Dell E172FPb

[ratdude747]

my first lcd to fix...

symptoms: powers on normally for 1 sec, then no picture. green power light stays on. a quick power cycle make it work for another second.

no VISIBLY bad caps.. but a mix of lelons and elite make me wonder.

no odd noises that i remember

any advice other than a recap? places to check?

edit- slight heat browning in one of the 2 cfcl inverter areas. seems centered around a transformer, diode, and PI coil (i think) area. It also has little jamicon caps.

[retiredcaps]

Originally Posted by ratdude747

my first lcd to fix...

"2 seconds to black" can be

1) bad caps
2) bad ccfls or bad wiring
3) bad inverter transformer
4) bad inverter sense circuitry

1, 2, and 3 are documented at starting with post 13 and 14.

http://www.badcaps.net/forum/showthread.php?t=10419

Pictures always help since it possible that Dell used a different power board and kept the same model number.

[retiredcaps]

I forgot to add that these Benq style boards have really poor soldering on the inverter transformers. You must resolder all the inverter transformer pins.

[alexanna]

Originally Posted by ratdude747

my first lcd to fix...

symptoms: powers on normally for 1 sec, then no picture. green power light stays on. a quick power cycle make it work for another second.

no VISIBLY bad caps.. but a mix of lelons and elite make me wonder.

no odd noises that i remember

any advice other than a recap? places to check?

edit- slight heat browning in one of the 2 cfcl inverter areas. seems centered around a transformer, diode, and PI coil (i think) area. It also has little jamicon caps.

Something I have learned on these monitors. And I am assuming there are 4 inverter transformers?
The symptoms you describe sounds like a shorted or open transistor for on one of the inverter transformers.
Unplug all the CCFLs and then plug one back in at a time [only have one CCFL plugged in at a time], power on the monitor, do you see a flash of light with each CCFL?I really suspect 2 of the transformers will not light the CCFL,Check the transistors associated with that transformer for a shorted or open condition.
And as is recommended resolder the transformers.

[smason]

Hey, who let Rat in here?

Welcome to the wonderful world of LCD repair.
Definitely check for shorted transistors, transformer soldering, resistance (if you have a ring tester, use it) and
probably re-cap if the transformers are ok.

[ratdude747]

resoldered all the transformers... the soldering looked shoddy...

i put it back together most of the way and tesed it. no lamps, blocks on screen, blinking green power light.

I opened it up again and found a solder bridge on one of the CFCL transformers. I fixed that and verified that it was the only bridge (that was on two normally disconnected pins). still gives me the same symptoms...

any advice still? i hope i did not blow something... no smoke, noises, or smells.

[retiredcaps]

Originally Posted by ratdude747

any advice still?

Post some pics so we can suggest test points. There could be multiple revisions of the power board inside a Dell.

[hoadle]

Here the website for you Dell E172FPb
http://syscon.wordpress.com/2007/07/...klight-repair/
go to ebay buy the repair kit
http://cgi.ebay.com/REPAIR-KIT-E172F...item3355ff735c

[ratdude747]

I don't do ebay repair kits. too many scams.

my camera died so no pics for now... I think i will start with a recap.

[ratdude747]

first faiure found

one of the 4 inveter area transistors is shorted out. the other 3 pass fine. i tested them out of circuit.

the part no. is c5707.


i had a sanyo 2SC4106 in my junk bin. popped it in, success.

I DONE FIXED IT!!!

i will run it a while to see if it holds.

[ratdude747]

spoke too soon.

it is now a case of 20sec to black... what now? caps?

[seanc]

Change all the 2SC5707 after having double checked the transformer soldering and other solder joints.

Transformers, 2SC5707 and a pico fuse is all that's *usually* needed in these.

[ratdude747]

the trannies already have been resoldered and checked.

3 of the 5707's were tested good. one was replaced.

[alexanna]

Originally Posted by ratdude747

the trannies already have been resoldered and checked.

3 of the 5707's were tested good. one was replaced.

Did you see if the transistors that you installed have not failed again? You do not have to remove from circuit.
Something I don’t see mentioned often on these monitors. Is to check and or re solder, the gray poly capacitors that sit right between the C5707 transistors. I have notice on more than one occasion loosens on the solder pads.

[alexanna]

I don’t know for sure but these transistors may need to be replaced in pairs.

[PlainBill]

Originally Posted by alexanna

I don’t know for sure but these transistors may need to be replaced in pairs.

Considering the application, they should ALWAYS be replaced in pairs, and with the proper part numbers.

PlainBill

[ratdude747]

Originally Posted by PlainBill

Considering the application, they should ALWAYS be replaced in pairs, and with the proper part numbers.

PlainBill

i think you are right. I feel stupid as all this time later, my failure to do things the right way may be why it still is dead.

I still have the unit in my possession and I tried it again.

it is good for maybe 10 sec before the backlights go out. after the test i smelled burning electronics near the transistors.

the 1 transistor that fried and i swapped for the larger transistor is a c5707. is it recommended that i replace all 4 with the exact same transistor or is there a compatible one that is less prone to failure? it seems they fail a lot and I would rather not have to replace them all the time (it belongs to a buddy who had it in his closet).

I thought that as long as I had the pnp/npn part good and it was the same or bigger all would be ok. i guess that and the fact I only replaced one may be why it still goes 10 sec to black?

[PlainBill]

Originally Posted by ratdude747

i think you are right. I feel stupid as all this time later, my failure to do things the right way may be why it still is dead.

I still have the unit in my possession and I tried it again.

it is good for maybe 10 sec before the backlights go out. after the test i smelled burning electronics near the transistors.

the 1 transistor that fried and i swapped for the larger transistor is a c5707. is it recommended that i replace all 4 with the exact same transistor or is there a compatible one that is less prone to failure? it seems they fail a lot and I would rather not have to replace them all the time (it belongs to a buddy who had it in his closet).

I thought that as long as I had the pnp/npn part good and it was the same or bigger all would be ok. i guess that and the fact I only replaced one may be why it still goes 10 sec to black?

This design uses a circuit called a Royer Oscillator. It requires two transistors with similar characteristics. With some care, it would be possible to select suitable transistors, but pulling some NPN transistor out of the drawer isn't going to cut it.

The root problem is not the transistors, it's the crappy solder job on the transformers.

PlainBill

[ratdude747]

i resoldered the transformers already. as in desoldering the transformer, removing it, and soldering it back in. the new solder should have fixed that.

[PlainBill]

Originally Posted by ratdude747

i resoldered the transformers already. as in desoldering the transformer, removing it, and soldering it back in. the new solder should have fixed that.

Yes, now they won't cause the transistors to fail. However, you must still have suitable transistors or the oscillator won't start.

My point was 'better' transistors aren't necessary, the originals are quite adequate. It's the poor job of soldering that is doing the damage.

PlainBill