(ET) Found the problem for that E12 guy

[Christopher Zach <czach computer org>]

To the guy who has the E12 that will go in reverse, but not forward, I 
think I have found your problem.

On page 4-4 of the owner's manual is the motor diagram for the E12 
tractor. The key here is the words "the 1R/2R resistors get very hot 
with no motion". This indicates the field is not there. However it does 
work in reverse, indicating the field is there.

The trick is this: The field is powered by a pair of diodes. One of them 
goes to the junction point of 1F/2R, the other at the junction of 1R/2F. 
Each diode (CR10, CR11) conducts power to the field in one direction. 
One conducts when you're going forward, the other backwards.

What's happened here is one of these diodes has blown open. It's the 
forward diode. Thus what happens is that you go to forward, there is no 
field, and the tractor armature does nothing but act as a conductor 
(heating the resistors).

Replace the blown diode and you will be set. Any old Radio shack 4 to 5 
amp silicon diode will probably work fine.

Chris