Another bad HP 1955 Inverter... probably

[ofcsilencer]

I have been watching the forums for some time now and I have been able to fix a few screens with the guidance I picked up here.

The screen I am having problems with is a benq 48.l1a02.a41, I think it's a version 94v-0. I saw several people had articles about this screen and how to fix it.

The problem I am having is the screens backlight kicks off after a second or two right after powering on.

I looked at the capacitors and they are brands that I remember people complaining about (elite for example). None of the caps seem bubbled but I guess that isn't really required.

I also heard people talking about the ccfl bulbs actually being bad or shorted, don't know if this would help but I tried running them one at a time to see if I got better results and no.

Would a capacitance meter help check these? If so I'll buy one since ill probably use it more often.

Strange thing I noticed with the 5707 transistors? I test them with a meter and I get one resistance reading and then the meter zeroes out. Is that normal?

Im not an electronics genius but I do have a basic knowledge of the parts and have taken a class on electronics. Spent a few months repairing overheads and other SIMPLE electronics but that's poor experience.

Any ideas?

[Krankshaft]

Re: Another bad HP 1955 Inverter... probably

It definitely sounds like a shorted C5707 to me.

You should be using the diode check function and not the resistance function on the meter.

From base to collector and reverse the probes you should only get a voltage drop in one direction. The same from base to emitter.

They can be measured in circuit (to determine which pair has a bad transistor) once you've found out which pair is bad. Remove and replace both transistors even if the other one is fine.

[ofcsilencer]

Re: Another bad HP 1955 Inverter... probably

ok i replaced all 4 just to be sure, meter i have didn't allow me to test them in circuit. but its still doing the same thing.

it will power up the display for a few seconds then when the dialog switches to check vga cable it then doesn't light but yet the light is green.

not sure where i should look next

[Toasty]

Re: Another bad HP 1955 Inverter... probably

I am not familiar with this particular board, but it sounds like it's tripping the over current protection.

[PlainBill]

Re: Another bad HP 1955 Inverter... probably

ok i replaced all 4 just to be sure, meter i have didn't allow me to test them in circuit. but its still doing the same thing.

it will power up the display for a few seconds then when the dialog switches to check vga cable it then doesn't light but yet the light is green.

not sure where i should look next

This is the typical symptom of the backlight inverter detecting a fault condition and shutting down. The problem is determining the nature of the fault and correcting it. As far as I know, it could be a CCFL drawing too much current - or too little current. It could be a bad cap, it could even be a resistor or cap that is out of tolerance.

I've got one with the same problem. I'm taking the approach of first replacing all questionable caps (the present caps were described as 'garbage'). That will cost less than $5 for the parts, and is a heck of a lot easier than replacing the CCFLs. After that - well, we'll see.

PlainBill

[EGuevarae]

Re: Another bad HP 1955 Inverter... probably

I have been watching the forums for some time now and I have been able to fix a few screens with the guidance I picked up here.

The screen I am having problems with is a benq 48.l1a02.a41, I think it's a version 94v-0. I saw several people had articles about this screen and how to fix it.

The problem I am having is the screens backlight kicks off after a second or two right after powering on.

I looked at the capacitors and they are brands that I remember people complaining about (elite for example). None of the caps seem bubbled but I guess that isn't really required.

I also heard people talking about the ccfl bulbs actually being bad or shorted, don't know if this would help but I tried running them one at a time to see if I got better results and no.

Would a capacitance meter help check these? If so I'll buy one since ill probably use it more often.

Strange thing I noticed with the 5707 transistors? I test them with a meter and I get one resistance reading and then the meter zeroes out. Is that normal?

Im not an electronics genius but I do have a basic knowledge of the parts and have taken a class on electronics. Spent a few months repairing overheads and other SIMPLE electronics but that's poor experience.

Any ideas?

Welcome to the forums

You say you've repaired some units so far. Can you get another LCD Panel/set of lamps to test with this inverter? That would rule out the CCFLs being bad.

Did you notice any red/orange/pink hue when they start up?
Also, did you check the removed transistors to see if they have failed?
One last thing. Even if the four lamps are OK, if you only connect one (or connect three or just two), the inverter would detect that as a fault and will shut down itself.

[FIXITNOW]

Re: Another bad HP 1955 Inverter... probably

these is a small fuse that blows when you have a shorted c5707 you will need to replace it

[ofcsilencer]

Re: Another bad HP 1955 Inverter... probably

i did try testing one lamp at a time, and i did get the result mentioned above, but each bulb did light, i was curious to see if all 4 would light and they did but then it would shutoff.

i do have another screen that a botched the replacement of some c5707s and destroyed a trace (handy i ordered alot of c5707s) so i might be able to use the bulbs from it. it is also a benq board.

i tried testing the c5707's , due to my poor abilit ywith a soldering iron i ripped the pin out of 2 of them, but the ones i was able to test all measured like 292 or something on the diode reading. not quite sure what that means.

i was looking at replacing the capacitors, but where would you recommend ordering them. i keep looking at mcmelectronics (out of habbit), and i read somewhere i should use esr caps.

also what is this fuse i should be looking for?

[Bobdee]

Re: Another bad HP 1955 Inverter... probably

Hello
The fuse you are after is on the right side of the big transformer in the picture of this link, its black and resembles a diode

https://www.badcaps.net/forum/showpo...58&postcount=3

the fuse number is PF703 on the board, the fuse in the picture is not the correct Amp, so go by what your Amp is

[PlainBill]

Re: Another bad HP 1955 Inverter... probably

The caps can be ordered from either Mouser or Digi-Key. Neither company has a minimum order, but I prefer Digi-Key. Their minimum shipping charge is less than $2.50; Mouser charges nearly $7.00. This makes quite a difference on a small order.

PlainBill

[EGuevarae]

Re: Another bad HP 1955 Inverter... probably

Hello
The fuse you are after is on the right side of the big transformer in the picture of this link, its black and resembles a diode

https://www.badcaps.net/forum/showpo...58&postcount=3

the fuse number is PF703 on the board, the fuse in the picture is not the correct Amp, so go by what your Amp is

Good one Bob. I didn't see it in the picture until you told where it was located. That would be the one with the problem if the lamps were dead, but he says that they lit and then shut off, and when I've ran into open fuses on inverter area, the lamps didn't lit at all. Anyone here has worked with this specific board before?

[Bobdee]

Re: Another bad HP 1955 Inverter... probably

. Anyone here has worked with this specific board before?

I have done a couple ,problems is always 2sc5707 + the fuse, and a lot of resoldering all over board, in particular components on h/sinks, but more so the 4 inverter transformers, is a must for resoldering ,loads of ring cracks, check the whole board for bad solder.

edit look at that picture in that link i gave earlier the board is brown by transformer and the 2sc5707 are pulled , usual problem

[ofcsilencer]

Re: Another bad HP 1955 Inverter... probably

ill give it a shot retouching the solder on the HV transformers. probably order the caps and fuse this weekend if that doesn't work.

thanks!

[EGuevarae]

Re: Another bad HP 1955 Inverter... probably

I have done a couple ,problems is always 2sc5707 + the fuse, and a lot of resoldering all over board, in particular components on h/sinks, but more so the 4 inverter transformers, is a must for resoldering ,loads of ring cracks, check the whole board for bad solder.

edit look at that picture in that link i gave earlier the board is brown by transformer and the 2sc5707 are pulled , usual problem

You're right. Now that you mention it, it makes sense. Thanks for the info, Bob.

[HOODLAM]

Re: Another bad HP 1955 Inverter... probably

Hi Guys.

I also have an HP1955 which gave problems, replaced the rf9024n transistors(high ferq switch) x4. Screen been running for over a year now with now problems.if you search for previous thread on bad caps where I spoke to Kranshaft and Davmax regarding the repairs and checks. It sounds to me as if could be the 9024n's which are "breaking down" on the starting current!

[SimonNZ]

Re: Another bad HP 1955 Inverter... probably

I have just recieved a dead HP 1955 monitor that once opened shows a spectacularly blown up c5707. Seems like a very common theme!

[Bobdee]

Re: Another bad HP 1955 Inverter... probably

I have just recieved a dead HP 1955 monitor that once opened shows a spectacularly blown up c5707. Seems like a very common theme!

Hello Simon
You may have a bigger problem here by what you say above,
2SC5707,s do not show any visual signs of damage even when shorted, when you examine them in my experiences, and I have never found a thread that had posted on that either.
Best get some help, post Pictures of top and bottom of board, a lot of the lads would like to see this, and they will help you out as well.
bob

[EGuevarae]

Re: Another bad HP 1955 Inverter... probably

Hello Simon
You may have a bigger problem here by what you say above,
2SC5707,s do not show any visual signs of damage even when shorted, when you examine them in my experiences, and I have never found a thread that had posted on that either.
Best get some help, post Pictures of top and bottom of board, a lot of the lads would like to see this, and they will help you out as well.
bob

Never seen one "exploded" myself, so the cause might be something around it. Pics would help as Bob said.

[SimonNZ]

Re: Another bad HP 1955 Inverter... probably

All right, bugger! Well here is the full carnage from this component letting out it's smoke. Any ideas? So you think if I replace it the same thing will happen again?

[SimonNZ]

Re: Another bad HP 1955 Inverter... probably

Just noticed - how do you like the misspelled Hight Voltage right next to it haha