They are also pretty tolerant of swapping the A8 board between 'scopes.
Apologies in advance for necroposting...
I now have two PM3394Bs.
The first I coincidentally bought just prior to Dave's video, and it came fully loaded with Math+, extra memory, GPIB and the External connectors, but sadly CH2 & CH4 have substantial offsets in digital mode (analog is fine), indicative of a broken hybrid on the A8 board.
Anyway, another PM3394B turned up on eBay a few days ago for a reasonable price, so I ordered that one too with a view to using it as a parts scope for the first one: the eBay description detail was rather sparse, so I was expecting the worst, and it was already clear from the pics it didn't have GPIB or the External connector options. Well, despite my prejudged misgivings, it turned out to work perfectly, and furthermore it's a far cleaner unit than the first, and, like the first unit, it also has the extra memory and Math plus. Even the graticule illumination bulbs are working.
Both units have the same firmware versions (EXT-V4.0, 1996-10-02, UHM-V1.0, UFO-V2.0), but the first one has 640KB ROM with the second one having 512KB ROM, presumably due to the GPIB being missing.
So the question is, do I take the working A8 digitiser board out of the new clean scope to fix the fully loaded but aesthetically displeasing first scope, or move the appropriate boards, daughter boards and connectors from the grotty example to the clean unit?
Bonus question: is there any significant advantage to updating the EXT ROM to 4.1?