Dell E172fpb

[HOODLAM]

Re: Dell E172fpb

Oooppps. Not Replacing Trannies, Ment Transistors( 2sc5707's)! All Caps Are Being Relpaces With Exactly The Same Spec, But Just A Different Make(hitano) Fuse And Rectifier All Okay. Did The Torch Test And Can See Image On Screen When On.

[Krankshaft]

Re: Dell E172fpb

It is likely that the 3 Amp fast fuse (PF701) needs replacing, do not use anything other than this or the transformers may flame.

A correction the fuse on my HP 19" was a 5 amp fuse not a 3 amp probably due to bigger backlights then the 17" Benq.

My monitor had the same symptoms before the repair and on mine at least the fuse was intact.

However the casing of the fuse was cracked so I replaced it just to be safe here is a pic.

https://www.badcaps.net/forum/attach...achmentid=6510

As you can see the board design called for 2 fuses but the bean counters jumpered the other one.

The amperage is clearly labeled on the fuse.

[davmax]

Re: Dell E172fpb

All sounds good. So where can you get the 2SC5707's from? Being in S Africa.

[HOODLAM]

Re: Dell E172fpb

I ordered the 2sc5707 from Blue Star Inter- https://www.bluestar-online.com
$1.49, not a bad price so i ordered 10 of the little critters! Got confirmation from UPS that they have been shipped off yesterday and should recieve Monday at the Latest. Hope the thing works. Blue Star Inter have a large range of components in stock. Check them out!

[HOODLAM]

Re: Dell E172fpb

I removed the transisitors lastnight and sure to be one of the 2sc5707 are faulty, but when you test it in curcuit itseemed fine!The one thing i dont understand is why is there one tranisitor different to the other 3, aren't they all supposes to be the same?

[Krankshaft]

Re: Dell E172fpb

Yes they are supposed to be C5707s it is possible that this may have been some kind of revision performed at the factory.

But obviously it failed miserably.

Mixing and matching transistors is never a good idea.

As I remember these transistors are tied together which makes this even more of a mistake. I can't remember exactly how they linked them up though.

The reason that your readings are erroneous when in circuit is because of this common tie between them.

[davmax]

Re: Dell E172fpb

The links between the pairs of 2SC5707 are the transformer windings that are low resistance. The bases are linked by the drive winding and the collectors are linked by the the primary winding. An ohmmeter test would make these seem to be connected together. So when one transistor goes short ciruit the other will appear to be shorted also.

It is possible that these transistors are not adequately protected against all overcurrent conditions. The circuitry in this area is not the same as experienced with the FP737s monitor, but I suspect that if a slow start circuit is employed there may be a weakness there.

[HOODLAM]

Re: Dell E172fpb

you can see on the bottom of the photo where i have removed the tranisitors(waiting with anticipation for replacements). hopefully the pic will give an understanding of how this inverter layout is! thanks guys for everthing and all the help!

[HOODLAM]

Re: Dell E172fpb

Does Anybody Know Where I Can Purchase Irfu9024n And Tl1451a Online?

[Krankshaft]

Re: Dell E172fpb

Ah finally all is clear the board my monitor had is a revision (it had SMT FETS for the power off circuit) and yours is an older version (with through hole parts).

However I've found some things for you to check in the meantime.

First check both diodes under the board I circled them in Red (D803 and D801 I think I can't read the numbers that clearly on that one).

Their job is to protect the FET if they measure ok check on.

If you trace the lead leaving the diodes (not the ground lead which connects to the ground plane) they both will lead to a 3 pin FET. Measure from source to drain if you get a voltage drop you're fine.

Next if you wish you can measure the soft start transistors circled in red however they should be fine if the FET is ok.

The IRFU9024N is a MOSFET you can check it with the diode checker of your DMM.

Just measure from source to drain you should only measure a voltage drop in one direction. Just follow the pinouts on this datasheet its on page 8 or 9 depending on the type of package they used.



As for the TL1451A thats the PWM for the high voltage inverter that should be fine.

[HOODLAM]

Re: Dell E172fpb

Just sitting here tracing out the board. Question-Shouldn't Q805(2sk2141) be the same as Q812(FU9024N) and shouldn't there be a left and right side inverter which should be the same?

[davmax]

Re: Dell E172fpb

Just sitting here tracing out the board. Question-Shouldn't Q805(2sk2141) be the same as Q812(FU9024N) and shouldn't there be a left and right side inverter which should be the same?

The answer is yes in both cases. The 2SK2141 is a bad replacement for the FU9024N it has ten times the ON resistance, lower DC and peak currents, it is also a 600V device (this is unnecessary) the circuit operates at a nominal 15 volts.(FU9024N is 55V) Worst on all 2SK2141 is N channel, completely wrong the FU 9024N is P channel. REPLACE IT.

THIS DOES CONFIRM THAT THE CIRCUIT HAS BEEN REPAIRED BY SOMEONE NOT SKILLED.

Are you sure that we are discussing the MOSFET controlling one inverter? The one at top right in your photo.

[davmax]

Re: Dell E172fpb

Caution. Make sure the two controlling MOSFET circuits are identical and that the 2SK2141 does indeed switch a +15V or similar supply just like the FU9024N.

[HOODLAM]

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.

[davmax]

Re: Dell E172fpb

Seems like you are on the right track. The repair has been revealed!!!

The two diodes are connected in reverse across the 15V supply switched by each MOSFET ie they are on the output side of these transistors and conduct current only when each MOSFET switches OFF and energy is returned to the circuit by the small choke(inductance).

[HOODLAM]

Re: Dell E172fpb

JUST ORDER THE IRFU9024N FROM Audio Lab shopping ,$1.75 EACH PLUS $20.00 SHIPPING AND ABOUT 4-6 DAYS DELIVERY! IF IT DOESNT WORK AFTER ALL THIS, THEN I DONT KNOW!!

[davmax]

Re: Dell E172fpb

Costing a bit I see. Perhaps New Year hols will delay delivery. It should all work with the correct components. What is the part number of the controller chip under the board and driving the inverter MOSFETs? 16 pin device on right of photo.

[HOODLAM]

Re: Dell E172fpb

The Controller Is A Tl1451a. I Have Attached The Data Sheet For This Chip. This Is The First Time I Am Attempting To Repair An Electonic Component Like This. I Am More An Mechanical Engineer. There A First Time For Everything!!!

[davmax]

Re: Dell E172fpb

Wrong data sheet!!! However I know this controller chip and have data sheet.

Having to close for day . Bye for now.

[Krankshaft]

Re: Dell E172fpb

Hey Davmax you were right someone please slap that repair man for me .

Rule one of electronics repair just because its working today doesn't mean that it will work tomorrow.

Subbing a component improperly may allow the monitor to work for awhile but it will just fail again.

Similar to replacing the vertical deflection IC in a monitor and not replacing the VCC resistor. It will just blow the IC again.

It seems like the "repair man" cough just threw in whatever he had in his scrap box to get it working.

If I can't find an exact component (and luckily I rarely need to do this) I painstakingly compare the datasheets of the old component with the possible new one BEFORE I ever solder anything.