A1200 iMac black screen fast fans problem

After fitting a known-good replacement hard drive, the iMac when switched on gives these symptoms (which I have read about elsewhere):

Black screen

Fans run at full speed

Front LED bright, steady

Diagnostic LEDS inside: only first 2 come on

Pressing power button for several seconds shuts iMac down, leaving one diagnostic LED on

Numerous SMC resets have had no effect

Leaving battery out overnight has no effect

Removing logic board and checking all plugs when reasembling has no effect

Removing graphics card and refitting carefully has no effect

All Mosfets near graphics card have likely-looking voltages on them

There were no graphics irregularities before the failure

Fitting a different hard drive has no effect

I know that a replacement graphics card or logic board will be recommended, but I hope to find out exactly what is likely to be wrong first. Is it definitely hardware, or can it be anything like firmware? Has anyone ever made a component level repair to put right this failure?