Forums | MacLife

You've answered your own question. You can't get power to the down converter board. It may be both parts but likely its the PAV.

In my experience with iMac/eMac this age the power analog video board is the most likely part to fail.

Last edited by car121_007 (2006-09-28 9:56 am)

I believe I have a similar issue. But I'm hoping to not have to replace a circuit board!

While attempting to turn on the iMac (same as original poster's) after a recent power outage, all I get is a click from the hard drive, the monitor flashes on then back off and the power light does the same.

The battery is fine, and I've reset the PMU

What happened?!?




Moi

This is an endemic CRT iMac problem. It used to be confined mostly to trayload imacs, however the issue is beginning to appear frequently now in slotloads too.

The specific cause is a faulty flyback transformer on the Anologue Video board. The FBT is the large black thing that supplies HT voltage to the CRT through the anode well on top of the tube.

it also kills a certain capacitor (cant remember which one)  when it goes,.

The symptoms are usually that the machine will turn on and bong, the screen will try to turn on, but the FBT shorts and shuts down the machine. On a trayload imac you get around the problem by unscrewing the monitor cable from underneath and unplugging it, then plugging in an external monitor into it. (it is a standard 15 pin trapezoidal plug under the bottom cover, like the one on older mac monitor cables). Ona slotload, the moniter is hardwired to the best of m knowledge.  You can more than likely get around it still by cracking open the imac and pulling the HT plug out of the CRT (making sure not to touch the contacts or anode well or it will ruin your day), then plugging in a ,monitor to the mirrored VGA port on the rear. The exception to this is the imac 350, which has no VGA port. If you have one of these your only option is to replace the FBT, replace the whole board, or replace the imac.

CHeers!

I replaced an FBT on an imac 266 man... dont bother. It's not worth the trouble... just get another PAV board

i imagine imac schematics would be near impossible to find.. but then again ive never looked.

Cheers!

eBay...

However in your case MOI, it could be a different problem. DO you get any response at all when you turn on the machine?... any hard disc noises?.... speaker clicks?

If so, you have probably had a surge through it and it has screwed over your mobo. try resetting PRAM tho with Option+Command+P+R keystroke numerous times... say about 5-10. This maaaaaaay fix it if your lucky... but it may not.

If it is totally dead, then you have more than likely still had a surge through it that has bent your PAV over and rooted it up the ass, and your mobo may still be OK... of course it may nopt be, but often a PAV board will take the belt from the surge before the mobo gets it.

CHeers!

Several attempts later, still the same nothing.

It seems like everything is getting power, but when you try to turn it on there is suddenly not enough power.
I've pushed the PMU button, with no luck.
Does any one know if there is a reset switch for the power supply?



Moi

Glad ya got the issue sorted nox... yeh... bad batteries can cause all manner of problem... however I can't honestly say Ive heard of a battery making the screen shut off... thats perplexing.

MOI, nope... no reset button for the PSU... I still am of the opinion that your problem is a bad FBT. replace the PAV board and it sohould work again.

however given that imacs are so cheap these days it may be better to just buy a whole working machine and swap drives over, then keep the buggered imac as a spare.

FBT= Flyback transformer.... the high voltage transformer that supplies voltage (20,000 volts too be exact ) to the CRT. It can kinda be described as similar to an ignition coil in a car.

PAV= Power/Analog Video board... the board on which all power supply components are mounted as well as all non-digital, non-logic components to do with the CRT (capacitors, transformers, rectifiers etc.)

Early (trayload imacs, and most all in one macs before it had the PAV and PSU boards seperated left and right, however the iMac G3 has all analog video and PSU components on a single board mounted above the logic board.


Sounds like you may have a toasted PSU... easier to buy another iMac than replace it.

CHeers!