Emac tried to start then stopped, finally after 3 attempts it started

Question

After a hard drive failure i run my emac on an external hard drive through a fire wire.
That worked OK till yesterday when it tried to start but after a few seconds of the normal fan noise and before the screen lit up, it stopped. I hit the switch again and it did the same thing 3 times but on the 4th attempt it started but a white screen came on and told me to type "mac-boot" and hit return.
It finally started.
The external hard drive was still running OK at the time it had stopped, it was internally that it had stopped.
I took the back off the emac and vacuumed the dust from inside and after that it has since started normally every time though it seems slower to start, but I could be mistaken about that as I can't now remember noticing how long it took to start before.
Someone said the problem could be the power supply failing.
Is there any way for me to diagnose what exactly is wrong, since an emac power supply is not same as a PC power supply?
Could a PC power supply be modified to replace the emac power supply as I have a failed PC with a good power supply I could use if it would work?
Any help or suggestions would be appreciated.
Ann

Answer 1

Hello and thanks for your reply.
I neglected to mention that at the time of the hard disk failure I put a new hard disk inside too with the intention of using it as a back up to the External Hard Drive but I found it was easier in the end to get another external hard drive and now I use Time Machine to back up on to that 2nd external hard drive every few days.
I disconnect the 2nd external hard drive when I'm not backing up data on to it.
Most of the time when I boot up the emac it somehow knows to go straight to the 1st External Hard Drive instead of the Internal Hard Drive but occasionally it will go to the Internal Hard Drive and when it does that I have to go to System Preferences and then to Startup Disk and actually select the External Hard Drive icon and that's what I had to do every time when it was having that weird starting then stopping phase, ie, every time during the starting then stopping phase, when it did start it went to the Internal Hard Drive even after I would go into Startup Disk and select External Hard Drive it would start on the Internal Hard Drive, so in the end I didn't bother trying any more and I just connected to the internet through the Internal Hard Drive until I took the back off and vacuumed the dust out and ever since I did that it's been running normally, ie, it's always selecting the External Hard Drive again and running at what seems to be normal speed for everything too.
I just don't know why it went into that starting then stopping phase.
When it was in the starting then stopping phase the internal fan noise would stop but the little white light next to the DVD door would stay lit up so it was definitely getting some power from somewhere but it was failing to go on and boot up because the power to the internal fan had failed ie, I could clearly hear the fan stop only 3 or 4 seconds after hitting the start button.
I've just had a look at the capacitors that you suggested and they look perfect, like new in fact, they have a shiny metallic top with a + indented in the top and they are flat and they show no sign of a dried up liquid or any bubbling up anywhere.
Anyway, your reply was helpful since you ruled out the Power Supply and the capacitors, so thank you for those.
The friend who advised me how to get going again after the initial hard disk failure, ie, via the External Hard Drive connected by fire wire, did warn me not to touch anything in the back that wasn't directly related to changing the hard drive but thanks for the heads up anyway.
Ann