HP L1940t turns on and off frequently

[bbeastz]

HP L1940t lcd monitor....when i push the power button it turns green and show display (for a very quick 1 second) then the power button turns yellow and display is gone...then repeats this process forever.
I am a complete noob to soldering so i want to get some feedback before buying replacement parts. My first thought was to replace caps but i'm not sure which size i'm suppose to get my board is benq (48.L1F02.a02)



http://www.flickr.com/photos/7255826...2621/lightbox/

http://www.flickr.com/photos/7255826...ream/lightbox/

http://www.flickr.com/photos/7255826...ream/lightbox/

http://www.flickr.com/photos/7255826...ream/lightbox/

[alexanna]

Re: HP L1940t turns on and off frequently

It could be caps,but try this.
Here is a very simple test you can try.
With the power/inverter and signal card installed and a VGA signal going to the logic board.
With all but 1 of the CCFL disconnected from the inverter, power on the monitor. Do you see a flash of the CFL that’s installed?
Repeat this with the remaining CFLs one at a time.
We are hoping to have at least one CCFL that does not flash.

[bbeastz]

Re: HP L1940t turns on and off frequently

cn801 pink/white
cn802blue/black
neither has flash

[Krankshaft]

Re: HP L1940t turns on and off frequently

It's rarely bad caps on these units but I recap them preemptively after I perform the common repairs. Check all four of the C5707 transistors for junction shorts or opens. These drivers are directly beside the two gray tombstone caps.

Q812 and the other identical part number transistor towards the edge of the board is the backlight pair on / off switch. Check them for opens or shorts as well.

They fail as a result of bad solder joints in the inverter section especially around the two inductors and gray tombstone caps.

[alexanna]

Re: HP L1940t turns on and off frequently

There are some 3 pin transistors marked 'Q' on the top side of the inverter,
Would you check to see if you read a very low resistance reading on any of the pins.

[bbeastz]

Re: HP L1940t turns on and off frequently

the one on the very top near edge has (j598 ks 56)
the one on the right of the top gray tombstone has (c5707 4m)
(and the rest has identical codes to these two)


i think i'm going to order a repair kit or two and update my thread
thanks for replies

http://cgi.ebay.com/ws/eBayISAPI.dll...m=220488412168
this kit should work right?

[Krankshaft]

Re: HP L1940t turns on and off frequently

You need to go over the inverter section with a magnifier and repair all of the cracked solder joints or they will blow again.

I always desolder and resolder all of the inverter transformer pins the tombstone caps, the and the two inductors. The rest is done by eye checking for bad joints. I always desolder first to get rid of that lead free crap before resoldering.

Yes that kit will work.

[Byeelectric]

Re: HP L1940t turns on and off frequently

Hello all,

Not to sound like things are hopeless I repaired four of these. I ended up replacing the power board on two of the four only to have one of the new power boards fail. As a tech I feel a backlight is failing(one or more) and the negative feedback (HVAC) causes the C5707 transistors to fail. Also when replacing the C5707 transistors replace the two FET's (replace all six transistors). When you power the unit keep the back covers off and listen for noise, if you hear a high pitch squeal while the power is off then you have the feedback issue mentioned above. As for replacing the backlights I cannot advise as there are diffusers that cannot get soiled or dust between them and thin glass CCFL's that shatter easily. Also check the .47uf capacitor on the digital board, located near the power supply connector as I found this part completely shorted. If you get a green to amber light with no picture and all six transisors test good then the main transformer is gone. You can spend $40.00 for a spare parts board here: http://www.e-techsiliconvalley.com/s...ducts_id=10912



Benq= :BSFLAG:

[jetadm123]

Re: HP L1940t turns on and off frequently

Hello all,

Not to sound like things are hopeless I repaired four of these. I ended up replacing the power board on two of the four only to have one of the new power boards fail. As a tech I feel a backlight is failing(one or more) and the negative feedback (HVAC) causes the C5707 transistors to fail. Also when replacing the C5707 transistors replace the two FET's (replace all six transistors). When you power the unit keep the back covers off and listen for noise, if you hear a high pitch squeal while the power is off then you have the feedback issue mentioned above. As for replacing the backlights I cannot advise as there are diffusers that cannot get soiled or dust between them and thin glass CCFL's that shatter easily. Also check the .47uf capacitor on the digital board, located near the power supply connector as I found this part completely shorted. If you get a green to amber light with no picture and all six transisors test good then the main transformer is gone. You can spend $40.00 for a spare parts board here: http://www.e-techsiliconvalley.com/s...ducts_id=10912



Benq= :BSFLAG:

IIRC, there are 4 inverter transformers that have poor solder joints that cause the transistors to fail. Are you also resoldering these joints?

[budm]

Re: HP L1940t turns on and off frequently

I still suspect that the tuning caps (Gray caps by the inverter transformer) for the Inverter transformer Royer Oscilator circuits are bad, and that can take out your new transistors since it will be running at the wrong frequency. Bad solder joints are also the known problem for these BenQ boards as suggested by other.

[selldoor]

Re: HP L1940t turns on and off frequently

In post 1 you said " and show display (for a very quick 1 second) "

Im not sure you would get this if the transistors were blown - have you tested anything with a multimeter?

You should do the tests and report the results.

Two important rules
Test resistance with power off ( for 5 mins)
Test voltage with power on ( and boards all connected.)

If you have no experience at all please ASK if you are unsure.


Suggest you read this if you have not already:https://www.badcaps.net/forum/showth...highlight=1940

Please post pictures of your board - use manage attachments - do not post in line or off site.
(max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Example
https://www.badcaps.net/forum/attach...1&d=1290283049

[RJARRRPCGP]

Re: HP L1940t turns on and off frequently

then the power button turns yellow and display is gone...then repeats this process forever.

Not the reported classic symptom type. Sounds more like it's a VGA input (or HDMI input)
issue.
Possibly a bad VGA or HDMI cable connection.

Sounds like it keeps going to standby and out of standby rapidly.