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Old 04-05-2011, 05:08 AM   #1
playman
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Unhappy Could use some help, trying to fix HP 1940 LCD

Hi there.
I know this type of LCD has been discussed here before, but they are old threads that I did not want to wake up and they didnít appear to have my exact symptoms.

I have few HP 1940 LCD's here that came from local company,
I'm not sure what it was that caused them to break, but I would point at the
power surge demon

Any way, they all seem to have the "same" trouble's, as in
having the 2 sec blackout and others have "flickering" (blinking on and off)

I've taken 1 apart and by quick inspection there seems to be no visible problems on it, other than from "normal" heat brownish dots.

Just to clarify it now, I have almost no experience in repairing electronics (as in testing transistors or knowing what is transistor and what is rectifier),
but I eager to learn and have learned 1 or 2 things about repairing some basic stuff.
English is not my native language so I apologize for any error's or wrong word use.

Pictures will come soon as the camera has finished charging.
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Old 04-05-2011, 06:42 AM   #2
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Default Re: Could use some help, trying to fix HP 1940 LCD

There is no problem with your English. It's obvious it's not your native language - it's too well written.

Pictures are a big help. We prefer as close to 2000 x 2000 pixels. I prefer a single shot showing the entire top of the board from directly overhead, and a similar shot showing the bottom. Three out of focus close-ups of the same capacitor are seldom useful .

Above all, attach them using 'Manage Attachments' (below the text entry area). Do not post them inline; it inconveniences those with slow internet connections and irritates the rest of us.

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Old 04-05-2011, 08:44 AM   #3
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Default Re: Could use some help, trying to fix HP 1940 LCD

Well I had some help with spelling with Google :P

I hope those pictures are good enough, photography is not exactly my best thing hehe.
If better pictures are needed just shout and I'll try my best :P
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Old 04-05-2011, 10:13 AM   #4
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Default Re: Could use some help, trying to fix HP 1940 LCD

Quote:
Originally Posted by playman View Post
but I eager to learn and have learned 1 or 2 things about repairing some basic stuff.
English is not my native language so I apologize for any error's or wrong word use.
See post #13 and #14 at

http://www.badcaps.net/forum/showthread.php?t=10419

to help you get started with the basics. It explains how to use the multimeter, take readings, and things to check.

Your english is a lot better than most posters here who write in twitter style or without punctuation.
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Old 04-05-2011, 10:44 AM   #5
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Default Re: Could use some help, trying to fix HP 1940 LCD

For Benq boards, you want to check the following:

1) transistors marked c5707 for a short (should be 4 of them)
2) fets marked fu9204 for a short (should be 2 of them)
3) picofuse to see if it is open (should be 1 of them)

You must (mandatory) resolder all 4 inverter transformer pins even though the soldering may look good.

I will leave it to you, for learning purposes, to identify each of the above. With the guide that I posted, you should be able to test items #1, #2, and #3.
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Old 04-06-2011, 03:04 AM   #6
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Default Re: Could use some help, trying to fix HP 1940 LCD

Dang........ I never expected to get so fast replies and "right on the spot" I LOVE IT!
any way, I guess I'll have to stop criticizing my English lol.

Before I start probing everything, I read the part about the multymeter test.

Quote:
If you have a manual ranging multimeter, set it to 200 (two hundred) ohms.
Touch the black and red probe together. It should read 0.3 or 0.4 ohms. If it is higher than 1.0 ohm, there is something wrong with your multimeter.
Mine reads 01.1-01.0 so I was wondering if it was still safe to use?

Not that it should matter I decided to post a picture of my multymeter, it was the cheapest one I could find, that could do what I wanted at that time.
I hope it's good enough.
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Old 04-06-2011, 03:21 AM   #7
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Default Re: Could use some help, trying to fix HP 1940 LCD

That means your probes suck (they have high internal resistance). But that doesn't matter as you won't have to measure current in a monitor. They are fine to use.
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Old 04-06-2011, 03:33 AM   #8
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Default Re: Could use some help, trying to fix HP 1940 LCD

The reading youíre getting from your ohmmeter is most likely caused by your leads. Remove the leads from the ohmmeter and find something to fit into the holes where the leads plug in, a small machine screw may work just donít force anything then short them together do you get closer to 0 ohms.
You may also have a weak battery.
As long as each time you short the leads together you get the same 1 ohm reading you will probably be ok for now, just use that reading as a base line.
If when you say youíre getting a flickering on and off of the backlights, pay particular attention to checking the transistors for a shorted condition. These being shorted can cause the flickering and also what is described as 2 seconds to black. This is described in post #13 step # 2 of the link that retiredcaps provided.
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Old 04-06-2011, 03:37 AM   #9
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Default Re: Could use some help, trying to fix HP 1940 LCD

Quote:
That means your probes suck (they have high internal resistance). But that doesn't matter as you won't have to measure current in a monitor. They are fine to use.
Allright.... could you clarify that alittle more to me?
As why they suck becouse they have high internal resistance.

Well then, I guess I better start probing then. (that sounded horrible lol )
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Old 04-06-2011, 04:06 AM   #10
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Default Re: Could use some help, trying to fix HP 1940 LCD

Quote:
The reading you’re getting from your ohmmeter is most likely caused by your leads. Remove the leads from the ohmmeter and find something to fit into the holes where the leads plug in, a small machine screw may work just don’t force anything then short them together do you get closer to 0 ohms.
Thanks for that tip, lol I cant belive I could not think of anything that easy
you were right, I inserted ~6cm copper wire into the holes and I got a reading of 0.4
So... anything I can about that? maybe change the leads, i'm not even sure if I can get replacement leads here :/
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Old 04-06-2011, 04:37 AM   #11
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Default Re: Could use some help, trying to fix HP 1940 LCD

You should get some new leads for your DMM,But for now as long as you continue to get the 1 ohm reading each time you touch the two leads together use that as your 0 ohms,I think you will be able to find the shorted transistors.
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Old 04-06-2011, 06:26 AM   #12
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Default Re: Could use some help, trying to fix HP 1940 LCD

Something to remember is that while good probes would read lower, the resistance of a good transistor would be several orders of magnitude higher. A reading of even 100 ohms across two leads of a transistor indicates either a parallel component is distorting the reading, or the transistor is defective. For decades electronics technicians used meters that couldn't resolve even 1 ohm. Certainly your dmm is not a top of the line model - frankly, it's definitely low end, but should be adequate for troubleshooting monitors.

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Old 04-06-2011, 08:27 AM   #13
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Default Re: Could use some help, trying to fix HP 1940 LCD

Thanks for that PlainBill.

Unfortunetly I have a sad announcement to make.
For some reason the stickers on the panels must have beens switched,
therefore the HP 1940 does not have the "2sec blackout"
instead they have a blinkink screen wich follows by a high frequency sound.
I was so certain that they had the "2sec blackout", unless the "2sec blackout" changed
into the blinking problem, wich happens about 2sec interval repeatetly.
(next time i'll personally check them before deciding what's wrong with them)

For some reason I cant find the edit button for my first post...

Last edited by playman; 04-06-2011 at 08:29 AM..
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Old 04-06-2011, 08:49 AM   #14
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Default Re: Could use some help, trying to fix HP 1940 LCD

Blinking screen with correct picture? Power supply or inverter in auto restart mode. More likely to be the PSU, haven't seen an inverter controller with auto restart yet.

If the picture is correct but just blinking, that means the active components are likely okay. Look for bad caps or sloppy soldering.
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Old 04-06-2011, 08:59 AM   #15
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Default Re: Could use some help, trying to fix HP 1940 LCD

Quote:
Originally Posted by Th3_uN1Qu3 View Post
Blinking screen with correct picture? Power supply or inverter in auto restart mode. More likely to be the PSU, haven't seen an inverter controller with auto restart yet.

If the picture is correct but just blinking, that means the active components are likely okay. Look for bad caps or sloppy soldering.
There is no picture, it's just going from black to light black,
it's just like the screen is starting up but then fails and turns off,
and the power light flickers with it.
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Old 04-06-2011, 09:15 AM   #16
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Default Re: Could use some help, trying to fix HP 1940 LCD

Check the transistors for shorted condition
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Old 04-06-2011, 10:29 AM   #17
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Default Re: Could use some help, trying to fix HP 1940 LCD

Quote:
Originally Posted by playman View Post
I decided to post a picture of my multymeter
Your black lead is broken on both ends and the wires are both showing outside the insulation.

This is fine for taking ohms measurements with POWER OFF and lcd unplugged. However, I would NOT use this black lead for anything that requires a power measurement. I highly suggest you get a new black test lead.

Cheap multimeters come with cheap test leads (I know from first hand experience).

Last edited by retiredcaps; 04-06-2011 at 10:34 AM..
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Old 04-06-2011, 12:11 PM   #18
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Default Re: Could use some help, trying to fix HP 1940 LCD

^also look how thin the exposed wires are.. no wonder the reading is off
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Old 04-08-2011, 06:24 AM   #19
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Default Re: Could use some help, trying to fix HP 1940 LCD

Code:
C5707   1
           ___
	  |___|
	|        |
	|_____|
	 |  |  |
	 |  |  |
	 2  3  4
Bah the explenation ASCii is out of order, even with the CODE but you should get the general idea.


#1
a) black probe on pin 1, red on pin 2 - 003
b) black probe on pin 1, red on pin 3 - 002
c) black probe on pin 1, red on pin 4 - 002

d) black probe on pin 2, red on pin 1 - 003
e) black probe on pin 2, red on pin 3 - 003
f) black probe on pin 2, red on pin 4 - 003

g) black probe on pin 3, red on pin 1 - 002
h) black probe on pin 3, red on pin 2 - 003
i) black probe on pin 3, red on pin 4 - 002

j) black probe on pin 4, red on pin 1 - 003
k) black probe on pin 4, red on pin 2 - 003
l) black probe on pin 4, red on pin 3 - 002

#2-3-4
a) black probe on pin 1, red on pin 2 - N/A
b) black probe on pin 1, red on pin 3 - 002
c) black probe on pin 1, red on pin 4 - N/A

d) black probe on pin 2, red on pin 1 - N/A
e) black probe on pin 2, red on pin 3 - N/A
f) black probe on pin 2, red on pin 4 - N/A

g) black probe on pin 3, red on pin 1 - 002
h) black probe on pin 3, red on pin 2 - N/A
i) black probe on pin 3, red on pin 4 - N/A

j) black probe on pin 4, red on pin 1 - N/A
k) black probe on pin 4, red on pin 2 - N/A
l) black probe on pin 4, red on pin 3 - N/A
(I just hope his was the "right" testing circle)

The DMM was set to 2k ohm, and becouse of the bad probes I shorted them
and got a reading of 002, just in case if needed for comparison.

I belive my first transistor is broken and 2-3-4 are ok. (wild guess)
I also tested the J598 (Q805 on the board) and it had the same reading as 2-3-4

Altough I did de-solder them of the board.
But just to be sure, can I test all components on the board or do I have to
de-solder them and test them that way? I was told that I had to desolder them, but I dont know more then that hehe.

Is there anything else I should check?
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Old 04-08-2011, 06:39 AM   #20
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Default Re: Could use some help, trying to fix HP 1940 LCD

You worked too hard. Pins 1 and 3 are connected internally. For anyone else doing this, simply test 2-3, 2-4, and 3-4. This test is simply to determine if there is a short.

This can be done with the transistor in circuit. It is also STRONGLY recommended that the companion transistor be replaced.

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