My PC is a mishmash of bits but usually works well. Very occasionally it has refused to start up, i.e., the monitor and HD will not power-up to the monitor "ON" & HD "ON" stage so that the initial DOS stuff can then begin to load Windows etc. Usually if this happeneds I switch off, poke a few connections and it then works fine after switching on. This time however it won't start at all. I have renewed the IDE ribbon cables, the HD(20gigs) is only one year old, the monitor(17in IBM) is not faulty. I have checked all other connections and all are clean and pushed home ok.
I don't think this is a case of bad connections though, has anyone got any ideas? Thanks.

Power Supply??

I checked the power supply it's ok (I'm an electrician), there's power at the monitor and at the HD.......when, switching "ON" normally the monitor comes "ON" as does the HD at which point the normal DOS stuff regarding the BIOS chip, RAM etc comes on the screen, this is not now happening on my PC. As I said, I've checked everythying I can think of, however there must be something I've missed. I don't think it's bad connections or power supply but I'm willing to double check everything.

Can anyone tell me what the sequence of events are when first switching on a PC, what exactly makes the monitor come on. It isn't a simple case of switching the power on to the monitor as this is on as soon as it's plugged in, there must be something else somewhere which 'tells' the monotor to switch on.

So you said the computer is old? Do you know if the power switch goes directly to the power supply or does it go to the motherboard? A bad motherboard can cause this. Also a cable on backwards can cause this too. Try disconnecting your drives and run the bare minimum just to see if it powers up.

A couple notions,The older AT style towers had relatively small PSU's and adding or upgrading hardware will tax them pretty much.

Also you may want to take out the RAM sticks and clean out the slots and re-seat them. Other than that not much else to check as the above post said it could be MOBO or CPU.

Smithdk, I'll check out the switch priority, I can see your line of reasoning, thanks. The computer is old yes, the CPU is about 6 years old but everything else including motherboard has been changed at some time.
FJB, I think the power supply is ok, it is after all a pretty simple piece of hardware with very little to go wrong, however there may indeed be some aspect of a PSU which gives problems that I am unaware of, if anyone knows anything please let me know. Umm yes, badly seated RAM can produce exactly this fault, I'll check it out too, thanks.

AS FJB stated the wattage may be too low for your newer components since they draw more power. Most of the time, the wattage is written somewhere on the power supply. Do you know the wattage of your power supply?

Did you change any settings?? Try clearing the CMos, with the jumper. Thats what i did when i fiddled, and it seemed to hang before my monitor kicked in.