That sounds like a good guess, but I can't say for sure.

I was worried it was something in the power supply.

It turned out to be something in one of the two internal SCSI hard drives, both of which were non-functional anyway.

I feel like I dodged a bullet. Replacing the power supply would have been a pain.

I pulled both drives and replaced them with one I ordered from We Love Macs. It boots.

The new drive was cheaper than the ACARD ARS-2000SUP adapter box I was considering, but I still wonder if I should have gotten the adapter.

The drive probably won't last as long as the adapter, and I had forgotten just how loud those old SCSI drives were. Wow.