Two 42PQ10s Power Boards

[Michale32086]

Sometimes the stars align.

I have 2 42PQ10s. One is a Zenith and the other is an LG.. They are identical in layout..

On the LG, when I power up the unit, the red LED comes on, then turns blue. Then 2 blue blinks, then goes solid red... The relay does a double click..

On the Zenith, when I power up the unit, there is no relay click and the LED just does a stuttered red blinking..

When I switch the Power boards on the units, the same symptoms occur. In other words, the symptoms follow the Power boards...

This tells me that the Power boards are bad.. Safe assumption??

One of the Power boards (the one that does the red then blue blinks then red) has MOSFET written on the heat sink.. A clue!??

So, my first thought is to simply replace both the power boards.. But that wouldn't be much fun and wouldn't help me this weekend...

So, my second thought was to take the mosfet off the one board and put it on the other board that is marked with (what I assume is) the mosfet problem..

How does my logic track???

[tom66]

Re: Two 42PQ10s Power Boards

Blown fuse & FET is somewhat common on these. But, you should check the P/S output voltages before condemning the power supply. Maybe both your Y-sustains are bad? Your swap the PSU test wouldn't make any difference in that case, and you'd be needlessly swapping boards.

[Michale32086]

Re: Two 42PQ10s Power Boards

Changing out the marked Mosfet didn't help..

On both units, VS and VA is zilch. zero...

[tom66]

Re: Two 42PQ10s Power Boards

Well... if the FET was shorted, what else might fail?

[Michale32086]

Re: Two 42PQ10s Power Boards

I am haggling with ShopJimmy on a pair of boards. But it sure would be nice to track down the problem on one board using parts from the other.

[tom66]

Re: Two 42PQ10s Power Boards

Think about the question I asked. I have fixed the same PSU. Easy fix.

(Don't just replace "it" straight away, though... some other things need to be tested)

[Michale32086]

Re: Two 42PQ10s Power Boards

Well... if the FET was shorted, what else might fail?

Without a schematic, i could only W.A.G.

[Michale32086]

Re: Two 42PQ10s Power Boards

Awwww, who am I kidding. Even with a schematic, it would be a Wild Assed Guess

[tom66]

Re: Two 42PQ10s Power Boards

Check the fuse with the blue band on it.
(One of them looks to have been replaced already... but did the previous guy miss something?)

[Michale32086]

Re: Two 42PQ10s Power Boards

Well... if the FET was shorted, what else might fail?

OK, lemme pull the board and start tracing..

[Michale32086]

Re: Two 42PQ10s Power Boards

The FET goes to the fuse on the mid board.. The fuse checks out, but looks a little hinky on the TRACE side..

[tom66]

Re: Two 42PQ10s Power Boards

Have both FETs been replaced?

When repairing a SMPS stage, it is always recommended to replace any FETs directly connected to each other. This is a half-bridge converter, so the SOURCE of the TOP FET is connected to the DRAIN of the BOTTOM FET. If the top FET fails, excess current can flow into the bottom FET, damaging that too; the same applies for the bottom FET damaging the top one. It is also often necessary to replace the IC that drives the FETs, which is a small surface mount device, on this board it has 14 pins.

[Michale32086]

Re: Two 42PQ10s Power Boards

Have both FETs been replaced?

When repairing a SMPS stage, it is always recommended to replace any FETs directly connected to each other. This is a half-bridge converter, so the SOURCE of the TOP FET is connected to the DRAIN of the BOTTOM FET. If the top FET fails, excess current can flow into the bottom FET, damaging that too; the same applies for the bottom FET damaging the top one. It is also often necessary to replace the IC that drives the FETs, which is a small surface mount device, on this board it has 14 pins.

Judging from the flux, it looks like 2 were replaced, plus the big part off to the left.. The one in red is the one that's marked on the component side of the board..

[tom66]

Re: Two 42PQ10s Power Boards

Are these devices shorted or OK?

[Michale32086]

Re: Two 42PQ10s Power Boards

Don't appear to be any shorts on any of them..

[tom66]

Re: Two 42PQ10s Power Boards

Ok, with the board installed & TV powered up (you will have to do power up whilst measuring), what is the voltage across the capacitor nearest the 14 pin IC (electrolytic, on the top side)?

[Michale32086]

Re: Two 42PQ10s Power Boards

The only IC er... I see.. on the topside (component) is a 28pin IC.. Is that the one you mean??

I am gonna get the wife. I'll take this unit home if yer gonna be around for a bit..

[tom66]

Re: Two 42PQ10s Power Boards

It's on the bottom - little surface mount thing - the capacitor is NEXT to it, but on the TOP... does that make any sense... if not I will highlight it.

[Michale32086]

Re: Two 42PQ10s Power Boards

It's on the bottom - little surface mount thing - the capacitor is NEXT to it, but on the TOP... does that make any sense... if not I will highlight it.

Yea, I know which one you are talking about. It was shown in one of the pics above..

Let me unpack the unit from the car and I'll check it..

Thanx..

[Michale32086]

Re: Two 42PQ10s Power Boards

The IC under the board is the red box so I assume the cap you need is the one in the green box..

One sec