iMac A1311 late 2009 - no backlight

Hi Drexus

Thank you for being so descriptive with your reply ... i stumbled across this thread looking for help and advice for my imac G4 ... i pretty much have the same issues and have done the same process ... iv changed the inverter and still get the flash of light at restart or after waking up in sleep mode ... does the imac G4 have the same bulbs as the imac A1311? or is there something else? ... if you could help in any way it would be greatly appreciated.

thanx in advance

Hi Harry Cover,

Thanks for using Apple Support Communities.

It is possible that the SMC could encounter an issue that may cause unusual system behavior

For more information on this, take a look at this article:

Intel-based Macs: Resetting the System Management Controller (SMC)

http://support.apple.com/kb/ht3964

Best of luck,

Mario

Harry.

I'm at the same point as you've described.

Have you found a solution?

This is my 23rd Mac. I currently have 9 in use at home. And yes, my first Mac was an SE30 (which I still have - working), So I know what you mean by getting extended warrenty for the new stuff.

Just to recap: iMac starts, display blinks briefly, then stays black. There is an image on the screen, but you need a flashlight to see it. External display works fine. All 3 lights on MB are on. Replaced inverter — no change. Note: sleeping the iMac followed by waking - will blink the backlight again. All 4 tubes fire at the same level, so it's unlikly all 4 have failed at the very same time.

The link you refered to as per re-routing a signal to the inverter may be an option. This is well within my capabilities and if I find this resolves the issue I will post the results.

But since this is an old thread, it would be good to know what became of your situation.

Thanks.

I've done some tests on what voltages the inverter is getting.

Of the 4 wires supplying the inverter I've measured the following: GND, 12V (these power the inverter) 3.3V & 3.1V (not sure why there's two here.

I've booted the iMac and then put it to sleep. When I wake it up, the backlight comes on very briefly. So I wanted to measer any possible spike that would signify a higher voltage resulting in the brief flash of the backlights.

Since my meter is too slow to capture the (possible) spike, I will have to use a scope to watch the voltage when waking the iMac.

What would be nice is to find what the real volateges are for the inverter... and save me guessing what to aim for.

Beofre I drag the scope out, I'm going to measure the 3.3V line again while adjusting the brightness controls on the keyboard. This should tell me how the screen is controlled (if at all).

For the record, I've found that the power supply is providing the correct power to the inverter. The inverter takes the following voltages as per Lutiana of the Vintage-computer forum:

Pin 1 (Grey Wire) - +12V

Pin 2 (Black Wire) - Ground

Pin 3 (Black Wire) - 3.3V for back light On/Off (0v = off; 3.3v = on)

Pin 4 (Black Wire) - 3.3V for back light brightness. I assume the actual volatage equates to the brightness level (ie 0v is 1% brightness and 3.3v is 100% brightness).

These are the values I've read on the inverter. So the inverter is good (and so is the old one).

When the display starts up, there is a momentary flash where the backlight works. I should note that the intensity of the flash is quite dim. I've read that when the inverter starts, it provides a boosted voltage to "ignite" the lamps around 600V. After this brief ignition, the lamps run at a much lower voltage. It's this starting voltage that produces the flash/blink at the begining. Given that it's dim when this happens, tells me the lamps are spent.

It's evident the inverter is getting the proper signal to run the display, but not able to excite the tube any longer.

Just for fun, I will re-measure the signal inputs for the inverter and see if I can get both trigger and intensity pins to read 3.3v.

Replacement lamps are as follows: 2.4mm diamater, (445 - 450mm) in length. The range is there due to variences in measurement methods. There is no part number for the tube as the diamater and general length pretty much defines the voltage etc.

Cost for getting tubes range dramatically. There are pleanty of places willing to charge you $10 per tube with a $30 shipping fee. A great business for these people having a cost <$1. To find a more competative product, look for tubes of 445mm in length. I found an eBay supplier offering 20 tubes for $18 + Free shipping. That's ~90¢ each.

A replacement guide can be found here on the web.