TFT monitor

[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.

 

[Preher TV]

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]

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]

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]

Use a scope to check the ripple on them.

 

[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?

 

[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]

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]

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]

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

 

[Chippie]

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]

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]

#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]

#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]

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

 

[unclejed613]

or if you have a 2 channel scope (most are these days) put it in ADD mode with ch2 inverted (this makes it subtract). hook ch2's input to the "hot" ground (do NOT connect the scope ground to the "hot" ground, connect the scope ground to the chassis) and adjust the V/div for full scale deflection. then connect ch1 to the "hot" ground (with ch2 still connected), and adjust ch1's V/div until you get a straight line. once that is done, you can use ch1's probe to see the drive signals, etc on the "hot" side of the supply, and have plenty of isolation to the scope.

most people don't know their scope has this capability, and get an isolation transformer instead. or they more commonly take the dangerous shortcut of cutting the ground pin off the scope's power cord.

 

[Nigel Goodwin]

most people don't know their scope has this capability, and get an isolation transformer instead. or they more commonly take the dangerous shortcut of cutting the ground pin off the scope's power cord.

This comes up time and time again - I would suggest using a twin beam scope in that way is probably the most dangerous of all three methods?.

But in all three cases you shouldn't be doing this unless you know EXACTLY why you're doing it that way, and the reasons why it's safe under some conditions, and not under others.

Personally I wouldn't consider using an earthed scope, it's far too dangerous an instrument if you know what you're doing.

For anyone who doesn't know what they are doing, use an isolation transformer - it's the safest method for beginners.

 

[DMW]

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

Sorry maybe I should have made it clear, the inverter is on the same PCB and right next to the secondary side of the SMPSU.



Thanks for the info the ground was a lapse of concentration on my behalf, I will look into getting an isolation transformer .

 

[Nigel Goodwin]

Sorry maybe I should have made it clear, the inverter is on the same PCB and right next to the secondary side of the SMPSU.

I think we have different ideas of what 'near' means