Author Topic: LCD monitor repair - inverter issue  (Read 9218 times)

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Offline dr_p

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LCD monitor repair - inverter issue
« on: September 13, 2013, 01:30:18 pm »
Hello!


For the last 3-4 months I've been trying to repair a broken LCD monitor. It's more of an ego thing now  :scared: so I won't give up. I read a couple of books about LCD repairs, I looked at tens of schematics, video tutorials, I figured thing out but now I'm stuck and I need help.


Long story short the LCD is externally powered by a 3.3A 12V brick. It worked for 3-4years, then a PCB fuse in the LCD blew. Someone shorted it with a piece of wire and it kept on working for 2-3 months (?!). Then it died, putting the power supply in current limit.


Now comes the funny part - someone worked on the inverter board (separate board) and replaced all the SOT-23 and SOIC-8 parts with other (random) parts. What?! They're the same size.  |O |O |O So I received an LCD unit with (basically) missing unknown transistors in the on/off (control) circuit, missing unknown MOSFETS and a yet unknown fault.

I replaced the fuse with a 5x20mm holder, replaced all capacitors (even if ESR, capacitance and looks were ok). I isolated the problem on the inverter board(except the 5V regulator, it's ok and happily powering the logic board).

I studied schematics and read stuff, and learned of the existence of digital transistors, so it's clear why there were missing resistors footprints on the PCB. As I had no equivalent parts, I soldered in TO-92s and 1/4W resistors. I used BC557, BC547 and 10K resistors.



The startup circuit now works and supplies 6.8V to the controller (BIT3713) that seems to want to start, but goes into shutdown. I then soldered in a pair of IRF7105PbF as they seem to be fine for the job, however they don't seem to open properly at 5V, so they are getting hot. After more investigation I found a schematic that is almost correct, except for slightly different values of passives. It's using FSD8958A MOSFETs so I went ahead and bought some and soldered them in.



I have picture now, but the controller still shuts down in 1-2 seconds. I checked DC resistance of both secondaries, they are the same - 1.5K or 15K, don't remember - so probably the transformers are fine. I checked the 4 feedback points (with 10M impedance DMM) and they were 0.25V, 0.17V, 0.25V, 0.35V. I then replaced the 1K resistor to ground (for the 0.17V point) with 2K. It rose to 0.40V and the LCD stayed on for 1-2 minutes drawing 1.5-2.2A from my bench supply. Mind you it looks too bright on the top, though. But then it draws more than 3.5A and constantly sends the PSU in current limit and shutdown :bullshit: so the LCD keeps power cycling. In that brief moment that is on I have picture.

I put a scope on the 12V input, the 5V rail for the logic board, and the 6.8V to the controller IC, all are relatively clean. MOSFETS seem to be fine, they are not shorted, they still get the 0-5V square wave that was enough to fully open them, capacitors are ok.

I reverted to the 1K feedback resistor and the problem is still there and I don't know what else to do. Now that I think of it there's a slight chance I only checked one MOSFET, not both. It was really late and I just gave up for the day.

Can someone please help?

Edit: P.S. this is the full inside layout, FYI:

« Last Edit: September 13, 2013, 01:54:27 pm by dr_p »
 

Offline wraper

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Re: LCD monitor repair - inverter issue
« Reply #1 on: September 13, 2013, 01:46:14 pm »
Probably one of the CCFL tubes is bad. Try to swap connectors and measure feedback points again.
 

Offline dr_p

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Re: LCD monitor repair - inverter issue
« Reply #2 on: September 13, 2013, 01:51:44 pm »
well now it's not working any more, except for 1-2 seconds. All the feedback points are over 1V now.  :-//
 

Online mariush

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Re: LCD monitor repair - inverter issue
« Reply #3 on: September 13, 2013, 02:02:41 pm »
You can buy inverter boards from eBay for under 10$ and know for sure the inverter is working properly.

For example, see this : http://www.ebay.com/itm/10-25V-Universal-LCD-Inverters-Mini-LCD-Inverter-Board-for-17-22-Widescreen-/390651669520?pt=US_Monitor_Replacement_Parts&hash=item5af4a76c10

The 1-2 second thing is usually a timeout... the controller tries to ignite a cfl lamp and if it notices the lamp doesn't work within certain parameters (too high current drawn from transformer or it just doesn't "ignite" etc) so you may want to check the cfl tubes... often it can be a simple case of loose wire to the end of the cfl tubes, they get desoldered from the heat sometimes.
They also go bad due to old age, lots of hours... if the cfl tubes are old the ends are black, dark brown or dark yellow.

You can make some improvized cfl tubes from simple cfl house bulbs ... just open up the plastic case, remove the electronics and connect wires directly to the glass tube wires. See this gallery  for some examples : http://s807.photobucket.com/user/budm/library/Inverter%20tester%20using%20old%20CFL?sort=3&page=1
 

Offline wraper

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Re: LCD monitor repair - inverter issue
« Reply #4 on: September 13, 2013, 02:10:21 pm »
You can try to fool protection by disconnecting Isen pin from the circuit and connecting it to regulated voltage divider. Try something like 100K resistor to VDD and ~30K pot to GND. Then adjust until it stops shutting down so you can measure what happens in the circuit.
 

Offline dr_p

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Re: LCD monitor repair - inverter issue
« Reply #5 on: September 13, 2013, 02:27:27 pm »
I know I can buy inverters on the cheap, but I want to fix this. Actually, no, I want to know what's wrong with it, I couldn't care less about an old LCD.


So, to sum things up, I power the LCD, 3-4 seconds pass, then the logic IC sends the ON/OFF command to the PWM controller to turn the backlight on, and that's when things go kaboom, it draws more than 3.5A and my power supply shuts down. Now that I think of it, the problem must be somewhere in the MOSFETS-transformers area, because that is the only thing not engaged in the first 3-4 seconds. And if only one of the P and N MOSFETs were to be shorted, the failure would occur only when you try to open the other one. And the transformers would still get half the input on the primary, so they still work somewhat, so I have picture AND a huge current draw on the power supply.
When I get home tomorrow I'll check the signal on the transformer(s), should be 24Vpp, since it's H-Bridge driven. Also, I'll double check the MOSFETS. But why would they burn out after only a couple of minutes?

@wraper: thanks for the tip
« Last Edit: September 13, 2013, 02:29:29 pm by dr_p »
 

Offline casinada

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Re: LCD monitor repair - inverter issue
« Reply #6 on: September 13, 2013, 04:19:48 pm »
Just like they told you, if one of the filaments on the Fluorescent lamps is bad, the screen will work for a couple of seconds and it will shutdown. I have several LCD displays stashed away like that with good everything and bad ccfl. It is too complicated to replace the ccfls so I have them for spares as the power supply and controller are good.
I haven't yet found an LCD display with east to replace fluorescent lamps :(
 

Offline dr_p

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Re: LCD monitor repair - inverter issue
« Reply #7 on: September 16, 2013, 01:03:44 pm »
I took it apart and the lamps are black at the ends. I think I learned all I can learn from this puppy, it's time lo let it go.
I'll keep the boards, just in case.

Thank you all for the help.
 

Offline TerraHertz

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Re: LCD monitor repair - inverter issue
« Reply #8 on: September 18, 2013, 05:18:55 am »
Hmm, an LCD screen with BNC video inputs, that's kind or rare, isn't it? Is it a large screen?
Perhaps you should at least look for a new power supply and CCFL tubes.
Incidentally, those tubes don't have any filaments. That's why they are called CCFL - COLD Cathode Fluorescent Lamps, and how come there's just one wire to each end of the tube.

Maybe the circuit regulation was broken, and over-driving the tubes? Also not reporting correct operation, hence the rapid shutdown.
Collecting old scopes, logic analyzers, and unfinished projects. http://everist.org
 

Offline dr_p

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Re: LCD monitor repair - inverter issue
« Reply #9 on: September 18, 2013, 06:51:28 am »
Hmm, an LCD screen with BNC video inputs, that's kind or rare, isn't it? Is it a large screen?


It was a 19" LCD used in a CCTV system, hence the BNC video inputs and RCA audio inputs.
I don't really need the monitor, I just wanted to fix it. But it bugs me to have it sit on my desk for months while I try dozens of fixes. I don't usually fix LCDs, so it doesn't pay to keep it for spares.
 
I salvaged some parts and threw it away, ultimately because that makes me feel better (not constantly being reminded it doesn't work).
 
I am however very pleased with all the things I learned in the process.
 

Lurch

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Re: LCD monitor repair - inverter issue
« Reply #10 on: September 18, 2013, 07:34:49 am »
Depending on where it came from the CCTV monitors are generally made with higher quality components as they are designed to be on 24/7 rather than the consumer rubbish which is designed t be on for a couple of hours a week. Generally this seems to be true as consumer grade monitors on CCTV systems tend to last a year or 2 whereas monitors designed for CCTV use can go on for years of constant use.

I would possibly keep hold of the unit/persevere with it as it is probably worth a bit more than an el cheapo consumer grade monitor.
 


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