On 5/18/10, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
  This is going to sound silly, but do you have one of
those DEC mounting
 skids fitted? I had problems with an RD53 that wouldn't work if the skid
 was on. Turns out some component leads were shorting to the ground plate
 on the skid. A piee of cardboard cured that :-) 
That's not silly at all.  That has happened to multiple people.  It
was a "well known" thing when RD53s were in use all over the world.
There's even a special DEC-made purpose-cut mylar insulator sheet,
IIRC, but cardboard works fine.
-ethan