Today I bought a well-used HP DJ 697c to print from DOS apps.

It came with no adapter. The label specifies 30 volts at 400ma.

I took an old plustek scanner 24v/600ma cube and a radioshack 3-12v
adjustable cube and wired them in series. I got 36v with no load, so
I figured that was probably about right. The printer powered up, but
after a few minutes the plustek power cube failed. The rshack cube is
still fine.

This group's FAQ reassured me that I wasn't doing anything too crazy,
but it mentioned a rectifier, which I don't have. I didn't get the
impression the rectifier was really essential, but I don't know much
about electronics.

Any ideas why this setup would fail?

Charlie