Hp 1940 flicker

[terminatorX]

I have one hp 1940 (Benq 48.L1G02.A00) that when plugged to power starts flicker rather rapidly and just keeps doing that. To mention not connected to PC. The power led is on.
It's not originally my screen so I can't really say anymore in the symtoms department and I removed the power board from the inside now. Nothing really seems to be blown or damaged, no bulging of capacitators. Is it the usual C5707 transistor problem?

[selldoor]

Re: Hp 1940 flicker

We cant tell if a transistor is faulty especially when we cant see it.

I suggest you read this https://www.badcaps.net/forum/showthread.php?t=14068 , read the 2 secs to black guide carry out the tests mentioned and post your results.

[terminatorX]

Re: Hp 1940 flicker

So i did some small reading and the guide seems to be really good!
I'm to tired to read it through and eventually do some testing..guess i'll do that later. But I tested the screen more properly just to make it easier for you to do some speculation. The flicker mode is more like...there is a picture of the desktop and it flashes quickly and then it's like one second delay (black screen) and then the same flash and I had it on for like 20 seconds so it seems it goes on like that indefinitely. The power led stays lit and it's possible to hold the button down to power down the monitor.

[retiredcaps]

Re: Hp 1940 flicker

You must, mandatory, resolder or reflow every solder joint circled in red regardless whether it looks good or not.

The Benq boards all suffer from bad/poor solder joints.

Retest after resolder/reflow.

[Hunt]

Re: Hp 1940 flicker

My HP 1940 for a long time made a loud click now and then, but the screen never showed signs of a problem; in fact I did not realise it was the screen till I move office and the only common item was the screen.

The screen is slow to shutdown the monitor dialogue that says what connection it has to the PC. So much so that it is an annoyance as I have to move the page around to see what is under/obscured by the dialogue.

Now the screen has developed a flickering on and off fault similar to the title of this thread.

The flicker is not occurring at this moment but seems to be related to the amount of time the monitor is switched on.

Since it is my second monitor that is used as an electronic document holder; I sometimes turn it off when not needed. Switching the screen back on seems to trigger the flicker.

Does all the above sound like a bad cap fault?

P.S. is it ok to add to this thread or should I have created another one?


(Thanks to this forum, I am still using a ViewSonic that I repaired a year ago)

[PlainBill]

Re: Hp 1940 flicker

My HP 1940 for a long time made a loud click now and then, but the screen never showed signs of a problem; in fact I did not realise it was the screen till I move office and the only common item was the screen.

The screen is slow to shutdown the monitor dialogue that says what connection it has to the PC. So much so that it is an annoyance as I have to move the page around to see what is under/obscured by the dialogue.

Now the screen has developed a flickering on and off fault similar to the title of this thread.

The flicker is not occurring at this moment but seems to be related to the amount of time the monitor is switched on.

Since it is my second monitor that is used as an electronic document holder; I sometimes turn it off when not needed. Switching the screen back on seems to trigger the flicker.

Does all the above sound like a bad cap fault?

P.S. is it ok to add to this thread or should I have created another one?


(Thanks to this forum, I am still using a ViewSonic that I repaired a year ago)

The flickering could be due to bad caps. The only way to find out is to open the monitor up.

I prefer to see all problems with a single monitor in a single thread. This gets confusing only if two unrelated faults are being troubleshot at the same time for two people.

PlainBill

[terminatorX]

Re: Hp 1940 flicker

You must, mandatory, resolder or reflow every solder joint circled in red regardless whether it looks good or not.

The Benq boards all suffer from bad/poor solder joints.

Hah interesting. Not sure about the reflow part. Should I get a heating gun for that? If so what is the best temperature? If I do decide to just resolder,
I think it would be smart testing the transistors outside the board maybe. If I would use my DMM what would you expect for ohms for working transistors?
Forgott I have I diode checker but not sure what to expect...
Maybe good to compare between these to check the DMM ability...

[selldoor]

Re: Hp 1940 flicker

I would just re solder the joints, you can add a little solder and flux to get the process going rather than just working on the old solder. Try and practice on something first if you have any old boards.

Just test the Transistors in position as follows ( and as you were going to read?):
2) Shorted Transistors/MOSFETs
------------------------------------

On some LCD brands (example: Benq) the transistors (C5707) are sometimes shorted. Transistors are usually marked with a "Q" designation.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for shorted transistors "in circuit" (with power off and unplugged)

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any ohm reading is less than 30 ohm you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

Only remove if any reading is less than 30ohm
Post the results and someone will say if they are good or not.

The picture of the back of your board is not really clear enough - can you retake and repost it

[terminatorX]

Re: Hp 1940 flicker

Really good help thanks!
Is there a particullar part of the board you want to see? Maybe I can concentrate in doing that part more clearer?

[selldoor]

Re: Hp 1940 flicker

Just the whole board - there are members on here who can spot faults that people with the boards in their hands cant see. It needs to be a bit sharper than the one you posted and without the blue dots? I was looking at the top left where there is some browning. If anyone wants a close up of a section they will ask.

[Hunt]

Re: Hp 1940 flicker

I have one hp 1940 (Benq 48.L1G02.A00) that when plugged to power starts flicker rather rapidly and just keeps doing that. To mention not connected to PC. The power led is on.
It's not originally my screen so I can't really say anymore in the symtoms department and I removed the power board from the inside now. Nothing really seems to be blown or damaged, no bulging of capacitators. Is it the usual C5707 transistor problem?

Hi TerminiatorX,
How did you get at the circuit boards?

I have the plastic back and front off and the screws holding the rectangular box with the powers socket and other sockets removed but the rectangular box will only lift a few millimetres.

It seems to now be held in place by a heat sink that is attracted to the removable box ;(

If I undo the other screws that hold the flat metal plate to the screen there are two short wires that go up to the metal box I am trying to remove.

The above seems like catch22 how did gain access to the board and caps?

[Hunt]

Re: Hp 1940 flicker

Well I lifted the tape off the wires holding the bundle to the panel going to the long connector at the top of the panel and removed the long slim plug.
That gained me nothing.
Going back to the rectangular metal box and peering in side to see how the heat sink was attached; the rectangular box fell off! I was not using enough force it seems, as the only thing holding the box on was some single sided adhesive tape that is attached to the (power transistors?).

Now I can see if any caps are bulging

[terminatorX]

Re: Hp 1940 flicker

Hello Hunt!

Was just about to try to help you Hunt =) Se you managed it yourself.
As I understand you are far from solving the problem buy just looking at bulging caps.

Over to my pictures. It's not that easy doing clear views since there is alot of reflection...

[Hunt]

Re: Hp 1940 flicker

Here are my photos of my faulty board...

[Hunt]

Re: Hp 1940 flicker

Here are my photos of my faulty board...

I cannot see any bulging caps so I am now not sure how to progress

What should I do next?

Thanks.

P.S. I had to crop photos to get them to upload; hope they are ok.

[Hunt]

Re: Hp 1940 flicker

Hello Hunt!

Was just about to try to help you Hunt =) Se you managed it yourself.
As I understand you are far from solving the problem buy just looking at bulging caps.

Over to my pictures. It's not that easy doing clear views since there is alot of reflection...

You board looks the same as mine with the same hot areas.

[selldoor]

Re: Hp 1940 flicker

Have you both resoldered the power boards?
There is little point in doing much else until you do.

Can either of you make out the part number on IC701 -both show some signs
of overheating.

@hunt - Can you update your profile so we know where you are. Can you check the voltage on the large capacitor. CAREFULLY. High Voltage
If you look at the Terminator X post for the HP1955 it tells you how to do it.

[Hunt]

Re: Hp 1940 flicker

Have you both resoldered the power boards?
There is little point in doing much else until you do.

I am reluctant to do any re-soldering since I cannot see any section that looks cracked by heating up and cooling down or any signs of dry joints when observed with a jeweller's eyeglass.

Sorry just seen the bit about joints Circled in red!

Can either of you make out the part number on IC701 -both show some signs
of overheating.

The IC seems to have a logo of ST at the top left to the right of that there is a circle that contains a lower case e and a number 3

In the middle on the left there is a circle that contains characters F2 that are oriented (-90deg) bottom to top, to the right of that there is L117

At the bottom of the chip could be the characters GK 16053
The G could be a 6.
The K is definitely K.
After that the letters are like tea leaves and could be anything that my brain comes up with

[Hunt]

Re: Hp 1940 flicker

You must, mandatory, resolder or reflow every solder joint circled in red regardless whether it looks good or not.

The Benq boards all suffer from bad/poor solder joints.

Retest after resolder/reflow.

Resolder all these top side (main component board side) solder joints yes?

[Hunt]

Re: Hp 1940 flicker

Resolder all these top side (main component board side) solder joints yes?

I've assumed yes; all upper mini transformers (what are they and what do they do?) have had there connections resoldered.

Voltage accross C605 is...
Power to monitor with switch off 326V
Monitor switched on 320V
Monitor left on and screen blanked 328V

Now going to replace all metal work and attach to stand without plastic case to do longer test to see if flicker has gone.