So far I am just thinking about setting up a workbench again, I have been away from really working on any radios for many years. I have quite a collection of Heathkit and Eico equipment that I would need to get going again. I understand that many folks change the power cords to 3 prong with ground. What are the ups and downs of doing this? Advice is appreciated. Jim

Having the cabinet of these testers grounded is a good idea. Then have an isolation transformer to plug the radio you are working into, nothing else. This is a must with an AC-DC set (transformerless). A radio that has a power transformer shouldn't need an isolation transformer to power it but there are usually a couple small caps from line to chassis. Of course these should be replaced but even with new caps there will be a small current that will flow from chassis to your grounded test equipment without the isolation transformer. I always check to make sure the power transformer in a set has no leakage current from the primary to case.

Second Lou's advice. My collectible test gear comprises mostly Clough Brengle and as such, is of all steel cabinet construction. After having been bit a couple times, even after a re-cap, I decided several years ago to ground the entire lot of them.

Since prior to 1938 the power cords were (mostly) of the cloth type, I was loathe to install modern rubber three wire cord. Fortunately of course, three-conductor cloth covered wire is available for the purpose, and is sufficiently close in appearance, size, and current rating to the original to suit me (if I was a little pickier though, I would have dyed the cord black as per the original, versus the brown it currently is).

The only mods to the units have been finding a convenient place to attach the ground wire to the chassis. Since the C-B stuff from the 1930's period is liberally festooned with fasteners, this has proven relatively easy to do, although on occasion I have had to anchor the ground some distance away from where it enters the chassis.

Lastly, I added a standard black rubber grounded plug . I feel that even if I show the gear, most people will not bother to trace the power cord to the end to find out what the plug looks like. They tend to be focused on that weird, glowing, dialed and metered box with all the switches.

On my later Eico, RCA, and other stuff, if it had the zip cord originally, whenever I have reason to delve into the entrails for repair or refurbishment I change the cord out to the round black grounded utility cord as a matter of course.

Replacing the two-wire cords found on many older test instruments with three-wire grounding cords solves one problem, but it creates others. It is therefore worth considering both sides of the argument before proceeding.

The problem that is solved is, providing safety should something goes wrong inside the instrument that causes its exterior to become "hot."

The two new problems created are:

(1) if you come into contact between something "live" and the now-grounded instrument, you will get a shock. This would not occur if the instrument was left ungrounded.

(2) the negative or ground clips on many instrument test leads are connected to their chassis. Should you connect a grounded lead to a "hot chassis" device on an isolation transformer, it will re-reference the power to ground and some of the shock hazard returns. In other words, it defeats part of the purpose of the isolation transformer!

Most service instruments that were originally sold with two-wire cords also came with stern warnings to connect external ground wires to case screws, lugs, or ground terminals, so grounding was at least highly recommended. But it was left up to the users to ground the instruments or not depending on what other test equipment they owned and what kind of work they were doing in the first place.

Anyway, those are the pros and the cons. In my opinion, safety features like equipment grounds are for protection against hazards which cannot be foreseen, such as insulation breakdowns, while ground clips or touching "hot" and ground at the same time have perfectly understandable consequences. Therefore, putting three-wire grounded cords on test instruments which did not have them originally is an excellent idea.

I have a Hickok 6000a that does not have the 3-wire cord or plug, I did not want to change it because the cord and plug was in excellent condition.
I have felt a tingle on the front panel of the tester and had to reverse the plug.
I have had sone scopes that did not have the 3-wire cord or plug, those I also left alone.
Everything else was changed over to the 3-wire.

If you have ground fault circuit interrupters the caps to ground will trip them occasionally.

1st I would trace the way the power cord comes in and from what I understand the wire going to the on/off switch should be the hot wire and the other the neutral. Another thing is that if you want to add an earth ground is that if your chassis is the potential you don't want to have the potential and the earth ground directly linked to each other, so you float the earth ground by using a 5 watt 100 ohm resistor in parallel with a cap that is .1uF and 250VDC or a little higher. It creates a buffer to keep the potential from running away down the earth ground but if you have a power transformer melt down it will over power the buffer and maybe save you till the fuse blows and shuts the thing down. I think the main idea is to have the earth ground there but not exactly on the same level as your potential so that you aren't running the B+ that is separated from the chassis by a cap or resister in many places and have it run off into your plumbing, or into the earth by a copper coated steel rod.
My old house has both and you don't want your wife to get zapped in the kitchen if you have something melt down on your bench.
The floating the earth ground when adding a 3 prong cord to something made when there were only two with both prongs the same size was shown to me by someone who has been rebuilding old theater amps from the 30s to Mac mono blocks and even upgrading new so called high tec tube amps like the Manley Stingrays and everything in between. To him it is a standard operation and for the most part 3 wire cords are done with the computer type plugs so that you can use expensive cables if you want.
For my better looking amps I go to Home Depot, get some 3/12 and cover it with some fancy looking sleeve, use some hospital grade plugs and it is basically the same thing as ones that cost big bucks. Looks like it is anyway and I can't tell the difference. The rest I use computer cords on, but I always do the float, maybe I don't always need it, but since I don't know when I just do it all the time. It costs about a buck to do it and it is a safety measure. Inexpensive one at that.
Sometimes I add the computer plugs to the outer case of a piece of gear and only make sure the hot and neutral are set up correctly and leave it at that. If the chassis is not the potential just getting the plug correct is enough and the computer cords are very convenient. If you want to stay with 2 wire cords the type called electric shaver with the plug and removable cord that can go in only one way is a good way to go.
Going the cloth covered to keep things as original as possible as something built in the 20s isn't something I do, and I do modify the mid 50s mono blocks to take modern cords. I think in some areas that safety overcomes originality.
What would be a good way to change the color of the cloth cord? Could you put it into a hot bath with Ritt dye? I imagine if the copper is covered in plastic that is rated to 185C* then a pot of water that is 150F* shouldn't be a problem. Quick dip, let it dry, repeat a few times and the cloth would be black and it won't bleed off on anything. If the wire is stranded I would tin it as much as possible before dipping it in liquid and wrap the ends of the wires to keep liquid from getting up and causing lots of oxidation later. Maybe silicone caulk? Interesting, maybe someone sells cloth spaghetti? Or should. My father had a lot of plastic spaghetti in the early 60s that he used. Well I have gotten OT, so I will quit this line of thought.
Main thing with going to 3 prong is that you have your hot and neutral correct and if the chassis is the potential to put a float on the earth before attaching it to the chassis. Other than that I guess is up to personal wants.
Thatch

From my days in the Navy and on the Tektronix workbench: if it ain't grounded, it ain't safe; per the NFPS NEC: if it ain't grounded, it ain't safe.

If your entire bench constellation of equipment is grounded, you have one solid fact in front of you: what you're working on may not be grounded and it should be isolated.

I've leaned my sweaty arm on top of enough two-wire-power-cord test instruments to know that grounding is a must if you don't like 60 vac or more on the chassis, whether it's current-limited by some jive capacitor or not. Enough of those line filter caps fail to force an unscheduled visit to the hereafter that you may consider grounding to be an important option.

There's nothing unusual about that "negative" lead on a VTVM. Any scope made with a 3-wire cord has a chassis and probe shield that is solidly connected to earth ground. Common sense says that connecting this probe lead to a chassis under test should be done only if the DUT is transformer-operated or connected to an isolation transformer. Same goes for any item of test equipment.

Zip cord for a power cord? Take a look down the extension cord aisle of your big-box store and see if you can find a 3-wire zip-cord version of a small-enough gauge to work on your vintage item. There isn't that much difference in the looks between 2- and 3-wire zip cord. You may even have a choice of brown, black or white.

Floating equipment? X-ray companies were notorious for cutting the grounds off their Tektronix scopes so that they could float them for some measurements. Trouble is, they were floating them lethally, sometimes to a couple of thousand volts. Not only is this incredibly dangerous (and downright stupid), but the scope's transformer isn't rated for such potential differences between primary and chassis and could easily fail and/or destroy portions of the scope's innards.

Following this topic as fairly new guy to the subject, Im a bit confused.
Ive been recapping radios for about a year now (probably have done more than a dozen or so by now). Ive always used an isolation transformer to plug in the set being worked on when doing any testing, have the knobs on the radio, ect. I also have the outlets on my workbench routed thru a GFCI, I have a thick rubber mat on the floor- -I follow the advise I get from the folks here. I am very cautious about the ever present danger.
Now, Im venturing out from just recapping and Im starting to get some basic testing equipment together to try my hand a alignment and such.
Case in point, Im refurbishing an Eico 315 signal generator with the original 2 prong plug. I am trying to learn whether I should or should not install a 3 prong grounded replacement cord. Further, I am trying to understand just how testing equipment (especially the old metal cased type) should be used safely when working on a live chassis. Its a given that the radio is plugged into an isolation transformer, but what about this metal cased signal generator thats plugged in and is connected to the live chassis when testing? Is it safe (reasonably safe anyway) because the radio being tested is isolated?
Jusst trying to learn without having to do so the hard way!!
So what is the final verdict on installing grounded 3 prong cords on the vintage test equipment?
Bill W

I'm from the old school when wood benches, wood stools, and rubber floor mats were mandatory, and ground was somewhat the exception rather than the rule.

Now days, this subject is very straight forward, as Dean said, every box on the bench should be grounded, and the only place on your bench where there is NO GROUND, is at the output receptical of your isolation transformer.

Use 3 wire power cords on all your test equipment, with the cabinet/chassis connected to the safety ground. Hot, the black wire, should go to the on/off switch. Leave the line bypass circuits in place as they at least reduce the noise and RF getting on the AC line.

Whenever you use the isolation transformer, YOU establish where ground is going to be on the Unit Under Test, and of course, the chassis is the logical choice. Personally, I don't depend on the ground clip on test leads, and use a separate clip lead to establish ground.

You'll get plenty of advice here and on other public forums, but remember that it is advice. You will do yourself a big favor by doing research on your own. This will give you the ability to understand the "why" of things and not just the "how". It also will allow you to sort out the often wide ranging opinions you will get on various topics. The more you learn the more fun you'll have with this hobby.

RRM

Dear Thatch, I hope I understood you.. I think you mean install a 2 lug terminal strip
and solder the Neutral AC, & one side of the Capacitor and Resistor to one terminal, the
other side of Capacitor and Resistor go to the other terminal and then to Chassis (Ground) ? Did I get it right ?? I'm thinking the Neutral side of the AC goes thru the Cap, Res
in parallel ONLY THEN to chasis ?? Can I apply this Modification to all my equipment ?? Please Clearify... FRED..