Plasma Tv powersupply repair

[imgaurav86]

Hello everyone,

I am working on a my own LG plasma TV which has some issue with the main power-supply board. The problem is its got Va(60v) and Vs(140v) both missing.

It works for a day or two if you heat up the power-supply with hot air gun. I have already replace the main filter capacitor and few others, re solder all the major components.

I would really appreciate if someone can help me fixing it. I have sound knowledge of electronics and I will be able to follow the instructions. Please let me know if you guys need more information regarding it.

Thanks in advance

 

[davenn]

Hi
welcome to the forums

just confirm, heating the board keeps it working or causes it to fail ?

normally heat would cause a board to fail

Dave

 

[(*steve*)]

Not necessarily true, but it is reported that the ESR of capacitors tends to decrease with increasing temperature, so a marginal capacitor may cause a circuit to fail until it reaches a certain temperature.

I would look at the caps anyway (because they're a common cause of power supply problems), so this doesn't necessarily make me think they're more likely to be at fault in this case.

 

[davenn]

if heating keeps the board working ... then its surprising that its self heating isn't enough to keep the board working

but if that's the case then you need to warm individual components when the board is cold and no go and find out which
component, when heat is applied, cause the PSU to activate


Dave

 

[(*steve*)]

Yeah, I think I originally heard this story based on "PC won't boot at first, but after a few goes it's OK". The explanation being that the ESR of the caps reduces as they heat. If you go beyond this it's conceivable that the heat generated by operating the circuit would be insufficient to get them to the required temperature.

It's a long shot, and I'd expect the results would be very temporary as the cap would either soon cool, or get worse enough that they could not be heated to a temperature above which they'd work without that temperature being sufficient to stop something else.

It's curious (i.e. it tends to suggest my explanation is wrong) that it works so long after the hot air trick. It could also be a dry joint.

If you were to replace the caps and the problem goes away, it was probably the caps. If it doesn't, it probably wasn't.

Before I did anything more, I'd follow Dave's advice to try to localise where the hot air is needed and I'd also see if the board is sensitive to pressure. Poke it with a wooden or plastic chopstick and see if the fault appears or disappears.

 

[debe]

This is what I use for heat sensitive parts. Warm every thing with a hair drier & spray individual parts with Rapid Freeze un till faulty part is found.

 

[imgaurav86]

Hi
welcome to the forums

just confirm, heating the board keeps it working or causes it to fail ?

normally heat would cause a board to fail

Dave

Hi Dave,

It only work if I apply the heat with the hot air, so heating the board makes it work for a while.

Thanks

 

[davenn]

ok no probs

then follow the comments I made in post #4 and see if you can find the faulty component(s)

carefully heat individual components, avoid having more than a couple warmed up at once else you will have to wait till it all cools down again.