On Fri, 9 Aug 2019 15:53:51 -0700, (Dave
Platt) wrote:

In article ,
wrote:

The power cord had been removed so I looked inside to see if I
could determine where it used to go. I saw what looked like a
capacitor in a rectangular can with hermetically sealed pins, 2 on
each end, coming out. One pin hole was empty and it looks like solder
is splashed on the can around the empty hole. This can is called a
"Filter" on the schematic.
I found the manual online and looked at the schematic. There are
two schematics in the manual showing input power. One has the power
going through the filter and the other does not. Each schematic is
labeled the same.

In looking at the manual, it appears to me that the original
AN/USM-177 version of the scope does not include this filter, while
the later AN/USB-117B versions do.

Since one schematic does not show the filter I want to bypass the
filter. Does this sound like a bad idea?

It is probably not _functionally_ required, but the scope's
performance and reliability might be slightly affected without it.

Filters like this serve two purposes: they help keep any RF noise on
the power lines from getting into the scope (coupling through the
power trnasformer and getting into the power supply rails) and noise
generated within the device (e.g. switching noise from the diodes in
the power supply) from getting back out into the power lines. In a
low-power device like this, the former role is a lot more significant.

The FL201 filter module looks like a "differential mode only"
filter... it's probably got one or more inductors in series with the
two power wires, and perhaps a capacitor or two between them. It
doesn't show a connection to chassis/ground and so may or may not
provide common-mode filtering.

I don't really care much about the old scope, but I would like to
get it going well enough to turn it into a scope clock.

It should work with the filter bypassed. If you want, you could pick
up a more modern filter module (Corcom makes 'em in many sizes) and
wire that one in, in its place.

A lot of the newer ones have both common- and differential-mode
filtering. On the line side, they have three leads - hot, neutral,
and chassis/ground - and would be wired to all three of the contacts
on the power plug.

You can also buy single-piece "power entry" modules, which take a
modern IEC power cable (the type you see plugged into computers and
the like) and which incorporate a filter of this sort inside the
module. All Electronics has a couple of 'em in stock. They might or
might not fit your chassis, with or without modification.


That's a good idea Dave, the IEC filter. I've seen them on the All
Electronics site. In the meantime I think I should try to check as
many caps as I can. I'm not sure about how well the scope will work if
any caps are replaced but maybe I can adjust the scope by using the
manual. It looks really comprehensive.
Thanks,
Eric