Thursday, February 19, 2015

Repair LJ44-00143A / PS-426-PH power supply out of Philips 42PFP5332D/37

This board is from the fairly old Philips 42PFP5332D/37 plasma TV, but we received it from a customer with request to repair as they said they couldn't find it anywhere and nobody wanted to help them.

The board had a blown main fuse so the whole TV was totally dead (which correctly pointed the customer to the power supply), but all power components bench-tested fine.

So we replaced the fuse, took cover and plugged it in.

It took a second, but the board started crackling and sparkling.
I actually can not testify to the later as I was careful watching elsewhere (I always do when I test power boards), but I definitely heard it.

All the commotion has somehow gotten the tech working on the board by surprise since I could hear the crackling sound for long enough to start debating in my head if it is from a failing thermistor or what.

The failing thermistor was the best I could come up with.

Then AC was stopped and the board went quiet. Strangely, the main fuse was still good.

The brave tech said that the sparks have come from the sub-module between the second and third heat sink left to right. You can see his mark and arrow pointing to it at the above pictures.

However, when inspecting the sub-module I found nothing that would match the fairly loud cracking I've heard for 5 or so seconds. A small board like that should have been fairly well smoked.

Nothing was - nothing on the sub-module, noting else around.

So, after some meditation and recollection of past experiences I opted to take out the two large filter caps (which seemed fairly well and were holding well too).

I found this:

Considering this was my first try on the board I figured today must be my lucky day and called my wife probing my chances for the night. She said she's planning to have a headache again so for your luck I continued doing the board instead.

Now, the extensive burn looks pretty bad at a first glance, but after some cleaning we got this:

You can now see the track going left to right that was almost fully burned. The coat has been cracked and wiped away and you can see the copper there as well as the burned PCB layer underneath.

I bring this up because when the PCB layer burns it is important to cut or scratch it out until you get to a good material. This is because burned PCB tends to have very low resistance and is a great risk for repeat arcs that can do more damage.

The task is not unlike that of a good dentist - all bad material has to be removed until you hit solid one and only then the actual restoration can begin.

This is true for all PCB burns. I've seen it a few times in sustain boards and many times in plasma buffer boards.

So we scratch the burned PCB and take extra care around and under the edges of the damaged track.
If necessary - and it is necessary - we break the track to make sure we eliminate the bad material underneath it:

As you can see I've cut the copper track and made the hole bigger only so I can be sure there's no more burnouts.

Now, there were several possibilities for restoration.
I considered running a cable at the back side of the board. It would have been the safest and easiest approach, but the track going to the LEFT (under the heat sink) splits into three smaller ones, each of which gets a contact point I could run a wire to, but it would have given current to the other two at 1/3rd of the width of the original track.

Chances are it would have been all right, but I decided to run a jumper there, right under the capacitor.

That means it'll have to be lifted (will have to use silicone to prevent vibrations) and that I'll have to take extra care for separating the jumper from the capacitor.

Here is how it went:

 The last step was covering the two soldered ends of the jumper with coating. I did that, but forgot to take a picture and then left the shop.

Tomorrow I may cover the whole thing once more with some insulation and debate on whether to place something at the other side of the center of the capacitor to make the raising even.

Either way, the important thing was to show a problem on a board that can't be found and which is likely going to happen on others due to their aging plus bring attention to the proper way of addressing the burnout PCB issue.

While looking at the board I noticed a few smaller capacitors that did show signs of aging . Tomorrow I'll go through some if not all of them with an ESR meter, replace the ones that are off and try the board again.

I am pretty certain the board will work.

In case you haven't noticed I am good at doing boards.

And not so much my wife.


Unknown said...

I have the same board and am having problems with repairing, could you give me an idea of what it could be, I have no volts on either va or vs but all the standby power points are giving the right voltages, I have changed the to filter caps like in this and a diode

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