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Old 11-05-2009, 11:03 PM   #21
George Ortiz
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Default Re: HP L1740 Lcd inverter/power board

All,
Strange problem with the my first inverter/power supply board. I re-capped it and installed into the monitor. Upon power up the Windows 7 logon screen shows for only about 10 seconds (green power led stays light constantly) and then goes out. I have a bright light on my desk and I can see the windows logon screen still. This is the same problem with this monitor and it's original inverter/power supply board. I removed this board and found only one blown cap. I took a another capacitor (same kind capxon) from another power supply board and then reinstalled the original board.
Turned it one and it does that same thing as the newly recapped power supply board.
Windows displays for about 10 seconds and goes out (the green power led is still lit).
I have reading other posts about other monitors, could this be a problem with the CCFL's??? Still new to troubleshooting these monitors. Thanks for all your help so far.
George
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Old 11-05-2009, 11:09 PM   #22
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Default Re: HP L1740 Lcd inverter/power board

Is the cap that blows in backwards?
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Old 11-05-2009, 11:44 PM   #23
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Default Re: HP L1740 Lcd inverter/power board

pcbonez,
I have the original inverter/power supply board out (this one is here at home). I checked against that picture I posted--this is a bad one from work and has not been repaired yet. The cap I replaced in the original inverter/power supply board is installed correctly.
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Old 11-05-2009, 11:49 PM   #24
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Default Re: HP L1740 Lcd inverter/power board

If you go cheap on the cap replacement the monitor will cheap out on you.

Without an ESR meter to verify all of the secondary lytics they will need to be replaced with new units before any real troubleshooting can continue. Bulging is a sign of failure not a 100% determining factor of failure.

I have replaced 85C Rubycons in one of my TVs with up to 85 ohms ESR and not a sign of bulging at the top or bottom.

If you want to check thing before caps come in check the inverter driver transistors (C5707 or similar) for shorts and if you have one power the CCFLs from an external case light inverter to test them.

But I wouldn't waste all my time with this till the secondaries are replaced.

If only one Crapxon was bulged then the other caps of the same series are most likely are bad too or on their way to failure.
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Last edited by Krankshaft; 11-05-2009 at 11:54 PM..
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Old 11-05-2009, 11:55 PM   #25
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Default Re: HP L1740 Lcd inverter/power board

Well the one inverter board which has been recapped (with all new panasonic caps) and the other (original) inverter (only had on bad cap replaced) board are both doing the same problem in the same monitor. Could it be CCFL's themselves causing the problem? It's getting late here, but I have another monitor at work I could try.
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Old 11-06-2009, 03:28 AM   #26
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by George Ortiz
Well the one inverter board which has been recapped (with all new panasonic caps) and the other (original) inverter (only had on bad cap replaced) board are both doing the same problem in the same monitor. Could it be CCFL's themselves causing the problem? It's getting late here, but I have another monitor at work I could try.
Two important points have been made here.

1. Caps can fail with no visible signs. If one cap failed, all from the same manufacturer are suspect, especially if it is a known bad brand. As a matter of routine, I replace them all.

2. CCFLs also fail. There is a recent thread describing problems with CCFLs with bad solder joints, and CCFLs arcing through the insulating boot.

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Old 11-12-2009, 08:09 PM   #27
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by PCBONEZ
I agree.

There is nothing -wrong- with using a higher volts caps, it's just that it makes things harder [fitting and on your wallet] and there aren't any benefits due to the higher voltage rating.
FALSE! It is VERY beneficial. Yes it makes it harder to fit because it is larger, but that increase in size means that at any given ESR, the thermal density is lower, it doesn't get as hot.

Also, look at the datasheet of any low ESR family of capacitor, and you will see that comparing two caps, for example 10V/1000uF and 25V/1000uF, the latter has significantly lower ESR. The best repair uses the largest capacitor that will physically fit, moving up in voltage instead of in capacitance.

Why not capacitance increase instead? Because that causes higher inrush current, takes longer for the PSU to stabilize it's output (might shut down in an extreme case due to being out of regulation/undervoltage too long, though fortunately most have a good margin before this happens).

Quote:
If you run a 50v cap in a 16v circuit the chemistry going on inside the cap changes the thickness of the oxide layer on the aluminum to be consistent with 16v.
[This is called reforming, as in reforming the oxide layer.]
That thickness is the primary characteristic the voltage rating is based on. When it adjusts to a thickness for 16v you no longer have a 50v cap, you have a 16v cap.
Which is fine, more of a bit of trivia than something to be concerned about.

Quote:
If you then put the reformed to 16v cap into a 50v circuit the thinner oxide layer will allow excessive currents and overheat the cap most likely to the point of it failing quickly.
Let's stop and think about this. Who uses a capacitor to do a repair then later when repairing or building a new circuit then take apart the thing they repaired and pull out the used cap to reuse it? I discount the likelihood of this happening, and if you choose to do this you know what you are doing, and in most higher-grade equipment the capacitors are rated higher in voltage than you might expect.

For example, typical PC PSU might have 16V caps on the 12V rails, but many industrial PSU that are far more robust have 25, even 35V capacitors on 12V rails.

Quote:
When you see an over rated cap [higher voltage than needed] the usual reason they did that is that higher voltage caps tend to be bigger [physically] and physically bigger caps [of some given grade] have lower ESR.
- They didn't go up in volts for the volts, they went up for the bigger can size.
It's the el'cheapo way to get better ESR.
No, it's the BEST way to get better ESR without a negative compromise, till you get into specialty caps, solid capacitors, etc that simply aren't offered in the higher voltage... in other words when given a choice, it is the better choice and not at all "el'cheapo", in fact you already stated they cost more.

Quote:
The two ways usually used to get better ESR:
- Use a physically bigger cap. [The el'cheapo way]
- Use a better grade of cap.
You use the better grade of cap AND the bigger one.

Quote:
You are much better off using a better grade than you are going to a higher voltage.
For screens Panasonic FM is an excellent choice.
In fact [for screens] Panasonic FC or Nichicon PW are better grades than what was originally in there maybe 80-90% of the time.
Who said it had to be either-or? Use that better grade, largest possible cap that will still fit. DO ALWAYS pick a higher voltage unless you designed the circuit yourself and know the margins on inrush current and timing so you can know if going with higher capacitance is an issue. Just don't go to extremes with it. If it's a 5V circuit 50V caps would be extreme for example.

Most of the time these circuits are built to a price point, there is no magic in the values they picked except they expected it to work till the warranty was up. Unfortunately in most monitors the PSU board scarely has room for larger, neither height nor diameter, capacitors but if/when they do it is beneficial to do so.

Last edited by 999999999; 11-12-2009 at 08:13 PM..
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Old 11-17-2009, 03:49 PM   #28
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Default Re: HP L1740 Lcd inverter/power board

From the picture I posted previously, can anyone tell me where the transformer (s) and inverter(s) are. I know that some users go ahead and say to look for cold solder joints. Let me know so I can look and then resolder them.
thanks,
George
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Old 11-17-2009, 04:10 PM   #29
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by George Ortiz
From the picture I posted previously, can anyone tell me where the transformer (s) and inverter(s) are. I know that some users go ahead and say to look for cold solder joints. Let me know so I can look and then resolder them.
thanks,
George
The main transformer is the largest yellow-clad component with the label having a first line that reads "22.0881..." I doubt it is the problem but it won't hurt to check it.

The inverter magnetics are the two flattened looking ones at the top, opposite side of the board and the small yellow wrapped at the top board edge.

However, seeing the dark discolored areas on the PCB you should inspect and possibly reflow solder joints on all components in those darkened PCB areas as the heat and thermal changes are what tend to break the joints or delaminate the copper tracks off the PCB, including the wire jumper joints and others.

Since you reported the monitor does light up acceptably to see images for 10 seconds it is not likely to be the CCFL tubes themselves but rather the inverter portion of the PSU board (focus on mid-right and top half regions on the PCB oriented as pictured). It is possible it is the connectors themselves that are fouled, but not nearly as common a problem as bad solder joints first or component windings shorting out second.

Considering how dark the board is in spots, you might think about whether it's possible to increase the vent slit area in the back of the monitor shell a bit.

Last edited by 999999999; 11-17-2009 at 04:19 PM..
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Old 11-17-2009, 04:24 PM   #30
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Default Re: HP L1740 Lcd inverter/power board

If you get it working again, ultimately you'll want to replace all of the CapXon caps with something better, they just have too high a failure potential.
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Old 01-11-2010, 09:15 PM   #31
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Default Re: HP L1740 Lcd inverter/power board

Hello there,fellows,kind of have the same problem,but my problem is with a
circuit which is underneat the cap mark with a x right underneat the x,but
on the other side of the board,and my problem is that the circuit is really
toast and don't have a readable number,does anybody know where I can get
the whole power board?

http://i45.tinypic.com/2uijdwk.jpg
is exactly the same as this one,for same HP lcd 1740
would appreciate any help to this email:
arturocr2@yahoo.com
LCD doesn't power on
Thanks

Last edited by arturocr67; 01-11-2010 at 09:17 PM..
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Old 01-12-2010, 07:39 AM   #32
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by arturocr67
Hello there,fellows,kind of have the same problem,but my problem is with a
circuit which is underneat the cap mark with a x right underneat the x,but
on the other side of the board,and my problem is that the circuit is really
toast and don't have a readable number,does anybody know where I can get
the whole power board?

http://i45.tinypic.com/2uijdwk.jpg
is exactly the same as this one,for same HP lcd 1740
would appreciate any help to this email:
arturocr2@yahoo.com
LCD doesn't power on
Thanks
Don't post email addresses on the forum : you might get spammed (not from us but from a harvesting bot...)
Hunt a broken screen unit on eBay....
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Old 01-12-2010, 02:55 PM   #33
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by arturocr67
Hello there,fellows,kind of have the same problem,but my problem is with a
circuit which is underneat the cap mark with a x right underneat the x,but
on the other side of the board,and my problem is that the circuit is really
toast and don't have a readable number,does anybody know where I can get
the whole power board?

http://i45.tinypic.com/2uijdwk.jpg
is exactly the same as this one,for same HP lcd 1740
would appreciate any help to this email:
[removed]
LCD doesn't power on
Thanks
Can you elaborate about what you mean?

I take what you wrote to mean that the copper trace on the other side under the "x" capacitor is burnt through delaminated/broken?

If so, you should be able to solder a jumper wire from the component solder joints on both sides of the trace, or scrape off the coating and solder to the bare copper trace itself though if it is delaminating off the PCB, you might need to secure it with epoxy or other heat resistant cement.

Otherwise, if you feel comfortable doing it (if you have experience probing live high(er) voltage mains supplied circuits safely) you can put the board on a non-conductive surface, apply power, and using a multimeter probe to find where the circuit is broken by measuring voltage backwards from the output pins that went to the logic board(s).

Quote:
Originally Posted by eguevarae
Don't post email addresses on the forum : you might get spammed ....
If you remove the email address you quoted, then if arturocr67 edits the post it will be gone. Best not to put in a quote, IMO.

Last edited by 999999999; 01-12-2010 at 03:03 PM..
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Old 02-19-2010, 12:56 PM   #34
George Ortiz
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Default Re: HP L1740 Lcd inverter/power board

Well, I've taken one of the other four bad inverter boards. Replaced all the capxon caps with the ones I got from digikey (panasonic fm's). Reinstalled the inverter board and voila. Still have the same problem. Display for 2 seconds, then black screen and only a solid green power light......
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Old 10-08-2010, 03:33 PM   #35
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Default Re: HP L1740 Lcd inverter/power board

Well,
I was finally able to get a good HP L1740 via my supervisor. He accidentally
hit the LCD screen and left a mark w/ cracks, but the screen is displaying.
I brought one of my bad 1740's back from home. On a hunch, I removed the power supply that I had recapped per the forum from the bad 1740. I placed it in the known good 1740 and it powered and the monitor is working.
I placed the other known good power supply into the bad 1740. It is still doing the same thing. Powers up briefly and then shuts down after a few seconds. So I know the repaired power supply is good. Looks like I have problem now with the CCFL's. What do ya all think??? I now know my all my power supplies are good.
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Old 10-08-2010, 03:41 PM   #36
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by George Ortiz View Post
Well,
I was finally able to get a good HP L1740 via my supervisor. He accidentally
hit the LCD screen and left a mark w/ cracks, but the screen is displaying.
I brought one of my bad 1740's back from home. On a hunch, I removed the power supply that I had recapped per the forum from the bad 1740. I placed it in the known good 1740 and it powered and the monitor is working.
I placed the other known good power supply into the bad 1740. It is still doing the same thing. Powers up briefly and then shuts down after a few seconds. So I know the repaired power supply is good. Looks like I have problem now with the CCFL's. What do ya all think??? I now know my all my power supplies are good.
Good job of diagnosing the problem. Yes, it's CCFLs, or the wires linking them to the power supply / inverter. And it's good to hear that you finally found something that a supervisor is good for.

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Old 10-08-2010, 03:56 PM   #37
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Default Re: HP L1740 Lcd inverter/power board

Thanks PlainBill,
Now that I know what the problem is, I know that I can either repair the wiring/connectors or replace the CCFL's themselves. Ebay is showing 4 ccfl lamps for sale, but I am gonna check on the connection/wiring for the ccfl's first.
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Old 10-08-2010, 11:08 PM   #38
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Default Re: HP L1740 Lcd inverter/power board

I already had one of the bad 1740 lcd screen disassembled. PlainBill you were right.
One of the pair of CCFL's is bad. I have another three lcd 1740 displays. I am going to disassemble these and pull out each good pair of lamps and then make two good displays from the mess. Man talk about finally figuring out what the problem was after a whole year.
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Old 10-09-2010, 07:21 AM   #39
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Default Re: HP L1740 Lcd inverter/power board

Quote:
Originally Posted by George Ortiz View Post
I already had one of the bad 1740 lcd screen disassembled. PlainBill you were right.
One of the pair of CCFL's is bad. I have another three lcd 1740 displays. I am going to disassemble these and pull out each good pair of lamps and then make two good displays from the mess. Man talk about finally figuring out what the problem was after a whole year.
Well, it's taken me more than a year. The 'Two seconds to black' symptom is one of the nastiest problems to solve I have encountered, chiefly because there are so many possible causes. Some of them are easy to check, some are expensive to check, some are effectively unrepairable. It's complicated because the inverter / CCFL circuit employs feedback, and will shut down if certain parameters get out of the expected range. Add in the fact that you are dealing with AC signals of over 35 Khz and 1000 volts, and you can get unpleasant surprises while testing.

From memory, here are a few of the reasons.

Bent pin on one of the CCFL connectors.
Bad caps in the power supply.
Replacement caps had too high an ESR.
Broken SMT cap in the voltage sense circuitry.
Leaky diode in the current sense circuitry.
Arcing through the insulating cap on the end of a CCFL.
Broken CCFL.
Aged CCFL.
Broken wire to CCFL.
Open transformer.
Shorted transformer.
Shorted transistor in the transformer drive circuit.
Bad solder joints.

PlainBill

Last edited by PlainBill; 10-09-2010 at 07:24 AM..
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Old 10-20-2010, 11:33 PM   #40
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Default Re: HP L1740 Lcd inverter/power board

Well it turns out I have been able to repair my own 1740, plus the one at work. Found during repair attempts to be carefull pulling out the screen and separating the lamps.
I tore the mylar connections on two other 1740 displays. Well two out of four aint bad.
One more to go and all is well.......whewwwww!!!!!!!!!!!!!!!
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