HP 1955 Monitor Repair Guide

[alexanna]

Re: HP 1955 Monitor Repair Guide

What series are the Rubycon's? It will be a 2-3 letter designator stamped on the cap. Something like: YXG, ZL, ZLH, etc.

like jetadm123 asked.
Add some solder to the gray caps,they have a tendency to come loose.

[krantz]

Re: HP 1955 Monitor Repair Guide

YES! We found the problem. I desoldered the gray box caps then resoldered them. I mounted the bord on the back of the LCD, started it and it worked!

Im gonna give it some time off and stuff till I feel sure that its really fixed.

I just want to say Thank You to all of you for helping me out debugging the board. I love this place!

[Krankshaft]

Re: HP 1955 Monitor Repair Guide

Remember way back in post 64 when I said to resolder those caps and the components around them ?

You're lucky it was caught early a joint failure there would eventually blow out one or more driver transistors and sometimes On / Off FET and inverter fuse. Sometimes the fuse blows other times it doesn't when there's a short. A fuse of too high an amperage was probably sized from the factory.

Later revisions got rid of the 5 amp fast blow fuse and replaced it with a 3 amp slow blow. Probably in hopes of it actually opening a greater majority of the time if there is a fault.

I always go over the entire board with a magnifier but focus mostly on the inverter section to check for any other suspect joints when I repair these boards.

I recently repaired one for a member here who said the solder joints looked fine. The moment I got it in my possession one of the joints on one of the gray caps was completely cracked. When I wiggled it I could see the lead move. In this boards case it was too late and the inverter drivers were blown. But you caught it in time.

[JOE765]

Re: HP 1955 Monitor Repair Guide

This thread is a great source. I am trying to repair my HP 1955 but I cannot identify the diode that is in the attached image. I believe the color code is brown gold but I am not sure. I would appreciate it if someone could point me in the right direction to identify this diode.
Thanks.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

Hi Badcaps, this is my first post here in Badcaps.
I bought a faulty HPL1955 and plugged it in after having it at home for one month without using it and it worked fine for one month. I disconnected the monitor from my computer and changed a display cable but the monitor did not work then.
I left the monitor for a few weeks then replaced one suspect looking 1000uF 25V capacitor that had a very slight but definite bulging top and the three power plug connector solder joints to the motherboard were all cracked so I resoldered them with flux and plenty of new solder. The monitor worked ok again for a few days until it stopped again. Then the monitor would power on ok and flash a good picture then the screen will go black.
To my short experience with LCD monitors this meant bad capacitors and that is about the limit of my LCD monitor repairs so far.
I read this thread looking for what I could try to do to repair the monitor and will read some more now and I will definitely look to buy a capacitor meter and with my budget it will be a cheap one for now.
I will read through this thread again to look for clues of what to try next. Any advice as to what to look for with the damage I mentioned above in mind will be gratefuly received.

[Krankshaft]

Re: HP 1955 Monitor Repair Guide

Cracked solder joints in the inverter section cause these displays to die especially on those two gray caps between the C5707 driver transistors. I always find at least one cracked joint on one of those two caps. Resolder both gray caps between the drivers, both inductors in the inverter section, and all of the inverter transformer pins. The rest of the board then has to be checked over with a magnifier for any other bad joints.

Unfortunately these bad joints blow out the drivers if the joints aren't repaired in time the shorted driver can then blow a 4431 Backlight On / Off FET due to an over current condition.

After the cracked joints are repaired get a DMM and check the drivers and the On / Off FETs for shorts or opens. You can get the pinouts for the transistors and ICs in the attachments on my first post.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

Wow, I did not look carefully enough. One of the grey caps does have a cracked solder joint and is more evident when I gently wobble the cap.
From seeing the bright display I noticed when I first plugged the monitor in and used it for about 4 weeks I would be completely confident the backlights all work well.
I will check the first post to identify the On/Off FETs and maybe get the things you posted about done today and post back later.
Thank you Krankshaft.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

I checked the board solder joints carefully this time looking around the joints instead of at only one side of the joints and found three more half cold solder joints which were on one of the transformers I think they are, the four transformer looking partsbetween the C5707 transistors and another broken solder joint like on the other grey cap. I resoldered all the recommended solder joints and the monitor is giving a no video signal message. This was the error message the previous owner told me the monitor was giving when he brought the monitor to me.

[PlainBill]

Re: HP 1955 Monitor Repair Guide

I checked the board solder joints carefully this time looking around the joints instead of at only one side of the joints and found three more half cold solder joints which were on one of the transformers I think they are, the four transformer looking partsbetween the C5707 transistors and another broken solder joint like on the other grey cap. I resoldered all the recommended solder joints and the monitor is giving a no video signal message. This was the error message the previous owner told me the monitor was giving when he brought the monitor to me.

If the monitor is connected to a working signal source (computer), check the voltage regulators on the signal board.

PlainBill

[nickBcap]

Re: HP 1955 Monitor Repair Guide

I have done something stupid and now need to replace two more parts on the board. I left the board plugged into the power and switched on and began unscrewing the board. A screw dropped onto the board and emitted two lots of sparks one after the other. Photos. I think the parts may be the only damage. the screw touched the two part and left a silver mark on the protection sheet betweeen the bottom of the board and the rear of the screen.
I will buy replacement parts later next week and I will test the C5707s later today.
The picture is of both sides of the two parts. There was a small browning on the motherboard around the pin which was nearest the screw that touched the two parts, so I removed that part as well as the burned part.

[Krankshaft]

Re: HP 1955 Monitor Repair Guide

It's rarely ever that simple. Where are those components from on the board?

By the looks of it they look like they came from the main power supply. If that's resistor was part of the startup circuit you may have blown the main power supplies PWM IC. It's a 4 pin DIP at the corner of the board.

Is the main fuse blown? If not replace the resistor and try powering it up preferably with a 120v bulb in series with the input so it will glow brightly if there are any other shorts.

The other component is an inductor hopefully it didn't get damaged.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

This is the first LCD monitor I am interested in repairing for myself. I am waiting to afford a cheap capacitor meter to test the capactitors after reading about dead capacitors being found on motherboards when thye do not appear to have an ybulge on the top.
I have not idea what the parts are and I have colosed the monitor for now but I will definitely come back for some advice when I have bought the capacitor meter.
No, the main fuse did not blow.
I will have to research to find what the parts are. Is there a schematic for htis monitor that I can look at. Can someone recommend a place to look to identify the parts used in elctronics or in monitors.
I repair things for a hobby and will repair for very low prices rather to keep busy than for profit.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

Hi, should I start a new thread about the L1955 monitor I want to repair or shall I continue in this thread?

[budm]

Re: HP 1955 Monitor Repair Guide

May I also add, those two gray caps are part of the Inverter output oscillator circuits (Royer Oscillator circuit), if its value went out of range, the Oscillator will be running at the wrong frequency and takes out the C5707 output transistors, check the value and leakage or bulging on the caps first otherwise you will take out your new transistors. A lot of HP and DELL monitors were made by BenQ.

[nickBcap]

Re: HP 1955 Monitor Repair Guide

Thank you for the tips and photos. I pickup all sorts of little electronic repair projects to keep myself busy and when I have a few dollars for replacement parts I start repairing. My interest drops on each project for a while but my interest is growing in electronic repairs. There is a sense of satisfaction I guess to have repaired something to make it work.

[ivank]

Re: HP 1955 Monitor Repair Guide

I have a bad HP L1955 monitor. I need to know how to get the plastic cover off. I removed the four scrws on the back and I removed the front bezel, but there are four connectors two on each side that are holding the back plastic cover. What do you do to release these?

Ivan

[selldoor]

Re: HP 1955 Monitor Repair Guide

[ivank]

Re: HP 1955 Monitor Repair Guide

Thanks Selldoor, I got it open and it has buldging caps.

Ivan

[nickBcap]

Re: HP 1955 Monitor Repair Guide

Thanks from me too Selldoor, I plan to repair a few HPL1955's sooner or later.

[Behemot]

Re: HP 1955 Monitor Repair Guide

I have a funny problem with this display. After swapping one 2SC5707 and changing caps it now works, but the backlight is on all the time. It happens even with signal board disconnected so the problem is on inverter somewhere.

Any tips?

ADD// found also one bad 2SJ598…that's probably it. Do you think I can use SFP9644 as a replacement? It has higher Rds(On), but it's also bigger and will be cooled better…