[Test-Equipment] Blank Spots In HP8640B Tuning??

[Steve Harrison]

Just dragged out my old 8640B which has been stored for around 7 years.
After some clean-up, it's working as well as I rember except there are two
blank spots in the tuning range as if the tunable oscillator stops
oscillating. Output drops to zero. It runs at the lower end of the range
then quits about 30% the way up, starts up again maybe at 35% through maybe
40% then quits again all the way up to the highest end of the range. The
multipliers or dividers (I forget which these things have) are working
fine. It locks up nicely although just as I rember from years ago, there's
a small amount of cyclic FM modulation, sounds like maybe 50 to 100 Hz and
I believe it's synchronized with the display counter's updating. Not too
concerned about that right now, but the blank spots happen to cover the HF
ham bands  8-((((((((((((((((((((

What to look for?? I no longer have a manual and haven't been able to
download the one on Bama.

The fan was also not running but I finally got it going after dripping a
cupla drops of Liquid Wrench's Super-Lubricant into the front and rear
bearing locations. Will have to keep an eye on it, though, in case it quits
again. Anyone know whether the fan driver circuit will survive if I simply
unplug the fan and install a small muffin fan on the outside?? The
alternative is to find another fan from a junked unit......  Looks like
it's a four-phase thingy made by Siemens.

Thanks,
Steve K0XP