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Okay, so the Apple II Plus I was using to test the VM4209 monitor worked fine for about a half hour, then smoked a RIFA cap in its power supply. These are a very common known failure in various old microcomputer power supplies, and I really should have already caught this on inspection and shotgunned it. Boy, do they generate a lot of smoke and stink when they go...
Electrolytics in here are also all pushing 40 years at this point and Astec opted for only 85C parts for many of these, so I went ahead and ordered a whole replacement kit from Console5. I replaced the front end filters and anything on the back end that was 85C or looked even slightly bulgy, which ended up being most of them:
Put it all back together and back in business, except noticed an additional problem: DOS 3.3 booted but neither recognized nor loaded the "language card" (soft ROM) present in the machine. Since my machine has Applesoft BASIC in system ROM, this meant no Integer BASIC (and no ROM mini-assembler) for me until this was fixed.
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