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Old 05-20-2010, 09:06 PM   #21
jnfurst
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Default Re: HP 1955 Monitor Repair Guide

I also have come across the same monitor, and I believe I have the same severity as Krankenshaft's second case. I will attach the pictures and maybe people can point stuff out. It is obvious that the FTE is just ruined, and I am unsure about the transistors however they don't look good. What else should I worry about or test? How should I test the transistors to see if they are performing as expected with the blown FTE? I want to know what parts to order so I can add them all in one order.
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Old 05-20-2010, 11:25 PM   #22
jnfurst
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Default Re: HP 1955 Monitor Repair Guide

I tested the transistors with a diode test on with my multimeter test attached and found the transistors near the center of the board (Q815 and Q816 on A02.A00) were shorted. So I will plan to replace those along with the FTE. Is there anything else I should check before I purchase these parts

http://mechatronics.mech.northwester...ulti-diode.gif
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Old 05-21-2010, 08:37 AM   #23
Bobdee
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by jnfurst
I tested the transistors with a diode test on with my multimeter test attached and found the transistors near the center of the board (Q815 and Q816 on A02.A00) were shorted. So I will plan to replace those along with the FTE. Is there anything else I should check before I purchase these parts

http://mechatronics.mech.northwester...ulti-diode.gif
Hello
If you read Krankshafts post 1 and 7 of this thread it tells you everything you need to check and also everything you need to do to get the Inveter working again
??? Can you tell us if the 5 Amp Inverter fuse PF701 failed or is it still good ???
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Old 05-21-2010, 08:44 AM   #24
jnfurst
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Default Re: HP 1955 Monitor Repair Guide

I will go through all of his steps when I get home from work and have the board in front of me again. However I don't remember the fuse to have any visible damage, along with before I tore the monitor apart the back light would flash on for about 1-2 seconds at startup before turning off, so according to Krankenshaft's post "wouldn't even get a backlight flash" had the fuse been blown, makes me think that it survived, however I am unsure how to fully check it besides for just visually.
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Old 05-21-2010, 10:54 AM   #25
Bobdee
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Default Re: HP 1955 Monitor Repair Guide

Jnfurst
You are totally correct if your B/Lights do flash on for a second then your fuse is OK (my silly error)
The reason why I asked was there is a lot of damage to your board, my thoughts are that this fuse should have opened before all of that failure occured on this particular PSU, all the other Benq boards that i have done in HPs, DELLs, BENQs have the usual transistor and fuse failure with plenty of cracked solder joints they have a 3 amp fuses yours has a 5 amp
This fuse seems to be over rated as Krankshaft has mentioned in his post earlier,I was wondering if he reads this, has he yet tried lowering fuse to a 4 amp with any success

Last edited by Bobdee; 05-21-2010 at 10:57 AM..
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Old 08-30-2010, 07:08 PM   #26
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Default Re: HP 1955 Monitor Repair Guide

Fantastic guide! A big thanks to Krankshaft for all the hard work! I managed to resurrect an HP L1955 from e-waste thanks to this guide alone.

I wanted to post the links I used to order my components as I depended on only 2 sites: eBay and Digi-Key and spent around $10 total incl shipping to get this monitor back up and running.

Parts list & replacement order:

1) Capacitors
(3)1000uF 25V 105C Radial
(1)1000uF 10V 105C Radial
(2)220uF 35V 105C Radial
Digikey Capacitor Filter Search - http://search.digikey.com/scripts/Dk...dll?Cat=131081
under $1 per cap

2) (2)2SC5707
eBay - http://shop.ebay.com/?_from=R40&_trk...All-Categories
pack of 4 is around $3.59 w/ free shipping

3) (1)SI4431BDY
Digikey - http://search.digikey.com/scripts/Dk...ords=SI4431BDY
around $1.38 ea, first class USPS was just over $2 for the entire order at Digikey

**If needed, the NPN and PNP bipolar transistors can be found at Digikey

I ordered extra parts just in case and actually didn't end up needing them. It took me around an hour to solder and replace these components, the overall disassembly and replacement was simple enough. Use this manual for those who need the additional guidance: http://www.hp.com/hpinfo/globalcitiz...6112713913.pdf
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Old 09-03-2010, 01:59 PM   #27
garrysoh
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Default Re: HP 1955 Monitor Repair Guide

Hi all,
Does anyone know the part number for the zener protection diodes for the FET? Help is much appreciated...
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Old 09-03-2010, 11:37 PM   #28
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Default Re: HP 1955 Monitor Repair Guide

very informative.
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Old 09-04-2010, 03:14 AM   #29
Krankshaft
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Default Re: HP 1955 Monitor Repair Guide

It seems I made a mistake based on my voltage drop readings the FET protection diodes are Schottky diodes since they have a 0.150 voltage drop. Actually makes more sense due to the high frequency inverter supply.

They're there to prevent the source to drain junction from being reverse biased. When the inverter shuts down and the magnetic fields in the inverter transformers collapse. They will generate a tiny high voltage pulse in opposition to the normal current flow (through the FET) when the inverter is on.

This pulse can exceed the reverse breakdown voltage of the FET and can potentially short the source to drain junction. Thereby killing a brand new component instantly. The diode cuts down this pulse so it won't damage the FET. This is standard procedure for switching inductive loads like transformers with transistors.

A standard Schottky should be able to replace it. If it doesn't read around 0.150 in one direction it's shot.
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Last edited by Krankshaft; 09-04-2010 at 03:35 AM..
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Old 09-13-2010, 05:30 PM   #30
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Default Re: HP 1955 Monitor Repair Guide

First time user, Very interesting sight! I also have an HP 1955 monitor and have the same problem, switches on for a second then blank. I read all of Krankshaft advice and found it very helpfull. But is there a good and accrate way to cheack the signaling transistors plus the backlight ON/OFF 4431 Fets on board with a good volt meter. Or do I have to remove them to do a proper test? bacause I get the same reading on all four on board.
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Old 09-13-2010, 05:34 PM   #31
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by Ron K View Post
But is there a good and accrate way to cheack the signaling transistors plus the backlight ON/OFF 4431 Fets on board with a good volt meter. Or do I have to remove them to do a proper test? bacause I get the same reading on all four on board.
You can test them "in circuit". If they look like they are shorted, then remove them to verify.

What readings are you getting?
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Old 09-14-2010, 12:23 PM   #32
Ron K
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Default Re: HP 1955 Monitor Repair Guide

Hello
they don't seem to be shorted, signaling on transistors I get 6.5 to 7, on the backlight ON/OFF 4431 Fets I get full 7. Thanks
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Old 09-14-2010, 12:38 PM   #33
Krankshaft
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Default Re: HP 1955 Monitor Repair Guide

Diode check function should only get a reading in one direction. From base to collector swap the probes then from base to emitter swap the probes. Use the same process to check all four of the C5707s.

On the FETs from gate to source swap the probes from gate to drain swap the probes. They like to short from source to drain when they fail.
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Old 09-14-2010, 01:13 PM   #34
retiredcaps
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Default Re: HP 1955 Monitor Repair Guide

Quote:
Originally Posted by Ron K View Post
Hello
they don't seem to be shorted, signaling on transistors I get 6.5 to 7, on the backlight ON/OFF 4431 Fets I get full 7. Thanks
What setting is your multimeter on? ohm? diode?

6.5 to 7 what. 6.5 to 7 ohm? Kohm? Mohm? 6.5V?
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Old 09-15-2010, 08:22 PM   #35
Ron K
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Default Re: HP 1955 Monitor Repair Guide

It is set on diode.
Thanks
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Old 09-16-2010, 06:45 PM   #36
Ron K
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Default Re: HP 1955 Monitor Repair Guide

If one of my 4431 Fet is burnt, Do the 2 signaling transistors usually go with 4431 fet?
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Old 09-17-2010, 03:16 PM   #37
jsaracinohp
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Default Re: HP 1955 Monitor Repair Guide

Did you ever find out what the part number is for the zener diodes?

Thanks
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Old 09-17-2010, 03:27 PM   #38
Krankshaft
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Default Re: HP 1955 Monitor Repair Guide

Find some generic surface mount Schottky diodes and install them in it's place.

After closer inspection and testing on a spare board I had they're Schottkys not zeners.

Are you sure it's blown? You may get a real high reading on reverse bias due to other components in circuit. If you get .150 -.170 on forward bias the component is fine. Only replace it if it's shorted.

To the other poster:

Sometimes yes and some times no check them with the diode check of a DMM if they got taken out the BJT will read shorted at one of the junctions.

If the FET isn't blown then the signaling transistors will be fine. This would only happen though if the fuse was fast enough to protect them unfortunately the FETs usually open before the fuse can react. I've also been in situations where the FET and fuse blew at the same time.

I had one monitor by some miracle nether the fuse or either FET blew replacing the C5707s fixed it. I have no idea how that happened.

Last edited by Krankshaft; 09-17-2010 at 03:33 PM..
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Old 09-21-2010, 01:35 PM   #39
Ron K
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Default Re: HP 1955 Monitor Repair Guide

Thanks for your help! I have another question, The HP L1955 models has an upgrade inverter board from the HP 1955 models, it looks a lot easyer to fix and work with it does't seem to have the 4431 fets and other components. I'm wondering if I could use the new upgradeed inverter L1955 models and put it into the 1955 model? Would you also know where I could purchase those new model inverter boards? I phoned HP and all I get is transferd from one department to the next with no results.
Thanks
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Old 09-21-2010, 02:28 PM   #40
Krankshaft
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Default Re: HP 1955 Monitor Repair Guide

The HP 1955 or L1955 are the same thing.

If the inverter doesn't have the FETs it must have them in a through hole package. Which would make that an older revision of the board.

These boards are pretty easy to fix only basic troubleshooting and tools required. With all the info given here it should be even easier.

Last edited by Krankshaft; 09-21-2010 at 02:34 PM..
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