Pretty amazing machine for it's time. Been toying around with the idea of getting a Rigol for every day use, but I've not been able to bring myself to the point of dissing my DSA :)
The service manual indicates that the executive processor tries to communicate with the digitizer (IO Board I believe) and the display card via the back plane. If this communication attempt fails, it gets stuck at the "Comm test in progress" point of the startup sequence. I did have this error once when I upgraded the ROMs in my 602A. After much frustration, it turned out to be a mis-seated ROM chip (one leg had bent under the chip) on the display card. You may want to try to reseat these cards in the back plane.
