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.
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.
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