I haven't ever laid my hands on an MK1, but I assume it has a VFD on it, possibly the same as the MK2s. Have you checked to see if the heater wires are still glowing red? It's possible you've blown a fuse (if it has any) and then these would likely not be glowing. To see them you would have to power the unit in a dark room and look at the bare VFD for dark red faintly glowing wires running across the display.

Good luck,
Stu