Switch mode power supply in LCD TV

argonrepublic · 20th April 2011, 04:24 PM

I have (had) a dead TV. I was checking power supply. I got 12V, 5V. When I checked the 24V backlight supply + to ground with my meter, I got a spark and the backlight turnded on. There was no arcing or wires touching, it was a pin to pin test. Could I have surged or spiked a lazy capacitor etc?

pluis · 20th April 2011, 04:46 PM

>Could I have surged or spiked a lazy capacitor etc?
I dont know but you can check the ESR parameter of capacitors, is common that capacitor wich works in hig frecuency and current (as motherboards and power supplie switching) increase the Equivalent Series Resistence (ESR).

Here one schematic for made yourself Capacitor ESR Tester

*see the final paragraph, very interesating.

And finally, excuse me for the english, im from argentina and not know type correctly in english.

Bye!

jkuetemann · 20th April 2011, 10:37 PM

Could have been a poor solder joint and applying a little mechanical persuasion with the meter probes re-established contact, or as I've seen many times there is likely a failing cap(s) in there. Replace any that appear swollen, with the next higher voltage rating if they fit. Did this last night to a dead TV my parents gave me. Works beautifully now.

Andrew Eckhardt · 20th April 2011, 11:15 PM

Inspect the solder around those pins under some magnification.

Not impossible maybe, but unlikely that you "jumpstarted" anything.

mastodon · 22nd April 2011, 09:25 AM

A bad contact is the first thing that comes to mind. Another possibility is as Andrew stated that you jumpstarted an oscillator that drives the backlight. These typically operate at a few kHz and some are relatively simple discrete circuits. But then you'd still remain with the question why the oscillator doesn't start by itself, so you'd have to troubleshoot that problem.

eem2am · 27th October 2011, 08:22 AM

i used to work in a flat screen tv place, and there was a noted problem with the high voltage backlight transformers........they were using very very fine magnet wire, and often it would snap away from the connection, or be only intermittently connected, and certainly your spark could have been from this HV.

We had to do a test to see if it would cause a fire.

We had to physically break the magnet wire, and see what happened.......

we found that the voltage went very high as it tried to light the backlight, but it hiccuped on and off as it kept realising that the backlight was not lighting........even though we had broken the connection, the voltage was so high that it crept along the surface of the bobbin, and carbonised it to keep remaking the connection by itself.......so basically it kept sparking away but did not ignite anything.

20th April 2011, 04:24 PM	#1
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	Switch mode power supply in LCD TV I have (had) a dead TV. I was checking power supply. I got 12V, 5V. When I checked the 24V backlight supply + to ground with my meter, I got a spark and the backlight turnded on. There was no arcing or wires touching, it was a pin to pin test. Could I have surged or spiked a lazy capacitor etc?

20th April 2011, 10:37 PM	#3
jkuetemann diyAudio Member Join Date: Dec 2010 Location: Paris, Ontario	Could have been a poor solder joint and applying a little mechanical persuasion with the meter probes re-established contact, or as I've seen many times there is likely a failing cap(s) in there. Replace any that appear swollen, with the next higher voltage rating if they fit. Did this last night to a dead TV my parents gave me. Works beautifully now. __________________ ---Jason Kuetemann--- The greatest trick the Devil ever pulled was convincing the world he didn't exist; and like that, he's gone.

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20th April 2011, 04:46 PM	#2
pluis diyAudio Member Join Date: Apr 2011 Location: La Plata, Bs.as., Argentina	>Could I have surged or spiked a lazy capacitor etc? I dont know but you can check the ESR parameter of capacitors, is common that capacitor wich works in hig frecuency and current (as motherboards and power supplie switching) increase the Equivalent Series Resistence (ESR). Here one schematic for made yourself Capacitor ESR Tester *see the final paragraph, very interesating. And finally, excuse me for the english, im from argentina and not know type correctly in english. Bye!

20th April 2011, 11:15 PM	#4
Andrew Eckhardt diyAudio Member Join Date: Nov 2005 Location: PA	Inspect the solder around those pins under some magnification. Not impossible maybe, but unlikely that you "jumpstarted" anything.

22nd April 2011, 09:25 AM	#5
mastodon diyAudio Member Join Date: Jan 2011	A bad contact is the first thing that comes to mind. Another possibility is as Andrew stated that you jumpstarted an oscillator that drives the backlight. These typically operate at a few kHz and some are relatively simple discrete circuits. But then you'd still remain with the question why the oscillator doesn't start by itself, so you'd have to troubleshoot that problem.

27th October 2011, 08:22 AM	#6
eem2am diyAudio Member Join Date: Jun 2011	i used to work in a flat screen tv place, and there was a noted problem with the high voltage backlight transformers........they were using very very fine magnet wire, and often it would snap away from the connection, or be only intermittently connected, and certainly your spark could have been from this HV. We had to do a test to see if it would cause a fire. We had to physically break the magnet wire, and see what happened....... we found that the voltage went very high as it tried to light the backlight, but it hiccuped on and off as it kept realising that the backlight was not lighting........even though we had broken the connection, the voltage was so high that it crept along the surface of the bobbin, and carbonised it to keep remaking the connection by itself.......so basically it kept sparking away but did not ignite anything.

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