All the circuit drawings seem to show a line bypass cap across the line
into the radio, or one from each side of the line to chassis ground.

I seem to recall someone advocating one cap across the hot and neutral
of the line, and one from neutral to chassis ground (on a polarized cord).
There would seem to be some advantage to that, in that there's no
voltage divider effect.

Any comments either way?

Nelson

A cap between hot and neutral is wrong. Should be two caps; between hot and chassis and between neutral and chassis.

That's the way I do it. Eliminates the tickle when you touch the chassis.

Nelson -
I wire my sets the same way as you do, for the same reason.

--Chuck K7MCG

Way kind and size cap would you recommend for that

This normally applies to transformer sets. You can actually have (additional) around 0.1 mfd across Neutral and Active with really dirty mains.

With the two caps terminating on chassis, that will bleed AC onto the chassis and theoritaically the other may remove it.
Better off with a mains grounded (3 wire) cable, several mains transformers have internal shields which also bleed a charge to chassis and that can give a gentle nip.

All of the tech's, in our Radio Club, who are fixing the oldies, retro fit earthed cables (transformerless sets rare). Do make sure that the caps are of a mains approved type.


Addendum due to post. The caps are generally aimed at getting rid of RF. I have seen them at 0.0047 to 0.047 mfd.
I have a lot of RF on lines, & get a lot of lightning so have 0.1 A to N and 0.0047 A to E , E to N permanently on the power board along with surge arrestors. Do make sure the set has a decoupler (after last filter) of around 0.047mfd on B+ some sets don't have this and can be suceptable to RF noise getting into B+.

Marc

http://www.justradios.com/capacitors.html#product

Scroll down and look at X1/Y2, X2, and Y2 ---there's a detailed writeup on each kind

>pix, that's one of the sites I reviewed but it just copies the same scheme
that seems to be all over the net.... a cap from hot to chassis, and neutral
to chassis. But again, that's a capacitative voltage divider, and there's still
a 60VAC tingle.

>magicclocks, the site sighted and cited by pixellany gives a good rundown
on the types and values of safety caps to use:
http://www.justradios.com/X2capacitors.html

>Jonnysan, one of the schematics they show has a cap directly across the line
in, with Y-caps downstream. Again, making a voltage divider.

The transformer powered radio I'm working on right now had no line caps of
any kind on it but I put a couple on... hot to chassis and neutral to chassis. I'm
not enamored with the tiny spark when I hook the ground lead of my scope to it,
but still would like to kill some line noise. The radio has a two wire polarized plug,
and for several reasons I'm reluctant to put a three-wire grounded cord on it.

I'm still mystified as to why the hot-to-neutral and neutral-to-chassis scheme is
not suggested as an option anywhere...
Nelson

The BIG problem with the way they did it with a 2 wire cord and hot to chassis/neutral to chassis, is what happens it the hot to chassis cap shorts. Answer the chassis is at full hot potential.


It has to be WAY WAY safer with a 2 wire line cord to do hot to neutral and neutral to chassis.


that being said, I like to live on the wild side!

Some radios were indeed made with a capacitor across hot and neutral.

Grandbrass has three wire cords with plugs for just $4 each. I've also used the three wire cloth-covered wire from Sundial wire. Many radios receive much better when the chassis is grounded.

How is such a decopuler connected? B+ to chassis?

Decoupling mainly refers to what the cap does. It removes the higher frequency signal that can be riding on the mains, or can be generated internally. This can manifest as background noise and the mains stuff has a bad habit of appearing on the B+. The manufacturers who put filters on the mains side, were the ones that recognised that there was a lot of crap on the mains and should not be condemed, because they at least made some attempt to deal with it.

If you look on the schematic you may see a cap that could be anywhere between 0.02 to 0.25mfd going from B+ to chassis (B-) basically in parallel with the last B+ filter. Paper filter caps will normally not have one. Electrolytic caps are not good at passing RF this is why they are there.

Normally when I replace Paper filter caps with electrolytics, I add one and I have plenty of RF riding on the mains here to test it's effectiveness and adjust. This just needs to be the normal 630V cap.

Marc

Nothing wrong with hot-to-neutral bypass. It has the most effect on tunable hum in radios with a wire antenna instead of (or in addition to) a loop. The cap effectively connects hot and neutral together, at RF. When this is so, any grounding the radio experiences via the line cord doesn't change impedance as the rectifier conducts and blocks on alternate half-cycles. Some sets put a cap across the rectifier for this exact reason.

Dave Wise