50ps3000 getting hot

[pete ling]

I Have an LG 50PS3000 I have just rebuilt the ZSUS board replacing several MOSFETS and still no picture. I then found the switching MOSFET on the Ysus board short that produces the DD voltage stated on the board as 15v. I know have a good picture and all was looking good however after running for a few mins i found the MOSFETS Q13,14 and 15 on the Zsus board where getting very hot so I turned off the set and disconnected the Zsus board to prevent any further damage. I measured the 15vDD voltage and it is about 18v. I have also changed all the caps on the control board after reading some stuff on the net but still getting hot. I have also read that replacement of the control board can cure this fault but other than the MOSFET switching wave form I fail to see how this could help. I have also read that other people have commented on line that there 15v dd is also high so not sure how accurate this has to be?
also that there is a newer version of the control board with a newer software version to cure the over heating issues ( mine is rev L). could any one advise on these issues
regards
peter

[tom66]

Re: 50ps3000 getting hot

It is normal for the high power transistors on these to run hot. Any temperature up to around 100°C on the case surface is safe. I recall burning myself on the back of a Panasonic plasma panel -- the heatsink for a PZ80 42" 1080p panel, which ran very hot (around 80°C measured by my thermocoupler.) While a low 15V line would likely cause problems with the MOSFETs and cause them to run a lot hotter than they should, a high 15V would actually be more beneficial (up to a limit of around 20~25V.) This is because it increases the saturation time and drive current of the transistors, meaning they waste less power during the high frequency switching period.

[pete ling]

Re: 50ps3000 getting hot

Tom66
Thank you for the fast reply I have purchased two manuals for this set of the net but both do no contain any information of on the boards I want. I think they may be all the same and if so they are a very poor excuse of a manual. I don't understand why the manufacturer does not put all the circuit diagrams in the manual?
I understand what you are saying about the MOSFETS I don't have a way of measuring the heat but mine are so hot you can smell them cooking?
Regards
peter

[tom66]

Re: 50ps3000 getting hot

That might be a fault but the only way you will know is by measuring their temperature. You could use something like this
http://www.ebay.co.uk/itm/Digital-In...item3cd201d4ac
if you need to satisfy you curiosity. I've used them with success before.

[pete ling]

Re: 50ps3000 getting hot

Thanks
Are they accurate enough? By the way good luck with the studies I am just finishing a masters in electronic product development currently working on the final thesis. I am doing mine distance learning so trying to fit that around a full time job it is hard work.
Cheers
peter

[tom66]

Re: 50ps3000 getting hot

They measure within +/-1.5°C of my calibrated, precision thermocouple reference. Only issue, you can only measure black surface (like surface of transistor or black heatsink.) As they cannot accurately measure reflective surfaces.

I've had a good first year. Good luck with your studies, too.

[pete ling]

Re: 50ps3000 getting hot

Yes I have got a unit on order
Thanks again