitor looked to be fine before replacing the transistors. So if the transformer is bad, why didn't it kill that capacitor before and what made it die this time?

[Wizard]

Re: Dell E172fpb

Can be looking nice and intact but is BAD. I have seen this happen.

And missed any caps?

Cheers, Wizard

[Spacedye69]

Re: Dell E172fpb

Might be bad solder joints. Did you re solder all the transformer connections? Just to be safe, you should re solder all the connections on that half of the board.

[juane414]

Re: Dell E172fpb

Might be bad solder joints. Did you re solder all the transformer connections? Just to be safe, you should re solder all the connections on that half of the board.

Actually I didn't re solder anything other than what I replaced. I will get a new high voltage capacitor to replace that blue one and replace those two burnt out transistors and then re solder everything again. If it goes out again then I'll assume its a transformer.

BTW thanks for all the great help guys

[juane414]

Re: Dell E172fpb

Those blue high voltage capacitors say "27J 3KV" Any idea where I can find a few of those?

Also, if the transformer was bad would there be visible evidence?

[s37d]

Re: Dell E172fpb

I'm having the same problem as a lot of people here - replaced all 4 transistors, now it goes on for 2 seconds then shuts off. Anyone come up with a solution for this yet? It has to be something else causing this right?

[Krankshaft]

Re: Dell E172fpb

Defective backlights can cause the inverter to shut down.

[Krankshaft]

Re: Dell E172fpb

Those blue high voltage capacitors say "27J 3KV" Any idea where I can find a few of those?

Also, if the transformer was bad would there be visible evidence?

Well don't quote me on this because I wasn't able to confirm it by Googling but I believe the coding system works like this.

The first two numbers are the pf rating and the letter is the tolerance.

The kv stands for kilovolts so 3kv is 3,000 volts.

So these caps are 27pf 5% tolerance at 3,000 volts.

Digikey and I'm sure any other major parts supplier should have them.