HP lcd L1502 15"

[leov]

Hi guys. I am very new to this, but was hoping you would help me fix my lcd. Originally, the screen started to turn itself off randomly. I was able to restart it, but after few days it shut down and I was not able to bring it back to life. I replaced all 25V 470 uF caps on this board. They were not blown, but looked like they were starting to "inflate". (sorry, i am not sure what the proper terminology is to describe it). That did not fix the problem though. When I connect the power cable I can hear a clicking noise coming from the parts that I circled on the picture. They turn really hot if the power cable is connected. Would someone please tell me what I should do next? How do I go about testing the other components on the board? Thank you.

[sabre504]

Re: HP lcd L1502 15"

Hi did you test the transistors that you have arrowed (probably 5707 or 5706 markings on them )
Also did you resolder the 4 x transformers

If you check threads that deal with Dell 172 monitors they have the same type of board and have the same symptoms

[PlainBill]

Re: HP lcd L1502 15"

Hi guys. I am very new to this, but was hoping you would help me fix my lcd. Originally, the screen started to turn itself off randomly. I was able to restart it, but after few days it shut down and I was not able to bring it back to life. I replaced all 25V 470 uF caps on this board. They were not blown, but looked like they were starting to "inflate". (sorry, i am not sure what the proper terminology is to describe it). That did not fix the problem though. When I connect the power cable I can hear a clicking noise coming from the parts that I circled on the picture. They turn really hot if the power cable is connected. Would someone please tell me what I should do next? How do I go about testing the other components on the board? Thank you.

This is a classic example of the problems with these monitors. As sabre504 said, replace both transistors (2SC5707). Also resolder all pins on the 4 inverter transformers. Just for luck, I'd also replace the cap that was clicking.

What brand and series capacitors did you use as replacements for the bulging caps?

PlainBill

[leov]

Re: HP lcd L1502 15"

Hi did you test the transistors that you have arrowed (probably 5707 or 5706 markings on them )
Also did you resolder the 4 x transformers

If you check threads that deal with Dell 172 monitors they have the same type of board and have the same symptoms

Hi sabre, Thank you for replying to my post. I appreciate your help. The transistors are 5707. How do I go about testing them? I have very limited understanding of these things. I did not resolder the transformers.

[leov]

Re: HP lcd L1502 15"

Hi PlainBill,

Thanx for taking time to look into this for me. I used united chemi-con caps from Digi-key.


Also, I found out that the problem occurs when I connect little circuit board that has VGA connector on it. If its not connected, there is no clicking heard on the big board.

[Th3_uN1Qu3]

Re: HP lcd L1502 15"

That's the logic board. It's kinda obvious that it won't do it without the logic board connected, since the logic is what instructs the inverter to turn on.

[leov]

Re: HP lcd L1502 15"

So i guess it would be safe to assume that the logic board is working fine.

[Th3_uN1Qu3]

Re: HP lcd L1502 15"

Yes. Your problem is in the inverter area. For now, take out the 5707 transistors and measure them with your multimeter set to 200 ohm scale. If any leg combinations read 0 ohms the transistors are bad and need to be replaced.

[leov]

Re: HP lcd L1502 15"

Thank you, I will try doing that as soon as i get a chance.

[retiredcaps]

Re: HP lcd L1502 15"

For now, take out the 5707 transistors and measure them with your multimeter set to 200 ohm scale.

I usually test components "in circuit". If the readings are suspicious, then I desolder and measure "out of circuit".

[leov]

Re: HP lcd L1502 15"

I usually test components "in circuit". If the readings are suspicious, then I desolder and measure "out of circuit".

Hi Retiredcaps,

I've read that you do that, but I don't know how to test it. I am going to read about it and try testing it right now.


Thanx for advice. I appreciate it.

[leov]

Re: HP lcd L1502 15"

So here is the pic of the transistors i need to test. How do I go about testing them? I tried setting my meter to 200 ohms. When i connect positive lead to B and negative to E or C i get random readings that stabilize around 0.9-8. If I test from E to B and from C to E i just get random readings that do not stabilize. I am not sure if I am doing this right. Thank you for putting up with me. Any help would be appreciated.

leo

[PlainBill]

Re: HP lcd L1502 15"

So here is the pic of the transistors i need to test. How do I go about testing them? I tried setting my meter to 200 ohms. When i connect positive lead to B and negative to E or C i get random readings that stabilize around 0.9-8. If I test from E to B and from C to E i just get random readings that do not stabilize. I am not sure if I am doing this right. Thank you for putting up with me. Any help would be appreciated.

leo

Are you testing with the monitor unplugged?

Please attach a picture of your meter with the function switch set in the position you are using, and with the probes plugged in..

PlainBill

[leov]

Re: HP lcd L1502 15"

Are you testing with the monitor unplugged?

Please attach a picture of your meter with the function switch set in the position you are using, and with the probes plugged in..

PlainBill

monitor is not plugged in. I will attach the picture in a bit.

OK here are the pics of my meter:

[PlainBill]

Re: HP lcd L1502 15"

monitor is not plugged in. I will attach the picture in a bit.

OK here are the pics of my meter:

You definitely have the meter set to the correct range, and the leads plugged in correctly for resistance measurements. The display shows a typical display for a resistance greater than 200 ohms.

I can't explain the strange behavior. Ordinarily I would expect to see this display between any two legs of a transistor. A jitter of the last digit isn't unusual. What reading do you get when you try to measure the resistance between two points on the circuit board that are obviously connected, for example, the emitter of the two transistors in post 12?

PlainBill

[alexanna]

Re: HP lcd L1502 15"

So here is the pic of the transistors i need to test. How do I go about testing them? I tried setting my meter to 200 ohms. When i connect positive lead to B and negative to E or C i get random readings that stabilize around 0.9-8. If I test from E to B and from C to E i just get random readings that do not stabilize. I am not sure if I am doing this right. Thank you for putting up with me. Any help would be appreciated.

leo

The readings your getting with your ohmmeter, the jumping and taking time to settle down makes me wonder if you may have a problem with your DMM leads?What reading do you get on the 200 ohm scale with the leads of your meter hooked together?
Don't forget to read PlainBill's above post.

[leov]

Re: HP lcd L1502 15"

The readings your getting with your ohmmeter, the jumping and taking time to settle down makes me wonder if you may have a problem with your DMM leads?What reading do you get on the 200 ohm scale with the leads of your meter hooked together?
Don't forget to read PlainBill's above post.

it reads 00.1, 00.3. The last digit is flirting between .4 and .1 and if i understand it correctly, Bill said that its ok if that happens. I am going to post a pic of the readings that I got when i tested another set of C5707 on the board.

[leov]

Re: HP lcd L1502 15"

You definitely have the meter set to the correct range, and the leads plugged in correctly for resistance measurements. The display shows a typical display for a resistance greater than 200 ohms.

I can't explain the strange behavior. Ordinarily I would expect to see this display between any two legs of a transistor. A jitter of the last digit isn't unusual. What reading do you get when you try to measure the resistance between two points on the circuit board that are obviously connected, for example, the emitter of the two transistors in post 12?

PlainBill

Bill, here is what i did. I think that I found the bad transistors. Please see the picture. Th3 has pointed me to those before, but with everyone's help and a little of reading here on the forum i think i got it. Please confirm my findings.

Thank you for your help guys.

leo

[steve2]

Re: HP lcd L1502 15"

When they read bad in circuit, you need to remove them from the board and recheck them. You may be reading transformers windings to get a reading that low or something else.

[PlainBill]

Re: HP lcd L1502 15"

Bill, here is what i did. I think that I found the bad transistors. Please see the picture. Th3 has pointed me to those before, but with everyone's help and a little of reading here on the forum i think i got it. Please confirm my findings.

Thank you for your help guys.

leo

You can verify the readings by removing them from the board, but they are almost certainly bad. This circuit is what is known as a Royer Oscillator. In it the two bases are tied together by a transformer winding and the two collectors are tied to the 12V supply through transformer windings. There should be no direct Collector-Emitter short.

PlainBill