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In Reply to: "Shocking" Scott 312C Tuner posted by Hewie on March 5, 2007 at 13:01:11:
YES you do have something faulty & DANGEROUS. READ ON: There is a capacitor & resistor connecting one side of the power cord & power transformer primary winding to the chassis & thus circut Common. This is known as a Death tap situation today because as you discovered if the cord is inserted wrong way then full wall-outlet AC voltage & current are present on the chassis & all metal jacks or controls. UNPLUG the tuner. REMOVE both of those parts. DO NOT use or plug in the tuner until you do remove those parts. While the 312 schematic wiring diagram is not displayed @ the hhscott web page, the 310 & 314 are and should be adequate to locate those parts. There is also a capacitor ACROSS the power transformer primary winding; this is NOT the offending part, the purpose of this capacitor is to mute turn-off pop sound heard thru speaker. It would be a good idea to replace this switch-muting cap after all these years with a new one.
Follow Ups:
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The original purpose was to get the chassis to ground potential at high frequencies in an era when grounded outlets were rare. A small value cap would usually be connected betw one side of the line and chassis, sometimes with a resistor in series. If the cord was plugged in the "right" way, the chassis would be connected to neutral thru the cap. If it was plugged in the wrong way, the chassis would be connected to the hot side of the line thru the cap. When connected this way under intended conditions, the cap would limit current to a safe level if for some reason there was a circuit betw the chassis and ground...such as thru a grounded person. Most persons would experience such a condition as a little tingle. Real problems arise if the cap is leaky or fails outright since the chassis can become totally hot. Thus the term "death" which is a bit of hyperbole IMO but the point is valid.Since modern home wiring always includes a ground, the best approach to this cap is to remove it and attach a grounded cord set and properly ground the chassis. However, if one believes in total authenticity of vintage circuits (as some do around here), there are caps made specifically for this purpose. These caps have defined failure modes and carry the proper approvals of testing agencies like UL. Check out archives under moniker of "SGMLAW" for the gory details of cap designations and where to find them if you choose this route.
Ahh, I get it now. Interesting that in the day when outlets were not grounded that they weren't polarized (hot is small hole, neutral is large hole) and plugs were not polaraized.
and didn't expect their thinking and learning to be done by the government for them :-(
Whoa. Looking at the schematic, that is a death trap! Thanks.What is the consensus on vintage gear like this to replace the 2-wire plug with 3-wire and properly arranging hot-fuse-switch-transformer-(cap-ground)-neutral. Then tie ground to the chassis. That way you don't float the chassis.
I'm pretty much a noob, so please bear with me.
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