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Jim Yanik

"hr(bob) " wrote in
:

On Feb 20, 10:01*am, Jim Yanik wrote:
JW wrote

:

Hello all,

I have a Tek 475A whose CH1 bandwidth falls off as the input
frequency increases. At 200MHz, the vertical amplitude is about
half of what it should be. CH2 is working fine. The fault seems to
exist no matter what the vertical sensitivity is so I don't think
that it's the extenuator, and the 100 and 20MHz bandwidth limiter
also appear to be working. Does this fault suggest anything in
particular to the experts out there?

Thanks!

If the problem is just on Ch1,then you have a problem in Channel 1.
Ch 1 and 2 don't combine until the delay line driver/channel
switch,IIRC.

the tiny HF trimmer caps in the 400 series had a habit of developing
a black oxide underneath them that effectively insulates the rotor
from the solder pin,making the cap -open-.(and no effect when
adjusted) They also would freeze so they could not be turned without
breaking the screwhead loose from the rotor disc .

essentially,you have to replace the bad trimcaps.
Then recal per the cal procedure.

--
Jim Yanik
jyanik
at
kua.net

Have you swapped probes??

you shouldn't be testing/calibrating BW using probes.
you use a levelled signal generator,50ohm coax,a 50 Ohm feedthru
termination,and appropriate 50ohm attenuators.

--
Jim Yanik
jyanik
at
kua.net