[R-390] HP 8640B Question

[2002tii]

Bob wrote:

>I see several up for auction with a
>bunch of pictures.
>
>One in particular shows zeroes across the display AND the switch on the
>rear is set to INT.

The relevant INT/EXT switch is on the front -- it selects the input 
to the counter.  The INT/EXT switch on the back determines whether 
the 8640 looks to an internal or external oscillator for its time base.

That said, most 8640s I've seen that read all zeros don't do so 
because the counter input is set to EXT, but rather because someone 
once used them to count external signals and fried the counter.  That 
is the single most common 8640 problem I've seen.  So much so that I 
advise folks NEVER to use the 8640's external counter.  It comes with 
a BNC cap -- leave it there.  Note that the military Option 323 units 
do not have an external counter input, for this very reason.

Best regards,

Don