(ET) E15, troubleshooting field weakening

[Graham Allan <allan physics umn edu>]

Lost field weakening on my E15 sometime last fall, and now would like to
restore the tractor's turbo power!

I've gone through a fair bit of troubleshooting, and can only conclude
there's a fault in card 1. But my electronics skills are rusty enough
that I'm a bit mystified about the nature of the fault.

The FW relay is fine - speeds 4-7 operate as normal if the relay is
engaged by pushing it closed, and the relay does engage itself if I
apply 36 volts (eg by swapping over the contacts from the reverse
relay). It measures iditically in every way to the reverse relay, so I'm
pretty sure the relay itself is fine.

If I measure voltage across the relay (wires 13 and 39, or contacts 12
and 15 on the card), I see 0.7 volts. Odd number... looks a bit like a
diode voltage drop? However if I measure this again, but with the relay
disconnected, I do see 32volts. So it somehow seems as if the FW output
from the card is "weak" in some way, and can't drive the relay.

To complete the testing, I can get all speeds if I jumper between card
contacts 14 and 15, which closes the FW relay using the 2A contactor
output. Doesn't seem like this can do any harm, other than losing the
time delay between 2A and FW.... but it's obviously no real fix.

So there are only about 7 components on card 1 which could be
resonsible for this failure, but I'm not sure where to look next! Has
anyone encountered or fixed this before?

The card is a pretty recent Bill Gunn model, by the way. My original
card basically didn't work (a few components looked as if they had
either vaporised or otherwise disappeared, and another spare worked but
also lacked field weakening - maybe the same fault). I really feel I
should be able to repair this one...

Graham
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