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Chris Jones[_3_]

On 22/10/2018 10:23, wrote:
I have a Tektronix 465B. I bought it used because it was one of the
scopes recommended by folks who post here and on the electronics basic
group. And I know hardly anything about oscilloscopes. It has worked
very well until two days ago. The volts/div switch has one position
where the display gets all wanky. I thinks that's the proper technical
term. When any other position is chosen on the switch the display
works perfectly. In the one bad position the display shows different
types of wankyness as I wiggle the selector knob slightly back and
forth. So I'm thinking about spraying some Deoxit D5 into the switch.
I'll probably need to remove the scope cover to do this. Is this a bad
idea? Is there a better solution?
Thanks,
Eric

Have a look in the service manual - it is online, or at least the one
for the 475 is, and that is the same construction style.

I recall reading that because the contacts are gold, they don't tarnish
and just need the dirt cleaned off, and would not benefit from any oily
residue (as left by many contact cleaner sprays). Also, since they are
open to the ventillation air, any oily residue will make dust stick to
it and bring back the problem sooner than with just a cleaning.

I expect that the recommended cleaning solvent is probably some sort of
Freon, but in the absence of that, pure isopropanol is probably ok.

The little contact forks that press against the PCB traces are very
delicate, so don't touch them with hard objects unless you know what you
are doing.

A problem that I had, with similar symptoms, is that the little
attenuators are socketed, and I had dirty sockets on one of mine.
Pulling out that attenuator and reseating it fixed it - the switches
were not the problem that time.

There is a table in the service manual telling you which attenuators are
switched in for which vertical volts/div setting. Using this table and
your observations about the "wankyness" on each range, you could
probably narrow it down to one attenuator, so that you have to mess with
it in the minimum number of places.