(ET) E20 down, only one speed forward

[Christopher Zach <cz alembic crystel com>]

Never dull. Drove the tractor out of the shed, got to the top of the 
hill, realize I only have speed 1.

Stop tractor, check. Sure enough when in F, I have speed 1 and that's 
it. Switch to Reverse and I have speeds 1-4 like normal.

So it looks like 1A,2A,3A are getting power in reverse but not in 
forward. Probably something blown in card 4, checked it out. Sure enough 
another mouse nest, and this time they chewed into a few wires of 
insulation on the high speed (5-8) wires.

Card4 has a blown trace where pin 20 goes to C405/406. Probably explains 
part of the lack of speeds 5-8. But the weird thing of course is why 
does things work in reverse.

Maybe the issue is that pin 16 from the card 4 (which seems to carry 
power to 1a,2a,3a) is only active when power is on 17 or 19. 17 and 19 
seem to go to the hot side of 1F,1R so my guess is the one that goes to 
1F isn't getting power.

Then why would the motor run at all in F since 17 and 19 both bridge to 
feed 18, which is of course the field. No field would mean a very 
unhappy tractor motor.

Or one of those two diodes CR402 or 403 crapped out however when I put 
in another Card4 I got the same result (it might be dead too).

Never dull.

Chris