iMac G4 17": black screen, backlight works

[blargg]

I bought an iMac G4 1.25 GHz (one with built-in LCD) for a discount because the LCD just shows glowing black. The backlight works, and can have its brightness changed. Computer has on-board video and VGA out works fine. LVDS cable to LCD looks fine on motherboard, and socket solder joints to motherboard look good. Unplugging it results in OS not detecting LCD, telling me that the cable is at least partially connected to panel. I have yet to get a 1.5 mm hex to open the panel's case to see whether LVDS has come unplugged there. I've done all the Mac-specific NVRAM and PMMU reset procedures, as did the previous owner.

Any ideas what could cause a panel to be black? It's my understanding that this indicates that the LCD drivers must be getting power, otherwise it'd be all white, but maybe some are "inverted" and all white only when totally driven. The OS doesn't turn on backlight when LVDS is disconnected, so it's hard to tell what "color" it is when unpowered.

[Toasty]

Re: iMac G4 17": black screen, backlight works

Loosen the tiny Phillips head screws on the connector from the panel to the mainboard. Play gently with the connector and see if any changes occur in the display while doing so. I had one where the wiring to the connector was intermittent. Gentle pressure or twisting would cause it to come on or blink/flash.

If that doesn't work, it's time for a new panel.

You're getting all 3 green LEDs, yes?

Please fill out your location in your forum profile.

Toast

[Toasty]

Re: iMac G4 17": black screen, backlight works

If you need the screen, the part number is 875-2052-C.

Flea Bay has a bunch up right now. Make sure the seller has a non-DOA policy and covered by Buyer Protection.

Here's one:

http://cgi.ebay.com/ws/eBayISAPI.dll...m=220926714217

Toast

[blargg]

Re: iMac G4 17": black screen, backlight works

Not sure what three green LEDs you're referring to. This is the iMac G4 with the half-sphere base and separate LCD panel connected via a really adjustable silver supporting arm ("flower power" Mac). It's not worth replacing the panel if it's bad; an external monitor works fine. I only paid $75 for the computer. I'm hoping it's just an unplugged cable once I get in there. Thanks for the link to a panel, which is useful in case I ever change my mind.

I tried filing a hex wrench down just a hair (English versus metric, ugh) but rounded it a bit and may have stripped the hex screw in the panel. Once I get a proper 1.5mm hex wrench I'll know. In the mean time, I plugged the panel back in and tried pressing on the back and moving the supporting arm all around to see whether that would jiggle the connection any. Black the whole time.

[Toasty]

Re: iMac G4 17": black screen, backlight works

Sorry! My bad. Have no idea on these units.

Panel link was for the iMac G5 AIO unit. Newer than yours.

[blargg]

Re: iMac G4 17": black screen, backlight works

I finally got the hex key to open the panel and tried unplugging and plugging back in the TMDS connector to the LCD, but no change. I verified that each line connected to the other end, so the cable seems good. I verified 3.3V to the LCD as well. With the computer on I did try unplugging the TMDS to the LCD, which made the screen go from glowing black to bright white (backlight is powered separately). That's further evidence that the LCD is getting proper power and intentionally making the LCD black. At this point it looks like it probably won't be worth any further time, as the next steps would be more involved (disassemble panel to check LCD AC voltages, figure out whether it's getting a valid TMDS signal, perhaps by connecting DVI connector and trying to run a normal display off it). I was hoping this was a common enough problem that the fix was well-known. I have a hard time accepting that the LCD driver just failed, or that the TMDS signal driver failed, but not all failures are simple loose connections or bulged capacitors, sadly.

[Toasty]

Re: iMac G4 17": black screen, backlight works

Keep an eye out for local school disposals/auctions. I saw these going for $40 a piece 3 years ago...

Craigslist?