We have an HP 8640b that will not work in phase lock mode. When you
turn on phase lock, the unit tries to phase lock for about two
seconds, then the display blinks. It does this on all frequency
ranges. Looking at the output, when phase lock turns on, the signal
jumps up in frequecy and keeps climing until the display blinks, then
the signal returns to the original frequency. I haven't opened this
one up yet, figured I'd ask if this is a normal problem before I dig
in.

I have checked for the obvious problems, i.e. cracked gears, loose
boards, burnt components..

All help appreciated.

Thanks,
Steve