Thanks for the follow up, actually I did figure out what was wrong and it was the adapter after all.
What threw me off at first was that the multimeter showed it was still outputting 24v, so I assumed it was OK at first. But it turns out it wasn't allowing any amperage to go through. I'm a little murky (maybe a lot murky, lol) on my understanding of AllThingsElectrical, but I thought a resistor would reduce both voltage and amps, not just amps. But maybe they just affect amps? If not, I'm not sure what it would be that would restrict amperage but keep voltage constant.
Here's how it played out...
I decided to try testing amps instead of volts so I switched the multimeter setting and touched the leads to the output. Got a reading of something like 0.001 mA. Then tried the working controller and measured that. Blew the 400mA fuse in my multimeter.
Lesson #1: I probably don't know how to properly measure amps. It occurs to me that a device tends to draw what it needs, if that happens to be over the rating of the power supply you'll burn out the power supply. (Learned that lesson the hard way a couple times.)
Lesson #2: Although Canadian Tire sells Mastercraft multimeter tools, they do not sell the fuses that Mastercraft multimeter tools happen to use. Nor does Home Depot, apparently. Surprising to me because these stores seem to be big, and have a lot of stuff in them for sale. Just apparently not 400mA 250V .75" fuses. But I digress.
Lesson #3: I did get two different readings from the two different devices before my ability to measure amperage was taken away from me, so I concluded it must be the adapter, went back to Concept, got mocked accordingly, and bought back the adapter I had returned to them a few days prior. And sure enough, now I have a working powerhead again.
The last piece of irony is that I have the old powerhead working now, so the new one I bought is still in a box. I'm tempted to go return it rather than have it sit and take up space (which is what it's doing right now, .... and it's what, about $150 give or take, which is too much for it to sit around unneeded). Except that I'm sort of afraid to go back to Concepts at this point and ask to return it...
