(ET) Got it in one guess (E20 problems)

[Christopher Zach <czach computer org>]

Yup, swapped in my old card4 (has a dead SCR) and speeds 1-5 are back 
online.

Haven't looked at this card yet, but my guess is it's two of the four 
diodes that are in the rear. They control field current regardless of 
direction, and I think they take a real beating.

What bothers me here is that this was a new old stock card I got last 
year or so. Which means the diodes on the E20 are either undersized or 
degrading due to physical age. If this is the case, then I really feel 
for people who have E15's; with those electronic controls you could have 
a real basket case on your hands.

Chris