Last November I asked for help to trace the source of 240 VDC and 122 VAC on the chassis of my 5S56. I received lots of help which I appreciate. We decided that it came from a short in the power transformer. Since the radio sounded good and the voltage didn't appear to be dangerous I decided to leave well enough alone. Today I found the real source of the voltage. There was a third wire connected to the on/off switch which isn't shown on the schematic. This was a shielded wire with a black woven cloth cover. It went from one post on the switch, not the volume pot, to the plate of the 6F6 power tube and then the shield went on to pin 1 of the tube. I cut the wire out of the circuit and the voltage on the chassis is gone. The radio plays even better than before. With this wire in place if I grounded the chassis to house ground I got a very loud 60 cycle hum with the line cord connected one way and a dead radio with it connected the other way. Now I can ground it with no problems. I bought this radio about 1967 and I'm certain that the wire was there then. Does anyone have an idea of why someone would make this change?

Very odd. I assume that pin 1 of the 6F6 tube is not connected to the chassis, otherwise you´d have a short between the plate and the chassis. The schematic shows pin 1 as being connected to the chassis. After removing this odd wire, it may be wise to connect pin 1 to the chassis for shielding purposes. The wire could have been a very odd method of hum bucking, though if removing the wire doesn´t increase hum, then not. You say that the wire was cloth. Does it look original? The wire could have even been a mis-wire from the factory.

Thomas

:Very odd. I assume that pin 1 of the 6F6 tube is not connected to the chassis, otherwise you´d have a short between the plate and the chassis. The schematic shows pin 1 as being connected to the chassis. After removing this odd wire, it may be wise to connect pin 1 to the chassis for shielding purposes. The wire could have been a very odd method of hum bucking, though if removing the wire doesn´t increase hum, then not. You say that the wire was cloth. Does it look original? The wire could have even been a mis-wire from the factory.
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:Thomas

Thomas,
The wire didn't look old enough to be original, however, it did look like an old style wire. It was a small rubber covered wire inside a braided metal shield and then the sheild covered with black woven heavy cloth material. First I snipped the wire at the switch and tried the radio. I did notice a little inter-station hum and I reasoned that was because the tube was no longer grounded. So, then I cut the wire away from the two pins and tried it again. There was a little less hum then. I plan to connect a wire from the chassis to pin 1 of the power tube tommorrow. The radio is in our basement ratskeller and I have it connected to a 75' long wire outdoor antenna. There are very few vacent places on the dial for the hum to sneak into. Also, I noticed that the tone control has a much greater range. Hum bucking may have been the intent of some long ago service man. This past weekend I rigged up an antenna switch to route the antenna from the upstairs computer/radio room to the basement Zenith. I'm really beginning to like this old timey radio stuff. My next quest is to find and restore a Truetone D726 like my parents had when I was a child.

Will