In article ,
"Jeff" wrote:

The doubler is an external device that screws onto the front panel output
socket, there is no provision for putting it internally prior to the
attenuator. The 512-1024 switch position merely changes the displayed
frequency.

Jeff-

I understand about the HP 11690A Scott mentioned. Are you saying that
it is not possible to retrofit an 8640B with the internal doubler, or
just not possible with the military version Scott has?

I have two 8640Bs. One has the doubler option, the other doesn't. I
would like to upgrade the second one if possible.

Fred
K4DII