I have been working on an Athearn "blue box" SW7.
My first attempt ended in failure because I had neglected to check,
to see if the chosen decoder would fit in the locomotive.
I had the decoder laying around.
I checked the Digitrax sizing app,
and they recommended the smallest decoder they make. Wow.
Anyway, after it arrived I determined it would fit
(I ended up mounting it on top of the front gear tower)
I did the install.
When I put it on the programming track it programmed perfectly.
But it wouldn't run. Just like the SW1200.
So I examined all the connections.
I had determined that the locomotive ran fine before starting the conversion.
I could find nothing wrong.
I ran a resistance check, between the motor frame and the chassis.
Completely open !
Though I had tugged on the bottom motor connection, confirming it was firmly soldered,
I hadn't pulled the motor to look at it. As a last resort, I did that. Very firmly soldered.
In fact, the wire was buried in a rather slovenly blob of solder.
Upon close examination, it looked like that blob of solder, might be able to contact the frame.
Without much hope that was the problem,
I applied some additional tape to the frame and reassembled everything.
Well, at least the chassis.
I wasn't about to finalize the install until I was sure it basically ran.
AND IT DID!
That bit of solder, had been defeating my efforts to isolate the motor.
You know where my mind is going.
Once I have the SW7 completed
(There are some minor issues. Something is still rubbing on one of the flywheels)
I am going to take a VERY close look at the SW1200's motor isolation.