[ARC5] Dynamotor Whine

[David Stinson]

Besides the good suggestions about the bypass caps across the 
brushes and grounding the baseplate to the radio-
I infer from your original post that the dynamotor has not been
reconditioned before running it?
If the bearings have not been lubricated, the armature could be
"dragging" and this will cause heavy brush sparking.
These dynamotors are 70+ years old.
They should never be powered before being reconditioned.

73 Dave S.