[R-390] HP 8640B Question

[Barry]

I think some folks on this list have HP 8640Bs so I thought I might indulge 
those fellas a second.

I think I understand that there's a rather delicate gear that frequently 
breaks in these units.  When that occurs, what are the symptoms?  Is this, 
by any chance, directly related to the frequency change knob?  In other 
words, if this gear breaks completely, would the frequency change knob work 
at all?

Also, I seem to recall that someone made some replacement gears.  Was it 
someone on this list?

Thanks in advance for any info on this,
Barry - N4BUQ