iMac G3 Won't Power On

[retrotech42]

I've got an iMac G3 450MHz that won't boot properly. When the power button is pressed, there is an audible pop noise and then everything goes silent. No power LED, no monitor activity, no hard disk activity. The popping noise will repeat itself as long as the button is pressed. I was thinking it could possibly be a power supply or flyback problem, but I'm not sure. Thank you for any help in advance!

 

[Macdrone]

could be right on both counts.  the flyback being bad could have popped the power board.

 

[Strimkind]

I remember going through many iMacs from a retired computer lab.  Many of the early slot drive models had failed this way, or just plain did not turn on without any pop.  On review of the internals, I did not see anything specifically bad on the board, or PSU.  After a few issues I just considered them to be generally bad and gutted/recycled any that did not boot.

However, you probably just have the one in this case.  I would pull it apart and look at the state of the boards.  Then if nothing is apparent, check the CRT.

 

[retrotech42]

Thank you guys for your responses. After some research, the problem appears to be a capacitor on the analog board. I disassembled the iMac and am attempting to discharge the CRT. I've followed the proper procedures (I think) to properly discharge it, but when I stuck the screwdriver underneath the anode cap, nothing happened. Not so much as a spark. I'm assuming I did something wrong and that the CRT still has some power. Any advice would be appreciated!

 

[Macdrone]

the imac discharges itself.  But by all means, safety first.

 

[Johnnya101]

You're fine! All models from SE up I think self discharged... But still good to do it yourself (As said above). If you are like me and don't want to get shocked for sure, simply leave the screwdriver there discharging the thing for like an hour... 

 

[techknight]

the horizontal output transistor is likely shorted from a bad flyback, which is why its doing what its doing. B+ attempts to come up, but shuts down on overcurrent. 

 

[Johnnya101]

Where can you even buy new flyback for these? I am hesitant on getting an iMac G3 since the speakers always fail and also the flyback are always dead or dying and the screen flickers...

 

[retrotech42]

Thanks techknight and Johnnya! Is it possible to replace just the flyback transformer and transistor by themselves, or would I need a new analog board? And is there any way to increase the longevity of a flyback? From what I've been reading, they're all ticking time bombs.

 

[Themk]

You can replace the fly back and transistors seperatly.

Finding the flyback may be slightly difficult, but either DigiKey or Mouser should have the transistor you need.

 

[Johnnya101]

Heres a new one possibly if yours is Rev 1...

http://www.ebay.co.uk/itm/6174Z-1003G-Flyback-Transformer-/172141820526?hash=item281473ba6e

Instructions:

http://web.archive.org/web/20050217191140/http://www.macopz.com:80/columns/imacrepair/repair.html

 

[retrotech42]

Thanks for the help everyone. Does anyone know how to completely remove the CRT from the iMac? The service source I have doesn't seem to mention it, and I can't find anything online either.

 

[techknight]

You should test the horizontal output transistor for shorts before we condemn the flyback. Its a pretty good bet, but lets make sure... 

You will have close to 0 ohms from B to E because of the drive transformer being coupled. But B to C, and C to E should not be shorted. 

 

[fyndr]

Regarding removing the CRT completely, there is a tear-down guide for the tray-loading iMac G3 which could probably also be applied to slot-loading Macs if that's the one you have: http://cool-mac.com/imacquarium/other/imac_take_apart.pdf

The tear-down starts on page 125 but the relevant section would be around page 184, after you have removed the motherboard and the lower cover/chassis.