YuSmart 209RP-H (aka Viewsonic JT229ZP6MR )White Screen

[caryg]

(thread copied from this one posted by mistake in the TV forum - please reply here)

OK I know this probably ain't the best quality monitor in the world, but I got it non-working for almost nothing, and getting it going has turned into a bit of a personal challenge. It came totally dead, but after changing the power mosfet, a couple of resistors and the PWM I/C it now lights up (I guessed that the blown SM resistor in the first pic below read 471, and soldering a 470 ohm 1/4 watt resistor over the top seems to work), but I just get a white screen.

Now looking at this thread on the Samsung Synchmaster and the earlier HP thread it refers to, it seems likely that there is a problem with the T-Con board. I've been through and tested all the caps I can recognise and none seem to be shorted (damn those oh so similar looking SM inductors for the hopes they gave and took away), and measuring the likely test points gives me the following readings:

VDDD 30KOhm 3.3v
VDDG 100KOhm 24.2V
VEEG 146KOhm -6.6V
VDDA 8KOhm 13V

There are a bunch of test points in the top right labelled POL, LP, VCOM, STH_F, VDDG_FLK, XDON, OE, STV, CLKV which all read open wrt ground, but I'm not sure if they are relevant.

Or have I got a problem on the video board itself? I'm not quite sure where to go now.

New in this thread:

FU101 tests fine for continuity. I've since found a service manual for download, and it looks like the boards inside this monitor were also used for the Viewsonic JT229ZP6MR, but here they've put in a 20" panel with the same resolution and sold it under a different name.

Service procedures for no picture are however a bit limited. First check I105, the big PQFP-128 chip in the middle. No mention of how, and that's a bit beyond me anyway. Then check the socket, then the cable. If those check out fine then look at the crystal oscillator. The cable I've pulled and given a good clean with an eraser, the socket, well what can one do? slide a cleaned cable in and out a few times?

Now I think both the big chip and the oscillator are OK, because of the way the screen behaves when I switch it on. Plugging it in with no VGA cable attached gives me a white screen for a couple of seconds before it goes to standby. Attaching a VGA cable means that the backlights stay on, so presumably that main chip is controlling that backlight and not dead. Leaving the monitor on for a couple of minutes gives me a greyish white (i.e. not 100% bright) all over the screen apart from a thin stripe of white 1-2 mm wide about 50 mm in from the right hand side of the screen. Now wouldn't the crystal be needed to get the timing for a "feature" like that.

So now I really have run out of ideas. And there's not much on the main board to test. All the big inductors and caps on the left are part of the audio circuitry for the built in speakers.

[selldoor]

Re: YuSmart 209RP-H (aka Viewsonic JT229ZP6MR )White Screen

There is a smudge on the top of C125 is it leakage.
Are these green caps Lelon brand - if so they may all need replacing.

[Th3_uN1Qu3]

Re: YuSmart 209RP-H (aka Viewsonic JT229ZP6MR )White Screen

Don't think it's the caps on this one. White screen means either bad LVDS cable or tcon failure. Check the output of all voltage regulators on tcon and logic board.

[caryg]

Re: YuSmart 209RP-H (aka Viewsonic JT229ZP6MR )White Screen

The green caps are indeed all Lelon, but the smudge is just an artefact on the photo. This is a particularly shiny PCB and it took a few tries to get a reasonably decent picture.

I've checked the power on all the pins of the main logic chip as specified on page 20 of that pdf I linked to. The ones that should have 3.3v going in read 3.28 on my meter, and the ones that should be 1.8v show 1.79. I listed the T-Con voltages that I can check in the first post, and I'm not sure where else to check there. Plus I have to fold it down to connect it, so there's not a lot on show and easily measurable when it's fired up.

I'm wondering whether it's worth changing the crystal to see what happens. Changing the monitor resolution when I've got it connected up may possibly be giving me a different pattern of grey fuzziness. Or maybe it's just starting to look that way. And the white line may be the sign of a bit of damage I've caused to the connection between the T-Con and the screen, but now that I've come this far I'd like to get the thing working anyway. Sheer stubbornness, if you will. Do crystals ever get zapped by a power problem, or are they immune to just about everything bar being dropped?

[momaka]

Re: YuSmart 209RP-H (aka Viewsonic JT229ZP6MR )White Screen

Good troubleshooting.

Unfortunately, this problem has me a little stumped as well. Generally a white screen is linked to a power failure on the t-con, but you have ruled that out already with your measurements. The only thing I can suggest is to check if anything gets really hot on the t-con or the video/logic board. If nothing, then perhaps just check every single ceramic cap and diode for short circuit on both the t-con and the video/logic board. I know it's quite a lot, but I'm also quite curious (and stubborn as well ) when it comes to issues like this. This is probably the 4th or 5th monitor thread I see with a monitor with a white screen where the t-con power rails weren't at fault. Would be very interesting to find a fix.

Let us know what you find.