[R-390] HP 8640B Question

[Jeff Anderson]

I know of a couple of reasons why an 8640B might be showing all zeros...

The first is very simple.  There's a switch on the back panel that 
allows the user to select between an external frequency reference and 
the internal frequency reference.  If it's in the Ext. Ref. position, 
and there's no reference attached, you can get an all-zero display.

An all-zero display can also be caused by moisture in the oscillator 
cavity (if I'm remembering this correctly).  Apparently, this can lower 
the Q to such an extent that the oscillator no longer oscillates (for 
example, if the display goes to zero while you're changing frequency, 
this might be the cause).  The "fix" for this involves pulling out the 
cavity assembly, removing a plug on it, and then putting the assembly 
into an oven for a period of time to out-gas the moisture.

- Jeff, k6jca


Barry wrote:
> I'm curious about something.  Many of the 8640Bs I see display all zeros - 
> apparently on start-up.  Some of the ads state the display simply shows all 
> zeros - apparently even when the frequency change knob is exercised.  Is 
> this a common?  Is there something one has to do to get the frequency to 
> display?
>
> Thanks!
> Barry - N4BUQ 
>
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