Recapping an EICO Model 324 RF signal gen

[jdutky]

I've got a EICO Model 324 RF signal generator that I want to fix up. One of the things that I want to do is rework the power in section to modern safety standards, including replacing the power cord with a polarized power cord, and putting proper safety capacitors in before the transformer. The current design calls for two 0.01mF paper caps between the power lines (hot and neutral) and ground. My understanding is that a modern design would use class Y safety caps in this same arrangement, but I can't find a class Y safety cap in more than 1 uF specs, only class X caps, which clearly are not the right choice for this application.

Could I use the much smaller (1 uF) class Y capacitors in place of the 10 uF caps in the original design?

Could I eliminate the ground connection and use a 10 uF or 20 uF class X cap across the hot and neutral wires for the same effect as the two capacitors in the original design?

I have other questions about routing of power through the power switch and placement of a fuse, but I haven't given them enough thought to pose the questions yet.

Here is the partial schematic showing the power in and transformer, along with a parts list. The capacitors in question are C1 and C2.

-- Jeff Dutky

[BrokenYugo]

That position calls for a .01 microfarad safety capacitor, or 10 nanofard if you prefer.

Old notation uses "mF" for microfarads, "mmF" for picofarads, or everything is unlabled and assumed uF, leading zeros be damned.

Capacitors in the millifarad range didn't exist back then, and putting one across the line would result in fire, remember that caps conduct AC, and the current goes up with capacity.

[WattsThat]

I hope you mean by stating a “polarized power cord” you actually mean a 3 wire cord set, not just a two wire cord with a plug with one taller blade.

Even with a 2 wire polarized cord set, you still would not/should not connect the neutral to the chassis. That leaves you with caps across the transformer with no actual earth per the original schematic, effectively making it an across the line or X cap rating application.

If you want to make it safe, that means a 3 wire grounded cord. It’s proper to ground anything metal that you can touch. Then your Y rated caps actually have a proper ground.

Moral of the story: Use a three wire cord set or just don’t bother.

I’m going to make an assumption here that you’re not 100% knowledgable of mains connections. Apologies in advance if I’m preaching to the choir.

Any switching of power must be the active or hot wire, never the neutral wire.

In the US, inside our standard 120vac receptacles, we use black for the hot wire, white for neutral and the bare wire for ground. Looking at the front of the receptacle, the ground is the D shaped opening at the bottom, the larger vertical slot on the left is the neutral and the small slot on the right is the hot. You’ll find the hot is connected to the brass colored screws, the neutral to the chrome or silver screws and the ground/earth is always green.

The standard in the rest of the world is that hot is brown, neutral is blue. The earth/ground is yellow with green stripe or green with yellow stripe.

If you use an old computer cord set and cut off the computer end (female end) to permanently wire it into a piece of gear, you’ll find the color coding is brown/blue/yellow-green about 98% of the time. Sometimes the ground is green-yellow and in rare cases, the colors will be black/white/green.

Re-cording old gear is always a good idea, just double check everything with an ohmmeter before plugging a unit back into mains. The last thing you want is to have the hot connected to the chassis.

[bob91343]

I disagree with much of this.  The original design was adequate.  Modernizing it is asking for trouble, as evinced by the variety of suggestions and assertions.

If you want to use a three wire cord, the way that has worked for me many times is simply connect the primary of the power transformer, via switch and fuse, to the two active power leads.  Remove the two capacitors.  Ground the green wire to the chassis.

[BrokenYugo]

Yeah, crappy paper caps aside the electrical safety of most 2 wire stuff, at least post WW2, isn't far off from modern standards so far as I can tell, exposed metalwork is usually isolated from mains by high value resistors or low value caps. The whole point of a class Y safety cap is it's safe to put one end to mains and have people touching the other, and this is done all the time in current production products.

Like if we're dealing with a old true "hot chassis" (one side of line direct to chassis) transformerless radio or something then yeah, rewiring the thing to float the chassis is warranted. But for something like the circuit in question you're doing more than due diligence just using the proper safety capacitors on the mains side of the transformer.

I'd leave this thing 2 wire, maybe add a fuse, and probably commit the sacrilege of switching the neutral side to reduce risk of hum issues. You can get nice 2 wire cords cheap by buying a cheap household extension cord and cutting the end off.