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??