Old filter question

Eric-

My impression is that the filter is not needed for home use. If it were mine, I would remove it.

Make sure you connect the 3-wire power cord safely. Assuming only one side of the line is switched, that would be the "hot" side. And be sure power-line ground connects to your chassis.

Fred

Greetings Fred, Even though both wires are switched I will for sure be connecting the ground to the chassis. Thanks, Eric

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.

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.

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

Yes, failed 'lytic caps are a prime suspect in equipment as old as this. You're likely to find a bunch that have dried out and/or leaked - many will probably show low capacitance and/or high ESR.

Be sure to be careful about high voltage - the HV filter caps and even the CRT itself can hold enough charge to lethally ruin your whole day (and all of the ones that follow).

Replace every one of them, paper and electrolytic Leave the ceramic and mica alone for now. Also, remove the filter. Connect directly, also for now.

Get the scope working, then decide if you want to add a filter.

Lytics are sometimes fine, sometimes bad. Papers mostly bad, and the ones that aren't soon will be 'Micamould' caps are paper not mica, they just imitate mica caps Ceramic & film are usually fine. Wholesale replacement is not usually a great policy.

NT