No power (blackscreen) with MD3021PH -> AI-0088 the reason?

[PlainBill]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

The AO4606 is actually two FETs in one package. Pins 1, 2, 7, and 8 are the N-channel FET while pins 3, 4, 5, and 6 are the P-channel FET. Of these, pins 2 and 4 are the gates. A problem on the gate drive signal would most likely be responsible for the premature failure. I would trace the signals back to the inverter controller IC, looking for a problem with a diode, resistor, or capacitor.

I was able to trace visually back to JP2 and JP3; beyond that I would investigate the components and solder joints in the area of D101 and D102.

PlainBill

[knurz]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Hello,
I did that so far, PlainBill. I resoldered almost every component in the mentioned area.
After that I replaced the broken U101 again and assembled the LCD. This time I got it working for several hours more than last time. But in the end the screen became black again.
I beg that it is U101 again.

I recognized a rising, high frequent but relatively silent beep/whistling-sound after each power up or changing the resolution.
But which part could cause that sound and how is it connected with the enduring short-circuit of U101?

By and by I think I'm gonna sell the whole LCD to people with better electrical engineering skills and more patience

[PlainBill]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

That may be a good idea. At this point I am out of ideas.

PlainBill

[Th3_uN1Qu3]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Could also be bad caps on the HV side of the inverter causing the transformers to work harder. They are small ceramic caps of a few pF in value. Also you could have a bad transformer. No one has asked you to check the transformers yet so i'll outline the procedure:

On the HV side, test the ends of the windings with your multimeter. This is done with the monitor unplugged. Use the 2 kOhm range. I assume your monitor has two transformers, so far i've only seen one 19" LCD with just one trafo. Both transformers should read +/-3% between each other, if one of them is significantly lower than the other this indicates shorted turns, and the transformer must be replaced.

[knurz]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Hello unique,
well because of your answer I became motivated again to test the HV transformers. T101 on the HV side (to LCD panel backlight) has 710Ohm, T102 has 705Ohm. The small 5 Ohm difference should be normal and indicates that the transformers are okay!
As I suspected, U101 has been roasted again and now short-circuit on every pin against each other pin.
Can you make something up for the rising sound after power-up? doesnt seem normal for me.

The small blue ceramic caps also seem to be optically OK, I only can check for short-circuit but can't measure the capacity...

[alexanna]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

I have noticed something when testing inverter transformers with an ohmmeter.
Measuring the secondary side will give readings that are easy to make sense out of.
But when I am measuring the primary side the reading is around 1 ohm.
If u101 continues to fail and it is driving one particular transformer, there may be a possibility that the primary windings are either opening up under a load, or they are shorted and the ohmmeter is not sensitive enough to pick it up.
I am guessing on the primary side an ammeter could be used to look at amperage draw????.Or simply replace the failed FET,But also change position of the inverter transformers and see if the problem follows the transformers

[knurz]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Hi alexanna,
thanks for the idea with changing position of T102 with T101. Will do that when I've got the new AO 4606 mosfets. Will take a while since I let them beeing shipped from hongkong.

[dankoh]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Hi alexanna,
thanks for the idea with changing position of T102 with T101. Will do that when I've got the new AO 4606 mosfets. Will take a while since I let them beeing shipped from hongkong.

Mr. knurz.
Did you manage to repair the monitor? I have a Xerox XA7-17i and same power supply board and apparently the same problem. U102 & U104 shorted, monitor is dead.

Please reply with your solution if you have repaired it.

Thanks in advance, Danijel.
dankoh is online now Report Post Edit/Delete Message

[Pentium4]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

He hasn't logged on for 10 months I don't think he's going to reply

[dankoh]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

He hasn't logged on for 10 months I don't think he's going to reply

Maybe he will get an email. It is a long shot but who knows.

[jetadm123]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Since you've already found the problem, why not go ahead and replace U102 and U104? My guess is that one or both of the timing capacitors (next to the smaller yellow inverter transformers) went out of tolerance and caused U102 and U104 to work much harder, which caused them to overheat and fail.

[dankoh]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Since you've already found the problem, why not go ahead and replace U102 and U104? My guess is that one or both of the timing capacitors (next to the smaller yellow inverter transformers) went out of tolerance and caused U102 and U104 to work much harder, which caused them to overheat and fail.

Thank you for your hint. I did change all the capacitors and fets are ordered.
Just one question. Two capacitors near transformers are 150uF 25V, but i could only find 220uF 25V. Do you think this could be a problem?

[jetadm123]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

If those 220uf caps were used in DC filtering, then it would probably be ok. However, in timing applications, you must use the original values. Possibly use a 100uf with a 47uf in parallel to give 147uf if you can't locate a 150uf.

[dankoh]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

I understand. Will post results. Thanks.

[dankoh]

Re: No power (blackscreen) with MD3021PH -> AI-0088 the reason?

Hi all!

I hope You people can help me a little more.

I have replaced all four mosfets and the monitor works now but with problems. Thing is that whole picture is shifted to the right for about 4 - 5 cm. But not in all places, in some places is like 2cm an in others 5 cm. But how he got hotter (work for a while) it got more uniform in all places, 4 cm shift. I can post a picture of it if it would help. Second thing is that there is a few horizontal lines across bottom and top of the screen. Again less when worm. The third one is that in some places, when you look at the screen the picture is like grainy in some places horizontally, not clear like it should be. A lot of things is wrong with it ..

I need to mention that i put old capacitors near mosfets 150uF because i could not find new ones. Is it possible that they can shift the picture?? I will try the combination mentioned before.

Any advice??



Happy New year!! !