CurtPalme.com Home Theater Forum

amigaman

Hello all.

My XG75 has been working reliably for quite a while, but today when I powered it up only the red and green tubes displayed a clean raster. The blue CRT was just glowing at about 1/3 normal brightness, with no focus. The entire CRT face was glowing at an intensity that changed along with the input signal. It is as if the focus is WAY off, to the point that no detail or variation across the phosphor is discernible.

The projector was working perfectly when I powered it down last night, and the issue was there the instant I turned it on this morning. I cleaned the connections on the neck board, G2 lead, horizontal and vertical deflection, and the focus/astig coils, and after that it worked for a short period of time (about 2 minutes), then the issue returned. It seems to occasionally work on power up now, but it fails after 30 seconds to a couple minutes.

The problem happens on all resolutions, as well as on the menus with no external sync applied. I usually run the projector at 1080i, so I'm not pushing anything. I was running it at 1080p, but after reading that 1080i often looks better on projectors with lower bandwidth, I tried it, and it looked good. 1080p is still below the max horizontal/vertical frequencies of an XG75, so it wasn't being pushed much anyway.

Any ideas?
–Tom

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

First question is which XG75 is it?

Next, if you have an input and the green and red shows it is locked to that entry, what do you get on the blue if you hit menu and then put up an internal grid test pattern?

amigaman · Posted: Fri Jul 08, 2011 11:10 pm Post subject:

It's an XG75A, or XG-751, depending on what you look at on the back (actual model appears to be XG-751).

The issue is with the whole image, OSD and internal test patterns included.

amigaman · Posted: Fri Jul 08, 2011 11:13 pm Post subject:

I just noticed that the issue happens the same way whether or not the focus and astig cables are connected.

So I'm thinking it's not the focus or astig boards.

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

I would first just make sure you dont have the blue turned of with the remote.

Next guess could be a neck board. You could try swapping it with another color.

amigaman · Posted: Sat Jul 09, 2011 12:10 am Post subject:

The blue wasn't cut off, which is what I checked first. It would be nice if it was simple like that.

So now I think it's a vertical deflection issue. I watched the projector with a test pattern on this time, and the blue raster only started shifting vertically slightly and randomly right before the image went out. Also upon further inspection of the blue CRT, after the image disappears there seems to be more light coming from the top of the CRT, suggesting that maybe the raster flew off the tube face completely, and the glow I'm seeing is the beam reflecting off an interior surface of the tube.

This is of course a little scary, so I'm testing it at 20% contrast, and I'm sure to shut the power off within a few seconds of the issue.

So, vertical drive board maybe? I have a scope nearby, so if there's a test point I could check to verify, I can do that. Any common component failures in this circuit that anyone knows about? If it's the vertical board, what's the easiest way to remove it for cleaning and inspection?

amigaman · Posted: Sat Jul 09, 2011 12:24 am Post subject:

Yep. Swapped the blue and red vertical deflection connectors and the red got the issue, and the blue was fine.

Seems to me like a failing capacitor or bad solder joint on the vertical drive board, since the problem tends to happen after it warms up for a bit.

Any ideas?

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

Hmm, odd that it doesn't go into V Stop. Must not be enough to trigger it.

Happy hunting.... Very Happy

amigaman · Posted: Sun Jul 10, 2011 1:20 am Post subject:

Found the culprit.

I traced the issue to the STK392-040 used as the vertical sweep output IC. After a few minutes of warm up time, its output begins to shift slightly in DC bias (causing the vertical position to move up and down), then it goes very negative (several volts) as a flat line with a pulse corresponding to the vertical sync. Since the circuit used to protect the system from loss of vertical sync is AC coupled, I assume this sync pulse makes the system think the sweep is still there, so no shutdown, and no error codes.

I see that the STK392-040 is still very available, but seeing as how there are apparently counterfeits out there, where should I buy from? MCM Electronics has them here:
http://www.mcmelectronics.com/product/STK392-040
and Electronic-Repair-Kits has them here:
http://www.electronic-repair-kits.com/STK392-040.htm

I'm not familiar with the latter, and I don't mind buying from MCM if it looks like it's a genuine part. Any other known reputable sources would be good as well.

My NEC 9PG Plus might need one too, which I'll verify tonight (it also has vertical sweep issues).

Thanks!

amigaman · Posted: Sun Jul 10, 2011 9:33 am Post subject:

Ok, all is good now!

I looked into my 9PG Plus, and found that it uses the STK392-040 in its astigmatism board. Since that projector only has active astigmatism correction for the red and green channels, only two out of the three channels of the STK392-040 are used. Luckily, they used channels 1 and 2 in the PG, and channel 3 was bad in my XG. So I swapped the IC's, and the XG works perfectly now, and the PG doesn't know the difference, since channel 3 isn't connected to anything anyway.

Hooray for making everything work without spending any money!
The PG has issues with its point convergence (not the vertical sweep like I thought), but that's for another thread...

macgyver655 · Joined: 22 Aug 2007 Posts: 8508

Good job.