[ntsc525]

Hi, everyone:

I believe this question applies to most transformer sets with AC line bypass capacitors between the line and chassis, that require a ground connection. The issue is AC "tingle" (leakage) between the chassis and ground, and introduction of noise when the chassis is grounded.

In the case of the 70 (and probably most sets from the same period), the value of the bypass condensors (#47 in the 70 schematics) is .015. When I did the first set, I used plain polyester caps at .015, but on the second one I had safety caps (X and Y), and combined a .01 and .0047 to keep the original values. These fit inside the block. When I pull the first chassis out again, I'll replace the yellow caps with safety ones.

On the first 70 I did, I decided to install a nice cloth covered three wire grounded power cord and ground the chassis internally so I wouldn't have to provide a ground connection to the lug on the chassis. I noticed the set picks up hum on some stations (especially my home transmitter, a Talking House prone to hum anyway), but didn't think much of it until I did my second set.

The second set has a nice original power cord that's still supple and intact, so I left it, and I built a small adapter to plug into a 3 prong outlet, tap the ground lug and run it as a separate ground wire to the chassis.

So now with all that background, we have this:

Radio on the bench with a short wire for an antenna, which is enough to pick up the So Cal major stations (KNX and KFI) plus a handful of weaker stations in between.

I noticed two things when I connect the ground cable: Some stations (KFI, whose antenna I can see from my house) get weaker, KNX has increased noise and hum, and on some of the weaker stations, they may get stronger, stay the same, or get more noise.

Also, making the connection draws sparks and makes noise on the radio. I get about 60-something AC volts, in the under 100 milliamp AC range, which is enough to "tickle" but not a major shock hazard.

Through experimentation, I determined that adding a .0022 uF capacitor between the ground wire and the lug improved reception, as it's a small enough value to provide fairly high resistance to 60 Hz but still reasonably pass 550 KHz. However, in this configuration, one would still get a small shock if they touched the chassis and ground, and they'd feel the same thing from the antenna lead, which will carry the same "tickle" voltage from the chassis through the first RF transformer.

So, my question is, do I leave the cap values as designed? Or, would an ideal bypass condenser value for the AC lines be smaller, say .01 to reduce the amount of leakage to ground?

I like the idea of a grounded plug on an AC set that may come into the hands of non-technical people, because it prevents people from feeling the leakage, and all they have to worry about is the long wire antenna. However, I don't like the noise it introduces. I thought about modding my set with the grounded plug to put .0022 uF between ground and the chassis, but that would expose people to the 60 V tingle which they may find objectionable. (They'll also feel the tingle from the antenna lug.)

If a set is going to have a grounded plug, I think the ground must be tied to the chassis, since the original design calls for grounding the chassis, but now I see what's really happening through the bypass caps.

I know some people object to the idea of a three wire plug on any antique radio, but electrically it is exactly the same thing as grounding the chassis to a water pipe which was the original design, and no doubt a good ground kept people from feeling the tingle from the chassis.

I'm no electrical engineer by any stretch, but I did discover this page for calculating a capacitor's impedance for a given frequency (https://www.allaboutcircuits.com/tools/c...alculator/), and at 60Hz, the .015 uF bypass caps provide about 177K Ohm "leakage" (impedence) to ground, which, I'm guessing is enough to feel a tickle from an ungrounded chassis.

Anyway, I'd like to hear opinions, because the ungrounded chassis is still out, and I'll have the grounded chassis out soon to move to a better cabinet, so I have an opportunity to make changes.

Thanks.

Dan