Harke,

I had the same thing happen to an 8640a. The failure was the output gold IC.
I found one for about $150 bucks. The failure happened when I had some
low voltage ac get back into the output, and I happened to have the attenuator
in the max out (no attenuation) setting.

As a note to anyone who has an 8640a or b, there is a reverse power option
that might save the power amp. I found the best preventative is to use a 20db
external N connector pad connected right at the output. Just divide the output
voltage level by 10 (20 db). eg, if you have .75 volt output, you actually
have .075 volt output from the 20 db pad.

Jack