[R-390] HP 8640B Question

[2002tii]

Barry wrote:

>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?

There are quite a number of Nylon gears, all readily apparent when 
you take one apart, and at least two basic failure modes.  The 
collars split, which means you can't tighten the gear on the shaft, 
and the large gears shrink (and some warp) over time so they skip 
teeth.  Once the teeth skip, you either park it and look for a new 
gear, or ruin the gears by trying to use it and ruining the 
teeth.  Any 8640B on eBay that is not guaranteed by an upstanding 
seller to be working properly will invariably have ruined teeth on 
several gears, and usually broken and slipping collars, as well.  I 
have a pile of them here that folks I know bought on eBay hoping they 
had good gears, and not one of them works due to gear failures -- 
probably $3000 worth of eBay mistakes in one pile.  Some of them have 
other problems, as well.  Personally, I would not risk buying any 
8640B I had not personally seen and played with.

I have repaired several gears with split collars, by machining new 
brass collars and attaching them to the existing Nylon gear 
flanges.  I have threatened to machine new large gears out of brass, 
but have not done so as yet.  Maybe when I retire....

Best regards,

Don