Lg 50ps 3000

[llonen]

So I have had this set in for a little while now, I have a lot of repairs come through each week and this one keeps getting pushed back. Set's history was that it was brought in with a defective z board, and sure enough there was a fault on the board, some of the SMD caps had leaked and there were a number of the usual collection of shorted mosfets and failed fusible resistors. Board was stripped; cleaned with the tracks made good, pre requisite repairs carried out and tested for further shorts.

Checked the Y board for shorted rails and or components and nothing of significance found, reassembled and nothing, no start up, no status lamps on the control board nothing from the screen and no start led. There was nothing coming out of the smps not even 5 volt standby.

Came back to the set today and bench tested the smps; reworked some areas of questionable soldering, relevant points jumper-ed and the supply checked out to be fine. Put back in the set and noting, removal of the main signal board and jumper-ing for auto gen brings about a good test picture.

A cursory examination of the main board (PD92A EAX575662002 (0) found at a pair of AZ1085S one 'appears' shorted of course this may be one or more other components causing this. Other than the typical BGA failure mode for these I was wondering if anyone has come across other common points of failure.

[llonen]

Re: Lg 50ps 3000

Will give this a bump before I go in search of a replacement main board

[dick_barton]

Re: Lg 50ps 3000

The say you think that one of the regulator appears shorted. Which pins? Is the short still there when you lift the pins.?

[llonen]

Re: Lg 50ps 3000

For device IC1102 AZ1085S it did rather look that pins 1, and 2 appeared shorted or leaky, having lifted pins on this device and device AZ1085S neither device appears shorted or leaky the values were in circuit readings as I suspected. So is anyone aware of common problems with these boards beside the BGA, which I am inclined to suspect since it is a common failure mode with these boards. I don't currently have the facilities to rework that anyway.

[dick_barton]

Re: Lg 50ps 3000

Can you check what capacitors are between those two pins. You can twist them off and replace them with ordinary electrolytics. They have a tendency to go low resistance when faulty.

[llonen]

Re: Lg 50ps 3000

Already removed and replaced those and it is not looking good for a matching replacement board.

[dick_barton]

Re: Lg 50ps 3000

Connect an external supply at that point but do not exceed the expected voltage and see what gets hot/very hot. If it's not quite a short circuit then something should start to heat up quickly.
I don't think you have anything to lose if you think there's nothing else you can do.

[llonen]

Re: Lg 50ps 3000

Well it does not appear to be doing anything ugly, there a three notable voltage rails coming in, a 17 volt, a 12 volt and a 5 volt, none of them seem to pull more than about 100ma. Then again I don't see any significant voltages appearing at the regulator(s) to board ground. Then again I don't have three seperate bench supplies to provide power to each rail and can only do one at a time. Not sure why its tripping up the smps.

[dick_barton]

Re: Lg 50ps 3000

My thought was of connecting it to the output of the voltage regulator where you say there is a short circuit.

[llonen]

Re: Lg 50ps 3000

Well clearly changing the capacitors did seem to solve the main board problem after all, there were no measurable changes in the measured values around the linear regulators but the board now starts up.

This should have been the end of this but no, I find that the three replacement mossfets on the Z board are running hot. I am aware there is a known issue with the control board and recapped this, with the exception of three that I don't have in stock (will order them). Very nice picture when switched on but the three fets still seem to run hot, am not really sure how hot these should run nothing else on the Z board seems to run hot.

[llonen]

Re: Lg 50ps 3000

Well I have just about had it with this sorry piece of *^$% I have recapped the control board completely and while there were some electrolytic's that measured well out of tolerance I see no particular reason this should make any difference to the Z board. And it did not, I have a good picture but still I find the replacement 20NF20 plus FDPF51N25 51N25 still running hot well. I suppose they could always be fake, who knows any thoughts on what else might be causing this thermal runaway.

[llonen]

Re: Lg 50ps 3000

So having re read around this subject I find people quoting these devices (20NF20 plus FDPF51N25 51N25) as reaching as much as 120℃ within the first two minutes of panel start up, something to do with the sus boards being initially run hard for to warm up the gasses. Looking at the data sheets I see these devices operate from between -50℃ to 170℃.

Any thoughts on this as I would rather not put the set on and wait till they fail after a few minutes, which I am fairly certain they will, at this point I was looking for a replacement Z board but there seems to be little availability for them.

thanking you in advance

[llonen]

Re: Lg 50ps 3000

Any more input on this ??

[llonen]

Re: Lg 50ps 3000

bump