Thanks, Mark. The technical details are beyond my knowledge, but I can clearly understand that there are important differences. Good job.
Personal Used Oven Issues
1) Used can equal stressed and that can be unrecognizable in simple does-it-power-up tests. The donor equipment was taken out of service for a reason; who knows why?
2) It seems to be that you grab from whatever is currently available on Ebay... There are lots of substitutions. I don't know what is acceptable and what isn't.
3) I read lots of stories about folks that ended up with the wrong oven, because of some minor difference in the model number that equaled a major difference in usability.
The answer to these issues is to buy from a dealer that has a good reputation and to pay the extra required to not buy on Ebay. So, I need to research four things... Typical makes and models that are used, reputable dealers, cost and a circuit design (I think I understand what is needed in the circuit and why. I just need to settle on one.) I'll take any tips, or suggestions, that anyone has to offer on those.
Another thing to consider is that I am a hobbyist. I really can't justify a major expense for this piece of equipment. But, it surely has my interest as a fun DIY challenge.
And, if anyone has an extra that they would like to swap/sell, I would be glad to know of it. As money is limited, maybe a couple of PIC18F4550 controller boards would be useful to someone... I have two unique designs... One is made to mate with a bread board. The other has multiple pin sockets per MCU pin, for easy plug-in access.
Thanks, Mark, for your help.