Confused about the "Death Cap"

I tried to research this myself and not bother anyone here, but I'm finding contrasting info and would like to get the facts. This concerns the Fisher 20A and 30A mono amps. The 20A schematic shows this cap from the fused AC wire to ground, and the 30A schematic shows the opposit. Now I realize that these amps use non polarized, two prong AC plugs, so, I guess Fisher couldn't predict which leg would be "hot". Still, there must be a correct way for this day and age. I was assuming the cap would be on the fused "hot" side, but info from Guitar amp techs say to go from neutral to chassis ground as in the 30A. I plug these amps in with the hot side to the fuse by the way, and probably will install two prong polarized plugs when I am done with them. While we are at it, I think I need a Y2 cap for this as opposed to an X2, is that correct?

HiFi -- You've pretty well got the gist of it. The very best approach is to use a polarized two wire plug, or (as a temporary measure only) a three wire with the green wire ground taped off. The hot lead should go to the fuse first, and then the power switch (if present), and then to one lead of the power transformer primary. The neutral lead should go to the other primary lead, and the bypass cap to chassis ground.

Some will cry what if the AC socket is wired wrong, or what if, what if, what if. If these things concern you, then get one of those little plug in test indicators to make sure the proposed socket is in fact wired correctly -- but do not use a green wire ground system with vintage equipment. They were not designed with separate chassis and circuit ground systems like modern equipment is, so using a green wire ground with vintage equipment is a recipe for bad ground loop problems when the equipment is used with other pieces of gear with green wire grounds, like cable and satellite equipment.

Dave

Neutral to chassis is preferred. I've often seen gear with 2 caps though, one from each leg to ground. At that point it wouldn't matter any which way the non-polarized cord was plugged in, you'd always be roughly halfway to ground potential on the chassis.

I'm starting to get into replacing cords with polarized, and wiring them the way Dave describes. It just makes sense, and half the time old gear needs a new cord anyway. I still do own a bunch with originals, including my 1928 Philco 511 radio which sports it's original cloth wrapped cord yet. I do not leave that stuff plugged in though.

Being an amplifier rebuilder, I always install a 3-wire cord to remove any liability issues.

I install the replacement 'death cap' from neutral to ground in case one needs to lift the ground due to a ground loop issue creating hum. The neutral capacitor provides a ground reference to the chassis thru the house distribution system ground. The neutral is grounded, but at the electrical distribution box. If the outlet is wired backwards, the amplifier power transformer primary wiring resistance becomes in series to the electrical distribution ground thru the neutral capcitor. Also, one can add the power company distribution transformer in series too. Thus, the neutral wire and neutral cap is electrically closer (lower resistance) to ground and should be a little more electrically noise free when the outlet is wired with the correct polarity.

A 'ground lift' adapter is a ground adapter plug normally used on two wire outlets with the outlet cover plate screw providing the ground. This assumes BX (shielded) cable was used on the original house wiring and provides a ground back to the electrical distribution box. BX cable is common in some areas such as Chicago and New York years ago as applied in houses.

The trick to lift the ground on an amplifier with a three wire power cord is using the ground adapter with tape over the cover plate screw lug on the adapter and not using the cover plate screw

I hope my reply is not too complicated as Dave does explain so well in common terms..

Thanks guys for clearing this up. I had it wrong thinking the cap should be from the hot side. I'm gonna stick with two prongs, as originally designed. The problem of the wall socket wired wrong is a good point though. Am I correct that I can also test this with my ancient neon test light? I'm thinking ground to neutral should not light up, and ground to hot will, right?

Sorry about that dumb question on the light. Just tried it, it works (of course). One other thing: I am having trouble sourcing out a .01 mfd. Y2 cap for this. How close do I need to stick to this value? I see a tech before me (the dreaded Fisher Doc) just used the same film cap as the coupling caps, but in the correct value. Is that proper? I don't think it can be expected to have the same results if it fails.

HiFiHarv said:

... One other thing: I am having trouble sourcing out a .01 mfd. Y2 cap for this. .

Click to expand...

Consider X1Y2 and cover all the bases. Pretty cheap and exactly the value you need at Mousies.

FYI: http://www.justradios.com/safetytips.html

notdigital said:

Consider X1Y2 and cover all the bases. Pretty cheap and exactly the value you need at Mousies.

FYI: http://www.justradios.com/safetytips.html

Click to expand...

Just radios will also sell you good quality inexpensive X1/Y2 caps in multiple values....:thmbsp:

Notdigital

Are you using those types of caps in your amp? Could you show us where are placed??

Thanks

Luis

Luis: I'll post a picture when I get home some time tonight. Basically, it takes the place of an existing cap coming right off the incoming ac line & fuse.

In an 800c, it's C-110, blue disc cap. Here's a picture to help see placement. 500c same position; I believe 400 as well.

Thanks George!!!:thmbsp:

Sorry to open up such an old thread, but there did not seem to be a newer, better alternative, and I do not want to start a new one since this seems appropriate.

I have 3 working 1970s vintage receivers and I would like to replace the cables along with a thorough cleaning, DeOxit, etc.

I am not an electrician but I know that grounding is mysterious and that there is art and voodoo involved as well as science. I would like to use a good modern 3-prong plug because in my ignorant mind it seems like grounding the chassis is the right thing to do, but that seems to be problematic because of grounding, hum, etc. So, my first question is: Should I simply use a 2-prong polarized plug and forget the ground? Or is there some component that I can insert to allow the good and stop the bad? And if, in the future, the ground wire is connected and I plugged it into a 2-prong extension cord via an adapter plug, has a danger been created?

The larger question is about the "death caps" because replacing them seems to be the thing to do while I am in there. I have all the schematics and only the Pioneer shows a cap in the circuit between the power main and the transformer. I have not taken them down and opened them up, so perhaps they will actually have them in reality even though they were not shown on the diagrams.

I am planning to order some replacements from justradios and if they provide some advantage I would be willing to add them even if they aren't in there now. As I understand it, "XY" means that it can go either way, so if I add one is it better between hot and neutral or between neutral and ground?

Thank you very much in advance for a dumbed-down simple answer and/or easy-to-follow instructions.

PS - Sherwood S-7300, Sansui AU-5500, Pioneer SX-450, Kenwood KR-1400

When I'm working on 1970's Pioneers and Sansui's, I just remove the old cap, and put an X1/Y2 in it's place, ensuring it's on the neutral side with a correctly oriented Polarized plug. . If there is not one installed , like on the SX-450, I put one on the side NOT FUSED, and run a new 2 prong polarized cord. The NOT FUSED side is Neutral (FAT BLADE). BTW, C1 is a snubber cap on the power switch. It's there to minimize arcing on the switch contacts which it does miserably! It is NOT the "Death cap".

Don't use a grounded 3 wire on these units. If you plug in a turntable or tape deck, YOU WILL GET A Ground loop that is impossible to get rid of until you remove the ground wire at the receiver. Read Dave G's explanation in post #2. Physics and Electrical theory and practice doesn't change.

Thank you! I will forget about the ground wire.

Maybe I am looking in the wrong place, or not understanding what I am looking at, but here it looks like the Pioneer has a hot-to-neutral "death cap", the Sansui has the "snubber", and the Sherwood and the Kenwood don't have any caps. But maybe I will just be surprised when I crack them open.

You are looking in the right place. What you see is 4 different ways of showing the same thing electrically. Schematic layout is different physically than it is electrically. None of the above have a "Death Cap". 2 of them have a high resistance resistor in place for lightning strikes to the antenna to chassis ground. 2 of them have a snubber cap on the switch to prevent or minimize switch contact arcing when activated.

I appreciate your taking the time to explain this. I am not going to worry with adding or changing resistors to the ground, but it looks to me like the Pioneer circuit has a capacitor between the hot and neutral, behind the switch(es? - I don't expect to actually see 2) but the Sansui cap seems to be located only around the switch, as I would have expected.

So a "death cap" has to be between the neutral and the ground, shorting closed and making the chassis hot? If it shorted closed between hot and neutral, is that even possible?

edit - I went to justradios and discovered that the minimum order is $20 + $8 shipping. Is there a US source for just half a dozen of them at a lower price?

You can get X1Y2 rated caps @ mouser. They run about $2.00 each if I remember correctly.

X-Y caps are designed to "FAIL SAFE". That means they OPEN when they fail, not short when failing. Regular caps fail SHORT. So when a regular cap fails in this type of circuit you can get full line voltage to the chassis. Not a nice feeling!!!

The snubber cap across the line switch should really also be X/Y rated because it has the full AC line voltage across it when the switch is off. X/Y caps are rated for AC while most others are DC voltage rated.