Had a similar experience a few weeks ago when an iMac wasn't booting up at all on a rather hot and humid afternoon. I left it, and promised myself that if I ever got it started again, I'd back up all the files on it.

The next morning, it worked as normal, and has worked fine since. I still backed it up, to be safe.

I attribute the problem to the heat.

My iMac's screen stopped displaying about a month later (i.e. October 2014): when I turned it on, the power button remained orange, and it would not boot up.

At first I took it to the local Macintosh suppliers - but they said they had stopped supporting this iMac for ages, and that there was nothing they could do. I thought it was a motherboard problem - as did they, and so I opted to purchase and install a new motherboard.

No noticeable effect. Discovered, after some further research, that this problem could be caused by the overheating of wires leading to the CRT screen. Working on this basis, I opted to attach the iMac to an external monitor - first by purchasing a Macintosh to VGA Monitor Adapter (from DV Warehouse) and later buying an extension cable (Monoprice 6ft DB15 M/F 1:1 Molded Cable - Beige) from Amazon.

It worked - and now I can use my old iMac again by linking it up with an external monitor. Though some games (like Terminal Velocity) don't seem to work under this new set up, the rest do.

Lesson: if replacing an apparently faulty part doesn't fix a machine, it is likely that the fault lies somewhere else.

The flyback high voltage can be check with a smal neon lamp by puting it near the big red wire.
it should glow if the horizontal section is ok.

Please keep one hand in your poket while doing this and dont touch anything "Nasty Shock Ahead"
If you have an High Voltage probe use it sould be near to 15 / 20 kvolts.

The high voltage rectifier can be check by replacing it cost less than a flyback an easier to find.
The driver and horizontal transistors output need to be check with a vom or dvm for short and open.
you need to remove them for a good check, also check the horizontal oscillator,the damper diode,
there could also be a high voltage regulator in there not shure in the last model tv set using a crt there was one and it was very hard to troubleshoot those model.
and if everything check ok the power supply need to be check for voltage,open and short,transitors,
diode and the big filter capacitors make a visual check for any trace of leakage on those if so replace
them with new one of the same value.
I use to do all this before replacing a flyback transformer.
Hope you find something useful in my post a long time ago i use to make repair on tv & guitar amplifier.

A few thing i miss last night for the iMac;
there might be a high voltage tripler in there,those replace the hv rectifier with a smaler flyback to cut the cost down i've replace quit a lot of these it' a smal gray box with 3 wires on top.

For the power supply;
Check the regulator voltage you sould have one 5 volt and one 12 volt dc sometime there a minus 5&12Vdc also you should find one at about 140 Vdc

Swapping board would be easier if you have access to a good iMac
There a very good information here that apply to repair the SE/30 have a look and good luck.
http://www.biwa.ne.jp/%7Eshamada/fullmac/repairEng.html

Thanks for all the information and instructions.

'The driver and horizontal transistors output need to be check with a vom or dvm for short and open.'

Pardon my ignorance, but what exactly are a vom or dvm?