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Old 01-02-2010, 02:52 PM   #1
Krankshaft
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Default HP 1955 Monitor Step By Step

Well another HP L1955 crossed my path and for awhile it got me stumped.

This should be the defacto guide for repairing this monitor and similar Benq inverter boards.

It came in with the classic symptoms backlight flashed on and off for a second. If the backlight doesn't come on even for a second then the inverter fuse is blown.

A flashlight on the LCD revealed the the main board and LCD driver circuitry was functioning.

So I pull out the inverter board did a check of the C5707s one of them was blown as was a 4431 backlight ON / OFF FET from source to drain as usual. As you can see it has a nice bubble in the package (See Picture). Also if a FET is blown be sure to check the protection diodes as well (see picture).

However the 5 amp inverter fuse wasn't blown if it was as I said before I wouldn't even get a backlight flash (See Picture For Fuse Location).

The board was packed with Elites known crap caps they weren't bulged but as a matter of course I replaced them. I also replaced all 4 C5707s and both of the 4431 backlight ON / OFF FETs.

I power the monitor on and I get the same symptom. Backlights flash for a second then go off.

So I ring all 4 inverter transformers with my ring tester they test fine. I test the CCFLs with an external inverter they're fine. Now I was officially stumped.

I get away from the board for a bit and then it hits me. Check the small signal general purpose transistors for the failed FET. Then what do I find? The NPN signaling transistor that switches the backlight FET on and off was shorted from base to collector.

Since it was shorted this way it was sending a positive voltage to the gate of the replaced FET irregardless of the signal from the mainboard.

Since this is a depletion FET when a voltage is applied to it's base it is off. The bad NPN was forcibly holding one backlight pair off. So when I tried to power up the inverters PWM protection circuit was sensing two backlights not coming on assuming a fault then shutting down.

I checked these transistors in a few of these monitors I've done already and they were fine but I got a little reckless on this one since all of those other times the signaling transistors were okay. They are a pair of two one PNP and one NPN be sure to check them both.

Here are the part numbers a special thanks to mrlooi who posted in this forum a year back. Giving out the part numbers for the signaling transistors and saving me hours of datasheet combing.

Where to buy and component info:

Sanyo 2SC5707 Bipolar Transistors: E-bay or B&D Electronics

Vishay Siliconix SI4431BDY FET: Digikey or Newark

W04 GP NPN Bipolar Transistor: Original part number PMBS3904 (SOT23 Package) NXP Semiconductor (2N3904). Mouser has a stock of the NXP originals as of this writing. If they don't here is an alternate part number from Fairchild Semiconductor 512-MMBT3904.

W06 GP PNP Bipolar Transistor: Original part number PMBS3906 (SOT23 Package) NXP Semiconductor (2N3906). Mouser was sold out so I found the same transistor from Fairchild Semiconductor part number 512-MMBT3904.

The W in the preceding part numbers is irrelevant it just means the transistor was made in China yours may have a different letter. T means made in Malaysia and P means Hong Kong FYI.

5 Amp 125V axial leaded fast blow inverter Fuse: OEM was made by Bussman but any standard Pico fuse from any name brand fuse manufacturer will work.

Edit: I have found that in some later model 1955s the 5 amp fast blow fuse has been replaced by a 3 amp 125V slow blow fuse. It's a Bel MS series 3 amp fuse. Again no need for the same manufacturer any slow blow pico fuse will work.

The fuse can be at either of the locations circled.

My only guess for the fuse amperage change is HP received complaints of monitor failures and upon investigation it was found that the inverter fuses weren't always blowing. As in the case of the inverter board pictured in post 12 where the overload condition actually burned the traces off one of the On / Off FETs.
Attached Images
File Type: jpg C5707s.JPG (253.9 KB, 1604 views)
File Type: jpg FETs.jpg (309.5 KB, 1395 views)
File Type: jpg Signaling Transistors.jpg (309.4 KB, 1293 views)
File Type: jpg Zeners.jpg (308.1 KB, 1194 views)
File Type: jpg Inverter Fuse.jpg (217.4 KB, 846 views)
Attached Files
File Type: pdf 2SC5707.pdf (41.5 KB, 853 views)
File Type: pdf SI4431BDY.pdf (75.8 KB, 635 views)
File Type: pdf PMBS3906.pdf (124.4 KB, 605 views)
File Type: pdf PMBS3904.pdf (125.0 KB, 553 views)
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Last edited by Krankshaft; 10-04-2011 at 07:03 AM..
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Old 01-02-2010, 03:24 PM   #2
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Default Re: HP 1955 Monitor Repair Guide

With all that was shorted and the fuse not blowing would it be worth trying a 4 amp fuse to see if that holds up and possibly save some components,
if it had a failure on the inverter in the future ???
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Old 01-02-2010, 04:29 PM   #3
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Default Re: HP 1955 Monitor Repair Guide

Good job troubleshooting, and a great job documenting the component's location and sources.

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Old 01-02-2010, 05:52 PM   #4
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Default Re: HP 1955 Monitor Repair Guide

>HP 1955 Monitor Step By Step

nice work there!
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Old 01-02-2010, 06:52 PM   #5
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by Bobdee
With all that was shorted and the fuse not blowing would it be worth trying a 4 amp fuse to see if that holds up and possibly save some components,
if it had a failure on the inverter in the future ???
Oddly enough after resoldering the inverter transformers recapping and doing the said repairs I haven't had a repeat failure so I haven't given it much thought. Been using my first L1955 since 07 and it's still going strong.

A current measurement would have to be done at the input of the inverter to see if a lower value fuse could be installed. You would also need to take startup surges into account otherwise it will blow for no reason.

Last edited by Krankshaft; 01-02-2010 at 06:54 PM..
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Old 01-02-2010, 11:24 PM   #6
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by i4004
>HP 1955 Monitor Step By Step

nice work there!
Threads like this should be on a special "Step-by-Step" section. Nice job, Krankshaft.
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Old 01-03-2010, 10:22 AM   #7
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Default Re: HP 1955 Monitor Repair Guide

Thanks I've been meaning to post this for over a week just been too busy building my new rig.

Oh yes and I forgot to write a complete resolder of all the inverter transformer pins, the C5707s (if you only replaced two), the poly caps between the C5707s and the 2 inductors.

Plus anywhere else on the board that looks suspect.

Last edited by Krankshaft; 01-03-2010 at 10:25 AM..
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Old 01-03-2010, 01:33 PM   #8
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by Krankshaft
Oddly enough after resoldering the inverter transformers recapping and doing the said repairs I haven't had a repeat failure so I haven't given it much thought. Been using my first L1955 since 07 and it's still going strong.

A current measurement would have to be done at the input of the inverter to see if a lower value fuse could be installed. You would also need to take startup surges into account otherwise it will blow for no reason.
Thanks for your reply Krankshaft
And as allready said a great fully informative thread
Bob
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Old 01-05-2010, 02:22 AM   #9
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Default Re: HP 1955 Monitor Repair Guide

I found a broken L1955 (Model: HSTND-2B01) for sale last night and I'm wondering, if it's worth repairing. The symptom is supposed to be that the monitor turns itself off immediately after it's been turned on. If you repeat the process several times, it stays on. Suggestions?

I might buy it, if it's cheap enough and have a look inside..

Cheers
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Old 01-05-2010, 07:30 AM   #10
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by Half-Saint
I found a broken L1955 (Model: HSTND-2B01) for sale last night and I'm wondering, if it's worth repairing. The symptom is supposed to be that the monitor turns itself off immediately after it's been turned on. If you repeat the process several times, it stays on. Suggestions?

I might buy it, if it's cheap enough and have a look inside..

Cheers
SainT
It depends on the cost of the monitor, and how much you value a 19" monitor. Off the top of my head, if all the parts Krankshaft lists are bad, they will cost about $20. I wouldn't pay more than $10 for the dead monitor. This would allow you to make a small profit if you wish to sell it, or give you a good monitor for under $30 and a good feeling about keeping something out of a landfill.

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Old 01-21-2010, 11:53 PM   #11
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Default Re: HP 1955 Monitor Repair Guide

If you already have tons of the parts like me already here then the total is under $10 dollars even after the recap. Around $20 bucks is about right with shipping.

I order in quantities of 10 or more from Digikey so they give me a price break.

The symptoms you describe are the exact symptoms the monitor described in the original post had. You're looking at one blown C5707 a burnt up FET and either one or two small signal transistors for that FET shorted.

If it has any Jamicons on the board you are likely to have a bulger or two as well. Purge all of the Elites and Jamicons (on some boards) on the secondary leave the Lelons on the mainboard and USB hub they aren't heavily stressed.

I like the clean lines and thin bezel of this monitor and the 1000:1 contrast ratio isn't too shabby either. 19" seems like the perfect display size for me large enough but not overbearing.
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Old 01-22-2010, 12:07 AM   #12
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Default Re: HP 1955 Monitor Repair Guide

Well another one of these crossed my path same symptoms.

However the FET on this one seemed to really flare up killed three of the source lead traces on the FET and took out both the PNP and NPN signaling transistors. As you can see on the plastic insulator on the back of the panel some serious overload happened here.

FYI the 4 top pins are all tied together and are drain the 3 bottom pins are the source while the last pin is gate.

So I removed the old component removed as much of the char as I could with alcohol and cut off what was left of the lifted mangled source trace with a razor knife.

I then stripped some insulated wire the soldered it to the collector of the replaced NPN transistor. A valid tap point for the source. All 3 bottom traces were destroyed but the rightmost gate pin was still fine.

As I said before normally all of these pins are tied together so I intentionally bridged the 3 source pins and soldered the jumper wire to it.

As you can see I left some insulation on the wire that is because the jumper goes over the gate pin.

It's not pretty but you didn't see it before I cleaned it up .

It makes me wonder if they overrated the 5 amp fuse? It again didn't blow. I've seen other 17" Benq builds with 3 amp fuses.

As for the cascading failure I am still thinking there was a bad batch of C5707s that Sanyo never admitted to.
Attached Images
File Type: jpg PCB Repair.JPG (305.9 KB, 730 views)
File Type: jpg Insulator.JPG (323.8 KB, 629 views)

Last edited by Krankshaft; 01-22-2010 at 12:14 AM..
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Old 01-22-2010, 12:49 AM   #13
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Default Re: HP 1955 Monitor Repair Guide

Sorry about the pics I didn't deflux the board before I took the pics and the flash made it look horrible.
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Old 02-11-2010, 05:26 PM   #14
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Default Re: HP 1955 Monitor Repair Guide

Krankshaft, do you happen to have the part number for the zener diodes? If not, how could I find it?
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Old 02-24-2010, 02:35 AM   #15
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Default Re: HP 1955 Monitor Repair Guide

I also have an HP 1955 LCD monitor, which would flash once on power on, and once again when the Windows XP flash screen appeared, then nothing. I read a lot of info in this thread, and pulled it apart, but could not find any caps bulging, or ICs blown. I checked all the 5707s, just by moving them a little, and did not notice any solder breaks. I reassembled it, and now it is working as good as new. Go figure
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Old 02-24-2010, 07:00 AM   #16
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by Kevin07
I also have an HP 1955 LCD monitor, which would flash once on power on, and once again when the Windows XP flash screen appeared, then nothing. I read a lot of info in this thread, and pulled it apart, but could not find any caps bulging, or ICs blown. I checked all the 5707s, just by moving them a little, and did not notice any solder breaks. I reassembled it, and now it is working as good as new. Go figure
The rule of thumb is "If it starts working by itself, it will stop working by itself."

There are two possibilities.
1. There is an intermittent connection to one of the CCFLs. When you opened the monitor you jostled the wires and it is now making contact. At sometime in the future the connection will once again open up.
2. There is a bad solder joint at one of the transformers. At some time in the future the solder joint will once again reopen, and eventually one or more transistors will short.

PlainBill
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Old 02-24-2010, 04:16 PM   #17
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Default Re: HP 1955 Monitor Repair Guide

I think a majority of these failures are from bad solder joints on the transformers.

I always wick all of the transformer pins and reflow with fresh solder. The two gray caps in between the C5707s and the two inductors in the inverter circuit. This gets done automatically whether or not the joints look suspect for insurance.

Afterward I go carefully over the whole board with a magnifier and touch up any remaining suspect joints.

It does sound like an inverter side issue the flash is the inverter trying to power up it sensing a fault and shutting down.

Loose CCFL connections or a bad joint on the inverter board as PlainBill said. It's the only explanation if it magically rectified itself.

Last edited by Krankshaft; 02-24-2010 at 04:20 PM..
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Old 02-24-2010, 08:20 PM   #18
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Default Re: HP 1955 Monitor Repair Guide

I agree with Krankshaft. I usually reflow all of bad solder joints around the inverters when I fix HP 1955. If you take a look at the solder joints of inductor in the inverter ciurcut, you can see a circular crack around solder pin. Replacing parts without reflowing bad solder joints doesn't fix the failure.
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Old 05-03-2010, 11:53 AM   #19
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Default Re: HP 1955 Monitor Repair Guide

Is there anyway i can replace the inverter board, seems i can't get a good part number. the board has BENQ 4H.L1A02.A41 on it. but can't get a hit from Google or alternative part number. The CN804 connector to the Back lite is completely burned up thru the board itself, and has quarter inch hole in it. Before i dis-assembled it would flash picture then quit. Could see picture on the display with a flashlight. So don't know what all could be involved and woudl rather just replace the complete board if possible.
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Old 05-06-2010, 11:59 PM   #20
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Unhappy Re: HP 1955 Monitor Repair Guide

I need some assistance or advice.

I have that same monitor here however where the 4431 MOSFET chip is located the Contacts are actually burned off on one side where the 2 center pins for the chip are located at.

The right side closest to the signal transistors are mounted close together but are staggered and not mounted side by side like the one near the top or back of the Power board. It looks like the surge was so bad that it burned pretty much through all copper contacts on that side of the board. It is just black like charcoal no matter how much I scrap the black soot (in this case might be the copper itself.)

1. Should I junk it and get a new monitor at this point?

2. Attempt the repair anyways with help from you guys?

Thanks in advance for any advice.
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