I am recapping a Crosley 648 which is an AC/DC radio from about 1938. I wanted to add an inline fuse to the radio but am a bit puzzled by the placement of the switch in the circuit.

[url]http://www.nostalgiaair.org/PagesByModel/550/M0003550.pdf[/url]

By my read of the schematic, it looks to me like the switch is on the side of the plug that I would have thought should be the neutral side of the input power. The switch has one side tied to the radio chassis. This is my first AC/DC set so I am wondering if this is common or am I reading things wrong?

Shouldn't the hot side of the plug be the side that is connected to the switch and also fused?

Thanks

Randy

You are reading things correctly. Where the switch was placed in the old radios' was at the whim of the designer. In my line of work I still run across houses that have both fused hot and fused neutrals. The power switch installed on the line side was simply not the standard at the time.
MRO

Almost all radios from this period have the line switch on the chassis side. Call it the neutral side if you wish. There is a definite reason for this. Household switches were always placed on the hot side, so it wasn't as though radio manufacturers were morons.

The reasoning is that if you place the switch on the "hot" side, the chassis is always connected to one side of the line cord. If you plug in the radio "backwards," the chassis will be hot, and you won't be able to turn it off. Now, if manufacturers wanted to go all out, they'd install a polarized plug and a fuse and wire the switch on the hot side, or perhaps do a whole lot more. Polarized plugs weren't common back then except in direct current installations, and fusing adds cost. So, to remedy a half baked situation, they put the switch on the chassis side. If you plug in the radio so that the chassis is cold, when you turn the switch off, current flows from the hot side through the tube filaments, causing the chassis to be hot enough to give a good shock. Now, if you accidently shorted the chassis to ground, all you'd do is light the tubes. However, the shock is the real problem. So, in this case you could simply turn the radio on....if you happened to be grabbing the bare shaft (hopefully the on-off shaft and not some other shaft), and though to turn it the other way. Normally when I'm getting shocked, I either pull away or drop whatever it is that's shocking me. Well, anyway, to give their thought some credit, we'll continue with the explanation anyway. If the chassis is ever plugged in so that it's hot, the simple solution is to turn the switch off. So, either way, if you have trouble with the chassis, all you need to do is turn the switch the other direction. Also, since the chassis is capable of being live, and it is the largest exposed electrified object in the radio, it would only make sense that you put a switch on it. Idealy they should have equipped the radios with double throw switches, which would allow switching of both sides of the line. These were readily available, and were put into most 3-way portables, so I don't see why they didn't put them into AC-DC sets. As always, though, if they didn't have to, they didn't.

I respect AC-DC radios, and have no problem with how they were originally wired. I take care of them and observe what I'm dealing with. However, if you wish to make the radio safer, adding a polarized plug (unattractive but effective) and putting the switch on the "hot" side will do the trick. Of course there's always the problem of someone plugging it into a non-polarized outlet (forcing the plug in), such as in an older extension cord or with an older 3-way outlet adapter. The best thing to do, in my opinion, is to simply respect them for what they are, and to observe proper safety precautions when working on them. You can be careless with them on your living room carpet or hardwood floor, but be extra cautious with them when working with other electrical equipment (signal generator, two radios side-by-side, etc.), or when working on them in a basement, or when using them in a basement, outside (terrible), or in a bathroom (also not a great idea unless placed on the floor, away from you and water), or perhaps in a kitchen, though a kitchen is fairly safe as long as you don't touch any metal parts while washing dishes or operating the stove. Keep the radio equipped with knobs. If you must "update" the radio, though, the polarized plug and switch wiring change will do some of the trick.

Also, regarding fuses, if you want to fuse the radio with its existing wiring, it is best to put the fuse on the chassis side of the line cord. This way if the chassis is plugged in hot, and is bumped against some "ground" source, the fuse will blow. The fuse will also protect the radio and your house against shorts that may occur within the radio. Actually I recommend putting the fuse on the chassis side even if you do update the line cord, as there still is that slight possibility that someone may plug it in backwards and then bump the chassis against something. I'm really stretching things, but if you're going to make the radio safe, you might as well go all out. Perhaps put a fuse on both sides of the line cord.

Thomas