inverters with C5707 benq hp 1940
 

I just found out about badcaps and am amazed at the amount of information here. I have been going through all the old posts and seeing how the experiences of other tech compares with mine.

One of the subjects I have seen discussed a lot are the inverters based on the C5707. Often these are found in 4 lamp combo inverter / power supplies that have a simple tank circuit with 2 C5707s driving 2 transformers.

Most often these boards are made by Benq and are found in Dell and HP 17",18" and 19" monitors.

My experience has been that one C5707 blows and shorts the power for the inverter. Then I replace the pair of C5707s and recap the whole thing and maybe half the time the same 2 transistors blow out after a few hours of use.

Some people have suggested this indicates a bad transformer or bad solder joints on the transformer. Almost always the solder joints look fine to me, but I flux them and touch them up anyway.

As far as testing a transformer is concerned, the only way I know is to see if the lamp attached to the transformer is nice and bright and fires up quickly. They always do.

I just pulled a handful of transformers and the primaries always measure dead short (as far as my meter can tell) and the secondaries measure 180 ohms or so.

Does anyone know a better way to test transformers?

Are there any other ideas out there of why this happens?

The attached pic is the power/inverter out of an HP 1940. One thing that might be a clue: Look at the brown discoloration on the circuit board. This is much darker and larger on the side that blew out. This is the area of the circuit board that has resistors (4 470 ohm) that are in series with the primaries.

Any thoughts, fellow geniuses? (genii?)

-Michael

Re: inverters with C5707 benq hp 1940
 

Well the best way to test transformers IMO is with Bob Parkers Ring tester. It recently went back into production you can find it here.

http://www.anatekcorp.com/bluert.htm

Re: inverters with C5707 benq hp 1940
 

What's under the brown spots, those 4 resistors?

Also, Toasty suggested me to check those blue High voltage caps at each output of the 4 CCFLs in another thread. Did you check those? Cold solder/loose, or damaged?

Also, on the infamous Dell E172FPb thread someone suggested replacing those 5707s with high gain transistors (he even wrote the part numbers). I don't know if that would apply here.

Re: inverters with C5707 benq hp 1940
 

I opened up one of those L1940s last weekend. On mine one of the Schottky barrier diodes (a SS24 SMB 40V 2A) at D803 on the solder side of the board was shorted. I haven't received the parts I've ordered, so I can't say anything about success yet. I'll be replacing the two C5707s, one FQU11P06 and all capacitors. I'll also replace both the Schottky barrier diodes, because even the one who still measured OK had heat damage to the package. On my board the two C5707s at Q815 & Q816 had blown with a splatter on to the board.

Re: inverters with C5707 benq hp 1940
 

The poly caps are .33uf and they look and measure nice. The brown spots are on the opposite side of the PCB from the resistors.

I'll have to get one of those ringer things. My friend who is an old-school CRT guy told me I need to get one too, so I suppose it is time.

sofTest: What is the FQU11P06? Is that the FET that drives the C5707s?

Re: inverters with C5707 benq hp 1940
 

Yes, the one at Q812 on my board. I've read Benq have used versions of this MOSFET from other manufacturers also.

Re: inverters with C5707 benq hp 1940
 

Well, I did the work on my monitor last night, and have so far run for about 9 hours. I've now set it up to run the burn in, in the service menu. When I started it, I noticed that it had about 73 hours of burn in from before. Anyone knows if this is the normal manufacturers burn in time?

Otherwise everything worked out as planned in post 4. One note though, when I re-flowed the transformer legs, 3 or 4 of them had so little solder, that I had to add solder for a proper joint to form. I had during examination of the board noticed fine lines in the solder around some of the transformer legs, but I'm not sure if those were the same, as I didn't take notes of them.

Re: inverters with C5707 benq hp 1940
 

Maybe that's the reason the "007"s went down the drain... :eek:

Re: inverters with C5707 benq hp 1940
 

Yes, with the experience others have had with the soldering of transformer legs on Benq inverter/power boards, that is a strong possibility. If it hadn't been for the observations others had done, I probably wouldn't have checked them out before the monitor failed again. The monitor is still working fine on the burn in.

Re: inverters with C5707 benq hp 1940
 

I have a HP L1940 LCD monitor, with a BenQ 48.L1G02.A01 inverters/power board. Seems it over heated some components and fried them. I would like to replace them but can't get any numbers. It looks like there were 2 FETs (red square boxes) that were completely disintegrated, and the gray rectangular capacitor(blue box) which I think is at C825. If someone, anyone could help with some info. I will try to load pics of the board.
Thanks,
Dan

Re: inverters with C5707 benq hp 1940
 

Hello cova33
Its in a bad way, never seen one so bad, maybe your best bet is buy a new board, heat damage to C802+C832+Q812+L802 looks possible as well, you also have suspect transformers damage to consider, also bad CCFLs tubes may have been the initial cause of the problem,
bob

If you are in the US, lcdrepair does a new board for 40usd, but you would need to check out CCFLs first for it to be viable,

Re: inverters with C5707 benq hp 1940
 

Hi,

Do you know where I can buy the BENQ inverter 48.L1G02.A00 for my HP 1940 - I live in the UK.

I spent ages looking for a fault, but think it will be easier to replace the board.

Cheers.

Re: inverters with C5707 benq hp 1940
 

I have also a problem with hp1740: replaced 3times q808 also q809 only q808 gets faulty, diode 602 is damaged can someone give me information to replace that one. Is the diode the reason for failure of q808?

Thx

Re: inverters with C5707 benq hp 1940
 

After replacement of q808 it works for 3days..

Re: inverters with C5707 benq hp 1940
 

Did you resolder the transformer solder points?

Re: inverters with C5707 benq hp 1940
 

I just tipped it with my iron before. Now i placed my last good c5707 & removed the solder on the transformer and used new solder to resolder it!

Now my screen is flashing white, black, white -- no image + powerled flashing green <> orange

And the c5707 is broken (short circuit)

Re: inverters with C5707 benq hp 1940
 

I found this article that explains, you have to get some good replacement with good Hfe value.

http://ezinearticles.com/?Problem-in...tor&id=1049341

I used now 4 x c5705 from another screen & hope that it will work longer then 3days on normal use (max 5h/day)

Re: inverters with C5707 benq hp 1940
 

hi everyone.
I have also the L1940 HP monitor, but the problem w/ mine is that the T601 transformer is whining (pulsating whining when pluged in) and when i turn the power on the sound of the whining changes. Also there are horizontal strips running on the left side about 1.5 inches wide from top to bottom (better seen using 65Hz refresh rate). Is the transformer dying an these things be releated.