My trusty Mac LC no longer boots from the HD

Question

I purchased a new Mac LC many years ago and it has served me well until recently when it would no longer boot from the hard drive. When I boot from a Disk Tools floppy, I get some limited success but do not ever hear the HD spin up. The startup chime also does not sound.  When used as the boot disk and trying to use the installer option, the program can't find a SCSI drive, only the floppy drive. The 5V & 12V to the HD are right on and a new PRAM battery is installed. I can load and run programs using the floppy drive but that requires a lot of disk swapping. I tried substuting a tested Apple LC HD with the same results and now I suspect the problem is not the HD but is somewhere else. Any help would be gratefully appreciated. Thanks!

Answer 1

Thanks to Jan & Jeff for their quick response to my question. Here is some additional background information which may give you both some more ideas.
The original 40 MB HD was replaced with a 120 MB HD along with an increase in RAM to 4 MB sometime in the first year after I bought the LC. The 40 MB HD was installed in an APS (?) external SCSI box with its own power supply. OS 7 was just becoming available and Apple gave me a set of disks even though OS 6.x was in the computer so I put OS 7 on the external (40 MB) HD. Everything worked great for 20 years although there were periods of intermittent use when I used other computers for work, Internet, and e-mail. I powered up the external HD infrequently but it worked fine the last time I tried it some months back.
Then when I fired the LC up 6 or 7 weeks ago, it would not boot unless I turned the power switch on  & off 2 or 3 times in succession then everything functioned normally. Then it would not boot at all even from the external HD.
Now, to address some of your comments and observations:
All connectors, plugs, SIMMs, and socketed RAM devices were removed, examined and replaced. All electrolytic caps showed no signs of leakage or swelling and the PWB looked in its original condition.
The +5, -5 and +12 voltages measured at the power supply connector were within 1%. When I originally thought I had a HD problem, I opened up the 120 MB HD and when trying to boot, the platter would give a slight twitch with no tendency to spin. A friend of a friend (a MAC serviceman) brought a couple of vintage drives SCSI over to try with similar results. The Apple HD I purchased was tested and guaranteed, although wiped clean. It has since been returned. As I said in my original question, the Installer on the floppy could not find a SCSI drive. In all of my efforts I could never get any of the hard drives to spin up.
Although I have a MacMini (OS 10.6.8), I still used the LC because for drawing schematics and floor plans, printing labels, etc., simple is better for me. Is there any hope that I can get my LC working again?
Many thanks for your help and suggestions. len-35