Shorted HP L1955

[mjsbv]

Hi all, I'm new on the fourm and am working to bring a HP L1955 LCD monitor back to life. Its probably identical on the outside as the HOODLAM describes but the power inverter board is a little different. The board is a benq 48.L1A02.A00.

Inital symptoms are power light comes on flashes yellow then turns green and stays solid green but no display either with out without video input. With alot of squinting I can make out the menu display when the menu button is pressed so it looks like the video logic must still work. I tried a flashlight but still couldn't make out any more than the menu.

Opened it up and see initally two 1000uf 25V caps with black smudges on the top but not bulged. The main fuse is good but secondary fuse PF703 is blown and there are two darkened areas near two of the inverter transformers where it got hot. I replaced the two caps and fuse and gave it a try and the fuse blows instantly. WIth some tracing about one half of the inverter section has .2 ohms to ground and the other is several K. I pulled out the two 5707's for that section and one seems shorted and the other not. But a short still exists on the board with the 5707's out. I understand from other threads there is a mosfet (FU9024N) that drives these but I need some help figuring out which one it is. The SMD side of my board is different than the picts hoodlam posted. I'll post a picture tomorrow when I get them downloaded. I took my board into work and looked at it under the microscope to try to read the numbers on the chips and didn't find any that looked like 9024. I found one each labled W064d (Q811) and W0452 (Q810) in the area I expected the mosfets to be. The 4d and 52 are turned 90 deg to the rest of the letters. Am I reading it upside down? Q812 is a 8 pin package nearby with all four pins on one side tied together and 3 on the other side (effectivly a 3 pin device) labled 4431 and L48C. This chip is connected across the shorted traces and could also be bad. Any idea what it is? There also is a diode that I thought might be the culpret so I detached one end and it tests just like the one in the other half of the circut. I concluded it was probably ok and reconnected it.

Also while looking through the microscope I found what appeared to be cracks around the inverter transformer pins like what others described so I reflowed the solder there.

I'd like to save this monitor from the grinder so any thoughts would be appreciated.

[Krankshaft]

Re: Shorted HP L1955

I am reading this post with the EXACT same monitor.

Most likely your power board is like mine. I fixed it months back and its still going strong.

Check out my post here for pics:

https://www.badcaps.net/forum/showth...4&page=3&pp=20

Your board is most likely like mine with more SMT parts.

The circled SMT packages are the CCFL ON/OFF transistors you are looking for. Also check the protection diodes circled as well.

Yup thats them 4431 measure them with the diode checker from source to drain one of mine was dead shorted 0.00. In addition to bad inverter transistors.

You can get the SMT fets at newark.com:

http://www.newark.com/jsp/search/pro...sp?SKU=06J7725

Here is the datasheet for the SMT FETs:



If it keeps blowing the secondary fuse check the secondary rectifiers for shorts as well.

[mjsbv]

Re: Shorted HP L1955

Thanks Krankshaft...I didn't find that thread and your board is identical to mine. One of the FET's seems good the other is shorted. I'll get enough to replace them several times if I don't get it right the first time. Did you replace all the 5707's also?

Here are some picts of my board

[Krankshaft]

Re: Shorted HP L1955

Yes I replaced all four of the 5707s while only one was kaput on my monitor.

I also replaced both SMT Fets to be safe the shorted one on my monitor had a bubble in it. When I replaced the inverter transistors the blown FET caused one of the backlights not to turn off when I powered the monitor down.

So its better you catch the FETs now rather then later like I did. Also check the large SMT diodes next to the fets that protect them you should get a voltage drop in one direction.

My fuse had a nasty crack in the casing right in the center exposing the core of the fuse so maybe it should have blown like yours and I was lucky I ended up replacing that fuse even though it still worked because the casing crack worried me. I think I have a pic of that somewhere in my thread.

The hot spots around the inverter transformers I had too its nothing to worry about since most inverter boards run quite hot.

[Krankshaft]

Re: Shorted HP L1955

A common failure point is cracking on the HV transformer joints so be sure to resolder them. Also check those gray caps in between the 5707 transistors on mine one of the solder joints on one of their legs cracked as well.

Also be sure when you solder in the new 5707s that the metal side of the transistor faces the white line on the PCB you don't want to install them backwards.

The Elite capacitors on the secondary should be junked as well in the future.

For now though just check them with a resistance test to make sure they are not shorted or open.

Were those Panasonic caps on the secondary stock or did you install them? Mine had Elites on the secondary and those radioactive puke green Lelons everywhere else on the board.

When you eventually decide to recap make sure that you use low ESR caps on the board and not General Purpose. Panasonic FC is a good example of a low ESR cap that works well in hot LCD inverters.

[starfury1]

Re: Shorted HP L1955

Were those Panasonic caps on the secondary stock or did you install them? Mine had Elites on the secondary and those radioactive puke green Lelons everywhere else on the board.

I dont know for sure and colours aren't much help, its possible too he may have recapped but

Dont let the can fool you they may be HERMIE's
(that name and inverter boards seem to go together too.)

I thought they were (possibly) Panasonics in the Linksys...same style "T" vent but when I had a look they weren't.

so mjsbv please post what they are, Thanks and welcome to the forum

Cheers

[starfury1]

Re: Shorted HP L1955

Doesn't look like there has been reworked joints on the PCB..but hard to tell
so they may possibly be original caps...

[kc8adu]

Re: Shorted HP L1955

i have replaced many hermei caps in flakey linksys routers.
many are panasonic lookalikes.

[mjsbv]

Re: Shorted HP L1955

Ok I'll fess up. The two caps in the secondary are not originals. The board was virgin before I started messing with it and was loaded with elite and lelon caps. I dug a couple Mallory CE's out of the bin at work that were the closest I could find to replace the two that had black smudges on the top. They are the correct uf value but are 63 volt and only 85 deg. temp rating. I figured I'd put them in temporarly to see if the board would work. It didn't, so I started looking elsewhere. I'll pull them out and replace with probably panasonic but I'll have to order them as we don't stock them at work. I've located the parts think I need just have to get the order in.

I did resolder the transformer pins but will look at the grey caps too. I should have taken a picture of the cracked transformer pins in the microscope but didn't think of it till it was too late.

[Krankshaft]

Re: Shorted HP L1955

Yeah they're fine for testing but 85 Cs are a no no in any inverter they just get too hot.

Mallory wow that company still makes caps?

I only see them in old tube radios I have to recap and 70s oscilloscopes.

Anyhow make sure that those thermal pads that are on the rectifier heatsinks are straight they radiate the heat from the small heatsinks to the metal chassis effectively making the metal case a large heatsink.

As the saying goes with thermal pads more surface area = better heat dissipation.

Here we go with the bean counters again I guess they saved a few bucks on aluminum by using this method.

Without these the inverter would probably overheat its fine to run it without the case for testing but long term would be a problem.

[starfury1]

Re: Shorted HP L1955

yes KC8, was wishful thinking in pentiums post on it with the linky they are as you say Hermei's

Mallory yeah I remember that name from long ago....mallory are the "T" vents? just checking. (yeah and replace them with the suggested or equivalent)

cheers

[mjsbv]

Re: Shorted HP L1955

A quick update to the L1955 repair. I ordered a collection of Panasonic FM caps from digikey along with replacement 5A fuse and Fairchild NDS9435ACT Mosfet to replace the 4431 (I didn't want to place another order from Newark just for that and they did'n't have small order quantities of Panasonic caps available). I think this is an equilivilant replacement, let me know if you don't think so. I got some 5707's from ebay. Replaced all the caps, fuse, one mosfet and 2 transisitors (only one was bad). Put it back together and it lit right up. I'm using it now, YEA. I resoldered the inverter transformers and the polyester caps next to them. Only problem with the soldering was the mosfet which wanted to lift the traces when I took the old one off. It wasn't a beautiful job putting it back but its working. It was enough of a pain I decided to not do the other mosfet if it still worked which it does. The bad one was shorted so the fuse blew instantly. I concluded the other half of the inverter circut was probably fine so except for the transformers and associated caps I left it alone. I'll keep the old monitor handy for a few weeks while this one burns in.

Thanks for the advise and help. You all definatly helped save this very nice monitor from the grinder.

mjsvb

[Krankshaft]

Re: Shorted HP L1955

Glad to help.

An old trick I use to remove 4 pin SMT ICs is this.

Bridge all 4 pins on one side of the IC with a blob of solder put a sewing needle under all four legs. Then melt the solder at the same time pull the needle up and one half of the IC will come off the board.

Do the same for the other side blob up the pins then melt the solder and remove the IC using tweezers.

Use desoldering braid to clean the excess solder up and you're good to go.

I haven't lifted any SMT traces yet with this technique.

I know first time SMT IC soldering is a real pain but you get better at it the more you do it.

[vbripple]

Re: Shorted HP L1955

Hello to everyone,

I have found this forum and related information very helpful and have managed to fix one 48.L1A02.A00 board, by replacing one set of the c5707s and the associated .22 cap. Everything works great and using this monitor today.

Now I have second monitor with exactly the same board. I have found the same condition, but also have found a blown fuse at FP701. Although this is a virgin board, FP701 is a fuse but FP703 is a wired jumper. I thought FP701 and FP703 were the primary and secondary fuses. Am I wrong? If not, why would FP703 be jumped from the MFG?

Regardless, the fuse must have blown for a reason. Could the C5705 cause this to happen or should I be looking for something else? All other capacitors are in very good condition, no swelling, discolor or leakage.

I don't want to power the unit back up just to have it blow again.

Thanks, VB

[Krankshaft]

Re: Shorted HP L1955

It probably blew because the one of the 4 C5707s is shorted mine had the same problem. I'm using it to read this post now.

As for jumpering one of the places where a fuse was supposed to be the manufacturer saved a few cents that's all.

Those pico fuses are to protect the inverter side of the supply against shorts. The fuse for the primary side is located around the power plug.

Just set your multimeter to the diode check function. Measure from base to collector and base to emitter you will find a dead short 0.00 on one of them.

I believe the leads have been labeled on the PCB from the manufacturer for you B for base C for collector and so on.

Replace the defective pair of transistors and replace the fuse.

Now onto that 8 pin (really 3 pin) SMT device you were talking about this is a MOSFET that is responsible for shutting down the backlight circuit when you press the power button.

On my monitor one of them was shorted (after replaceing the C5707s and the fuse the backlight wouldn't go off when I pressed the power button).

To check them measure with the diode check function of your multimeter from source to drain if you read a short then that FET will need to be replaced as well.

[Krankshaft]

Re: Shorted HP L1955

To get the pinouts on the semis here are your datasheets.

Here is the datasheet for the C5707s:



Here is the datasheet for the FET:



And a link where to buy it if one of them is shorted:

http://www.newark.com/jsp/search/pro...sp?SKU=06J7725

That's an OEM part the original has a triangle logo on it that is Vishay's logo.

As for the C5707s go OEM on those too they are made by Sanyo.

[vbripple]

Re: Shorted HP L1955

Krankshaft, sorry I haven't replied, so here is an update.

I repaired the original monitor and it works great and have repaired two more. All seem to work well till.... the last two worked for about 3 days and now come on then picture quickly goes off, then on and off. This just repeats over and over.

I only replaced the 5707's in pairs. If a 5707 was out for the lower lamps I replaced the lower set. If the upper lamps, the upper 5707 pair. Should I have replaced all four?

Any ideas as to why the protection circuit is jumping in and cycling the lamps?

thanks,
vb

[Krankshaft]

Re: Shorted HP L1955

Well for safetys sake I would have replaced all four but that doesn't sound like the problem. In sets of two (on the same lamp) should be fine.

First check the 5707s for shorts if they aren't shorted continue on.

Resolder all the pins on the inverter transformers and check the joints around them for breaks.

What you are referring to could also be a cap problem.

My 1955 had crappy Elites and Lelons on it they weren't bulging but I replaced all of the secondaries.

Without an ESR meter cap diagnosis is a hit or miss some caps can fail without visible signs of fatigue.

If all of these turn up zilch I would check the inverter lamps bad lamps can shut the inverter down.

[vbripple]

Re: Shorted HP L1955

Krankshaft

You have a source for those .22 uF 160V caps? Digikey only has the 85°C versions in stock. The 105°C's are large Qty non-stock and Newark is a no go on those.

Would be nice if we could find a single source for the 5707s, FET and caps. The minimum purchase and shipping is killing me.

[Krankshaft]

Re: Shorted HP L1955

.22 uf at 160 volts?

That sounds like a metalized film cap check mouser.com they will probably have it in stock.

Yeah I hate being at the mercy of Flea bay for my electronic components I have found other Internet sources for the 5707s but they mostly sell ICs.

That's the Internet loads of crap strewn in all directions .

The term one stop shopping doesn't apply here.