I bought a used iMac 7.1 without original installation disks and have upgraded the HD, so the original Hardware Test is unavailable. Can anyone help me download AHT 3A125 ?

3A125 is an older AHT for your iMac.
Another choice is 3A141, which is the most up-to-date AHT for your model.

Thanks DeltaMac. At least I'm chasing the right target, but where to find AHT 3A141 to download?