L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

[Alastair E]

This was the fiest plasma that Ive worked on, since I got this, Ive played with a Philips --that has no adjustments at all --with its own issues.....

This L.G. had a blown IPM on the YSUS as well as D28 and a MOSFET near the top small heatsink when I first checked it.

A friend located a spare board that didn't have any shorts present on the IPM or other parts. We dont know if it was a good or bad board....

Fitting it I was presented with a black screen with just a sprinkling of random illuminated pixels of different colours.

Any ideas?

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Can you see the screen glow in a dark room?

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Well--To check the screen--Ive got to lift it a little upright--Its screen down on the floor. I did look for the usual glow--but there appeared there was none, only these random lit pixels sprinkled around, some winking a little...

The room isn't strongly lit and the screen was well shaded as its face-down....

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

It's quite important, because no glow with sparklies is often a Z-sus or Vzbias fault.

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

OK--I'll check now in a bit!

Ive put the set aside in a corridor--that can be made completely dark. I'll power it up and check for the glow! and let you know. Probably be a half-hour or so....

Forgot to add, The IPM's (Both Z and Y) get mildly warm.....

Cheers Tom, Thanks for your great work here!

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

No--There's no glow at all. Ive lost the pixels too this time around...

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Ive checked and set the Vy and Vsc voltage across the two resistors on the Ysus--they were only slightly out, didnt make a bit of difference.

Where would I be checking for Zbias on the Zsus?

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

I think this set does not use Vzbias but I'd need pictures to be sure.

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

These are the same type boards used in the set--

http://www.allmatrix.fr/boutique/ima...p_lg42v7-z.jpg

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

The above pic shows my YSUS but wrong ZSUS.

Here's my ZSUS--
http://www.ebay.co.uk/itm/42-HITACHI...item460b981140

There's very little on the Zsus asnd nothing there appears faulty...

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

I think what I'll do later, is go through all the settings again, make sure they are all as correct as possible--Then dig the Scope out, see what drive waveforms we find in YSUS and ZSUS.

That should at least localise the fault to one or the other boards, and go from there....

Hope my old Tek 454 150MHz 'scope will have sufficient bandwidth to cope with the waveforms...

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

No Z bias on this set.

Your scope is more than sufficient (I have used a 20MHz analog scope no problem, the maximum sustain frequency is around 200kHz.) But you must use a 10X probe.

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Cheers Tom, I didnt think there was any Zbias in this version--there's no DC/DC converter on the ZSUS board. As I said--There's not much apart from the IPM and a chip on there....

You reckon its still a 'zsus' board issue...?

There appears to be no panel 'priming' (Glow) at all....

Good to know the scope will be good, I have a couple of x10 probes so no probs there....

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

No, without Zbias I am not sure.
This display is not even 720p capable, I'm surprised people still bother with repairing these.

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Ha!--Yes, Only doing it to gain experience on these things!

So--we have some (No!) Progress....

The voltages are all present and correct.

The ZSUS is outputting its square-wave and is in time with the YSUS 'frame' square-waves.

The 'Ramp waveform--Which is a bugger to trig on! is only half right.

The 'set-up' ramp and flat-top are correct and set to 10uS, as per manual--pot alters it normally.

There is No set-down ramp, it returns direct to base-line after the 10uS flat-top of the set-up ramp.
Adjusting the set-dwn pot just moves the ramp in relation to whatever the scope is triggering to.

This ramp waveform appears to be repeated twice--although are slightly different- before the 'frames'

Looks like this fault is on the YSUS. The IPM is mildly warm, the block of 10 MOSFETS under the heatsink are cold.

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Can I see a picture of these waveforms?
Z-sus should have a ramp too.

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Here you go--Pic quality is crap and I'm not sure the timing from the top (ysus) to bottom (zsus) waveform is accurate--it shifts about with trig-sync of scope...

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

BTW--There doesnt appear to be any ramps on zsus. Looks like I was hasty saying there was no set-dwn ramp--as I say, it was hard to trigger on these waves, took ages to get much sense outta my old scope...

Looking at service-manual--there should be a square-wave directly under the ramp wave--which doesnt seem to be there I'll check its not the scope telling me lies...

[Alastair E]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

Now got to grips with the scope. Looks like the bottom ZSUS wave is Inverted to what it should be in the manual diagram--its not the scope doing it...

Here you go--

[tom66]

Re: L.G. PDP42V7 chassis Black screen, only a few random pixels here and there...

To get clearer shots of scope, try using sport or fast shutter mode on camera.
I don't see why the waveforms would be different unless something is wrong with the control board.