--- In TekScopes@yahoogroups.com, "George Headley" <georgeheadley@m...> wrote:
Hello Group: This is my first post but I feel that I know all of the regulars. I'm very impressed with your knowledge and have learneda
lot from your experiences. I an an old computer tech , retired but not ready to give up the challenges relating to Tektronix scopes. I
have been buying them mostly for the challenges found in the repairs but somewhat out of respect for equipment that is fun to work
because of the quality. I have several 465B that have poor focus. Not really bad but not good enough. There is some improvement
when I limit to 20MHz. I dont have a HV probe but I suspect HV problems. I know from experience that low HV will cause a blooming
of the trace. I have adjusted the CRT bias as per the manual. Slight improvement, astig and sweep ckts within spec. If I can trouble
you for a minute of your time do you have any suggestions. one more thing , it may be my imagination but the rise and fall lines seem
to be sharper than the time base lines..
George HeadleyThe Broadband noise of the amplifiers often makes the beam to apear "thick" and "out of focus".
As you describe, if you limit the bandwith, it reduces the tracewith significant, than the tracewith is
caused by the broadband (johnson et al) noise from the vertical amplifiers and other noise sources (resistors)
inside the vertical chain.
In my opinion, if you could see a significant difference between ful bandwith and bwl it to abt 20MHz than
the focus is usualy good.
But also not to forget is the astigmatism control, also often correctible (inside) which influences the trace thickness,
but not soo big as the focus does.