TFT monitor - Electronic Circuits Projects Diagrams Free

DMW · 16th June 2009, 08:53 PM

HI

I have 2 broken TFT monitors, one I haven't tested yet, the other turns on for a few seconds [more than 6 seconds is lucky] and then turns itself off again.

Ive opened the latter up, and all the caps look good, no discoloration, bad odour, burnt PCB area etc.

I was going to probe it with my scope but what voltages may I encounter with a TFT monitor? I would assume they are low voltage but I don't want a nasty surprise [either getting a shock or blowing up my oscilloscope].

Any ideas why it might be turning itself off, and is it safe to run while opened up.

**Preher TV** · 16th June 2009, 10:39 PM

Quote:

Originally Posted by DMW

HI

I have 2 broken TFT monitors, one I haven't tested yet, the other turns on for a few seconds [more than 6 seconds is lucky] and then turns itself off again.

Ive opened the latter up, and all the caps look good, no discoloration, bad odour, burnt PCB area etc.

I was going to probe it with my scope but what voltages may I encounter with a TFT monitor? I would assume they are low voltage but I don't want a nasty surprise [either getting a shock or blowing up my oscilloscope].

Any ideas why it might be turning itself off, and is it safe to run while opened up.

If you don't want a shock, first thing to do if you remove and trouble shoot the PSU or PSU/Inverter board, is to discharge the filter cap.....Also visual inspection of the capacitors is not enough, you need to check them with an ESR meter.

Does the unit power on and then actually power off or does the display shutdown but the power light is still on?

**Nigel Goodwin** · 17th June 2009, 09:23 AM

Yes, check the capacitors with an ESR meter - but inverter failure is common, with faulty transformers apparently been a prime cause (personally though I've never seen a faulty transformer in one yet).

You need to be aware that the inverters produce quite a high voltage, a couple of thousand or so.

DMW · 17th June 2009, 04:53 PM

Thanks, It appears the monitor turns itself completely off, reboots, and then off again [repeats forever].

Im afraid I dont have an ESR meter, i was going to make a quick one from a uC but typically my programmer cable has broken and i don't have any spare RJ11 sockets.

Any ideas what I should do next or give up on it?

note:
it first turns on for a few seconds - say 6,
the it might turn on for 3 seconds
then 1 second
and then on and off every half a second or so..

**Nigel Goodwin** · 17th June 2009, 05:08 PM

Use a scope to check the ripple on them.

DMW · 17th June 2009, 05:34 PM

Well as i say my scopes only 240V maximum, and i don't really want to risk putting a few thousand volts into it?

**Preher TV** · 17th June 2009, 06:43 PM

don't check the ripple on the ballast caps......check for ripple on the secondary side caps of the SMPS, just check the output on the secondary diodes, you should have a straight line no ripple...if you can even make this measurement as it sounds liek the tv isnot on for very long, 6 sec. should give you time though......you may just want to check a few things with the power off, check that the secondary diodes are OK....you should be able to figyre a way to do a ring test with your scope.....ring the secondary side of the HVTs on the inverter board, also just check for dry joints on the inverter board, look especially on the HVT pins and buck diode leads..(if it has a buck royer inverter).......check value of tuning capacitors....loose backlight connection...inveter ICs not to common to fail but check FETs......there is a start i can give some more places to look...hope this helps

**Preher TV** · 17th June 2009, 06:46 PM

also check the value of the ballast caps..with a capacitance meter, these can change value and cause shutdown......also check push pull transistors on the inverter board....

DMW · 17th June 2009, 06:52 PM

Well ill have to own up, im rather embarrassed this happend but you can say goodbye to that powersupply,

basically i was getting ripple voltage on the output, and In the end I got rather carried away at looking at how the SMPSU worked, so I was testing all different components, when I decided to measure the voltage across a transistor, bit of a mistake as the probes ground touched one of the leads of the transistor, bang, chunk of power resistor flys across the room and the fuse blows.

My first priority was to check the scope was undamaged, then once that was Ok I thought i better check all my limbs where still working - still are, anyway the power resistor is blown and a bit of a burn mark that covers a few SMT resistors on the other side, and the fuse where the mains comes in has blown, ill think ill be putting this in the parts bin now lol.

Apparently to make a common ground the DUT has to have an ground isolated from mains lol

.

**Preher TV** · 17th June 2009, 07:00 PM

guess it's time to pop open that other monitor......LOL...

Chippie · 17th June 2009, 07:24 PM

Quote:

Originally Posted by DMW

Apparently to make a common ground the DUT has to have an ground isolated from mains lol

.

We were remiss in not telling you to remove the ground connection from your scope...or you should use a 1:1 isolation transformer..

I did a simialr thing myself many years back...Working on a Thorn 4000 chassis...I touched the pcb with my energised soldering iron to touch up a dry joint while the set was on...There was a bang and a flash as part of the print was blown away..OH this was on the small signals side of things too....didnt do that again....

**Nigel Goodwin** · 17th June 2009, 10:19 PM

Quote:

Originally Posted by DMW

Well as i say my scopes only 240V maximum, and i don't really want to risk putting a few thousand volts into it?

Sounds like you've already done something silly - but the outputs (where you should be checking) are all low voltage.

DMW · 18th June 2009, 12:24 AM

#12:
True but its still handlying and probing very close to high voltage, would only take a slight slip for something nasty to happen.

Anyway the good news is ive used the inverter on the second monitor with the VGA decoder part on the first monitor and the screen and back light of the first monitor successfully creating a hybrid which works, only thing is i have to use my bench PSU, so ive ordered parts to make an independent power supply for it, I think things turned out aright in the end

**Nigel Goodwin** · 18th June 2009, 09:17 AM

Quote:

Originally Posted by DMW

#12:
True but its still handlying and probing very close to high voltage, would only take a slight slip for something nasty to happen.

No, there should be no high voltages anywhere near where you should be scoping, on the secondary side of the PSU.

sheldonstv · 18th June 2009, 10:29 AM

that is one of the first things that catch people out-always use an isolation transformer when connecting up test gear(scopes ....)because of the way the smps is designed it isnt mains isolated(chassis wise)connect a scope with no isolation transformer and you will get a nasty surprise....faults like that usually are due to hi esr caps-sometimes tho u do get invertor transformers fail-iv had that countless times and with small lcd monitors sometimes the smps is also home of the invertor as well
get yrself a 1:1 isolation transformer

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16th June 2009, 08:53 PM	#1
DMW	TFT monitor HI I have 2 broken TFT monitors, one I haven't tested yet, the other turns on for a few seconds [more than 6 seconds is lucky] and then turns itself off again. Ive opened the latter up, and all the caps look good, no discoloration, bad odour, burnt PCB area etc. I was going to probe it with my scope but what voltages may I encounter with a TFT monitor? I would assume they are low voltage but I don't want a nasty surprise [either getting a shock or blowing up my oscilloscope]. Any ideas why it might be turning itself off, and is it safe to run while opened up.

17th June 2009, 09:23 AM	#3
Nigel Goodwin	Yes, check the capacitors with an ESR meter - but inverter failure is common, with faulty transformers apparently been a prime cause (personally though I've never seen a faulty transformer in one yet). You need to be aware that the inverters produce quite a high voltage, a couple of thousand or so. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

17th June 2009, 04:53 PM	#4
DMW	Thanks, It appears the monitor turns itself completely off, reboots, and then off again [repeats forever]. Im afraid I dont have an ESR meter, i was going to make a quick one from a uC but typically my programmer cable has broken and i don't have any spare RJ11 sockets. Any ideas what I should do next or give up on it? note: it first turns on for a few seconds - say 6, the it might turn on for 3 seconds then 1 second and then on and off every half a second or so.. Last edited by DMW; 17th June 2009 at 04:56 PM.

17th June 2009, 05:08 PM	#5
Nigel Goodwin	Use a scope to check the ripple on them. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

17th June 2009, 05:34 PM	#6
DMW	Well as i say my scopes only 240V maximum, and i don't really want to risk putting a few thousand volts into it?

17th June 2009, 06:43 PM	#7
Preher TV	don't check the ripple on the ballast caps......check for ripple on the secondary side caps of the SMPS, just check the output on the secondary diodes, you should have a straight line no ripple...if you can even make this measurement as it sounds liek the tv isnot on for very long, 6 sec. should give you time though......you may just want to check a few things with the power off, check that the secondary diodes are OK....you should be able to figyre a way to do a ring test with your scope.....ring the secondary side of the HVTs on the inverter board, also just check for dry joints on the inverter board, look especially on the HVT pins and buck diode leads..(if it has a buck royer inverter).......check value of tuning capacitors....loose backlight connection...inveter ICs not to common to fail but check FETs......there is a start i can give some more places to look...hope this helps __________________ PREHER TV Medford,OR 97501 (541)772-0326 preher-tech.com

17th June 2009, 06:46 PM	#8
Preher TV	also check the value of the ballast caps..with a capacitance meter, these can change value and cause shutdown......also check push pull transistors on the inverter board.... __________________ PREHER TV Medford,OR 97501 (541)772-0326 preher-tech.com

17th June 2009, 06:52 PM	#9
DMW	Well ill have to own up, im rather embarrassed this happend but you can say goodbye to that powersupply, basically i was getting ripple voltage on the output, and In the end I got rather carried away at looking at how the SMPSU worked, so I was testing all different components, when I decided to measure the voltage across a transistor, bit of a mistake as the probes ground touched one of the leads of the transistor, bang, chunk of power resistor flys across the room and the fuse blows. My first priority was to check the scope was undamaged, then once that was Ok I thought i better check all my limbs where still working - still are, anyway the power resistor is blown and a bit of a burn mark that covers a few SMT resistors on the other side, and the fuse where the mains comes in has blown, ill think ill be putting this in the parts bin now lol. Apparently to make a common ground the DUT has to have an ground isolated from mains lol .

17th June 2009, 07:00 PM	#10
Preher TV	guess it's time to pop open that other monitor......LOL... __________________ PREHER TV Medford,OR 97501 (541)772-0326 preher-tech.com

18th June 2009, 12:24 AM	#13
DMW	#12: True but its still handlying and probing very close to high voltage, would only take a slight slip for something nasty to happen. Anyway the good news is ive used the inverter on the second monitor with the VGA decoder part on the first monitor and the screen and back light of the first monitor successfully creating a hybrid which works, only thing is i have to use my bench PSU, so ive ordered parts to make an independent power supply for it, I think things turned out aright in the end Last edited by DMW; 18th June 2009 at 12:25 AM.

18th June 2009, 10:29 AM	#15
sheldonstv	that is one of the first things that catch people out-always use an isolation transformer when connecting up test gear(scopes ....)because of the way the smps is designed it isnt mains isolated(chassis wise)connect a scope with no isolation transformer and you will get a nasty surprise....faults like that usually are due to hi esr caps-sometimes tho u do get invertor transformers fail-iv had that countless times and with small lcd monitors sometimes the smps is also home of the invertor as well get yrself a 1:1 isolation transformer __________________ all "txt speak" has been removed from this post for the benefit of all other forum members who may get confused!!!