Possibly, the high impedance buffer amplifier is oscillating.
I was the project manager on the 5334B frequency counter,
which was derived from the 5334A. The 5334A used a
gold plated PC board. Gold plating requires that you first
nickel plate the copper. It turns out that the nickel is quite
lossy at RF, and the gold is less than a skin depth, so it
is transparent to the RF. For the 5334B, we initially used the
identical PC layout as the 5334A, except we went to the
then-new process called "soldermask over bare copper" (SMOBC).
AFAIK, the 5334B was the first HP intrument to use a
non gold plated PC board. Anyway, the front end oscillated
on the new boards, because the lossy nickel was no longer
damping out the oscillations. We had to insert a discrete resistor
at the input of the buffer amplifier to damp out a 100 MHz
resonance in a long trace.

I don't know for sure that the 5341 has the same basic front end,
but knowing how things went at the Santa Clara Division, it is
likely to be very similar. I never worked on the 5341 nor do I
have a manual.

Rick N6RK





"Francesco IZ5DWF" wrote in message
m...
Hello,
I got recently a (very) used HP-5341A frequency counter and it seems to
have
a problem with the high impedence input (which counts something even when