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Transformer Set Shock - Printable Version

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Transformer Set Shock - dconant - 09-13-2026

Hello, I always work on radios using isolation transformer. Once I am through I might play the radio for a while in my shop using the wall plug. Once when I did so I saw small spark when I connected the antenna ground. That ground is to a ground rod outside the house. Why would this be and is it a concern? The radio was a Philco 60. 

Thank you, Dan

462ron - 462ron - 09-14-2026

Dan, it looks like there is a .015mfd going from one leg of the AC line to chassis ground. Be sure that has been replaced with a safety cap. If it has, that .015 is to remove some interference from the incoming line but also it can allow enough current to cause a slight spark when trying to attach an external ground wire but not enough to give you a shock when touching the chassis!

Ron

RE: Transformer Set Shock - dconant - 09-14-2026

Thanks Ron. Yes I always use a safety cap(s).

Dan

RE: Transformer Set Shock - Arran - 09-16-2026

I don't think it is really a concern, there may have been a small spark because whilst the neutral side of the household wiring is at zero, with respect to the two hot lines, it still may be above where an outside ground is, or even vice versa, so there may be a potential there rather then an electrical fault. This is also why I wince at people adding a grounded cable to an old radio with a grounded plug, the ground line runs all through the house and could be picking up electrical hash from anywhere. I've tried connected the ground terminal on an old radio to the power line ground but the results are mixed, sometimes it helps, often it does nothing but make the gain drop. On some sets you have to add am external ground because the volume control is in the antenna circuit and it won't work properly without it.
Regards
Arran

RE: Transformer Set Shock - MrFixr55 - 09-16-2026

On the Philco 60 schematic, there is that 0.015uF cap from chassis ground to the unswitched leg of the power line. One way of ensuring no "tickle" from the chassis is to install a polarized plug to ensure that the switch is on the hot side and the cap is on the neutral side.

A lot of sets have 2 caps. One each from each leg of the power inlet to chassis ground. This essentially puts the chassis at approximately 60V. I had one blow a fuse when the chassis touched the grounded frame of the display case that the chassis was on top of. (Edited 09/21/26 to make sense.) This is because one of the caps was shorted, a common problem with old paper caps across the AC Line. I suppose that on these sets grounding the chassis is important to allow for "common mode rejection" of incoming power line noise as well as completing the antenna circuit. Not grounding the chassis allows for the completion of the antenna circuit through the 2 caps to the power line, likely inducing noise.

RE: Transformer Set Shock - dconant - 09-16-2026

Not sure anymore how I wired it. I sometimes use two safety caps on single cap sets. One on each leg. Not mine but the radio Icon_lol . Seems like all my 30's Philco sets like my antenna ground. It quiets them right up.

RE: Transformer Set Shock - Arran - 09-16-2026

i think it's worth observing that Philco sets, unlike RCA or G.E, did not have an electrostatic shield on the power transformer, it's a band of copper foil wrapped around the outside of the core, and grounded to chassis, so Philcos, not having this, needed to have caps on the power line. Philco seemed to really like adding capacitors anyhow, they always seemed to use more paper caps in their circuits then other makes, at least in the 1930s and 40s, especially their early TV sets.
Regards
Arran