[Hallicrafters] SX-100 ground issue
Roy Morgan
roy.morgan at nist.gov
Wed Apr 6 10:00:13 EDT 2005
At 08:50 AM 4/5/2005, you wrote:
>I recently purchased a SX-100 and was attaching a ground connection and
>got "zapped". I measured the voltage potential between the chassis and my
>ground lead at 120 VAC; power cord is two prong. Would I help the floating
>ground situation by re-wiring the power cord to a three prong, or counld I
>have other issues; leaky cap, etc? Any suggestions, insight would be
>appreciated.
Charles and others,
At the risk of flooding the list with repeated diatribes, I include below
my assembled wisdom and opinion on line cords and bypass capacitors. If
you read it carefully and do what it says your wife is less likely to
become a widow.
In short: YES, install a three wire grounded line cord, install bypass caps
if you must one from line to neutral and one from neutral to chassis, and
ensure your outlets are wired correctly by using an outlet tester.
Complete details follow:
Roy's Diatribe on Fused Line Cord Plugs and Line Cords and RF Bypassing
powercordsandbypassing.txt
From k1LKY
Fused Line Cord Plugs:
Under no circumstances should you ever use a fused line cord plug,
period. It can kill you in a variety of ways. The Johnson company put
them on Rangers, Valiants, and other equipment. I have a Heath VHF-1 6-
and 2-Meter transmitter with one on it. These line cords and fused plugs
are the first thing to go when I start returning it to serviceable
condition. Note that there seem to be two applications for fused line cord
plugs: Electric fence energizers and decorative electric holiday candles
and light strings. The electric fence situation is based on long history,
and safety may well rely on the idea that the case of the energizer is
grounded with a ground rod to make the fence work properly. The window
candles and light strings have no chassis, no switch, no transformer, and
very little exposure of energized conductors to people.
Some time ago I wrote imaginative but quite serious descriptions of some of
the many ways fused line cords can make a widow out of your wife. I want
to re-write that thing and put in back into circulation, but that will have
to wait for another day.
In summary, however, the way your wife gets to be a widow is as follows:
1) The equipment with the fused line cord plug suffers an internal short
such as in a transformer or RFI bypass capacitor, with the short circuit
more or less to the chassis.
2) ONE of the line cord plug fuses blows (almost never will both blow
unless the fault is a dead short.)
3) You unplug the thing, unhook the "good station ground" wire and antenna,
move it to a work bench to figure out what is wrong. Notice that the
ground you *might* have had on the chassis is removed.
4) You plug it back in and haplessly insert the unpolarized plug so the
intact fuse puts line voltage on the chassis.
5) You reach for the power switch, the current kills you and your wife
becomes a widow.
This is a topic sure to generate much traffic on any radio mailing
list. People's attitudes seem to fall into four groups:
1) "Problem? What problem? There's no problem here." Duuuhhhh!
2) "Originality forever." To hell with the fact that it may kill me or
someone else, I will use the original fused line cord and my equipment is
authentic.
3) "Hmmm..." I'm glad to know about all this (but I may not DO anything to
prevent my death or that of any other hapless and innocent person.)
4) "But of course!" Safety in line cords is easy to understand and worth
paying attention to. I'm going to get busy and fix this situation now.
Be safe, live long. Do not use fused line cord plugs.
Install a three-wire grounded line cord, and make sure your outlets are
working right.
Line cords and how to install one safely:
(in US standard line cords):
GREEN is safety ground, and should be tied directly to the chassis.
In European (IEC) cords, the safety ground is Green/Yellow.
BLACK is "hot" or "line" - it goes directly to the rear of the fuse holder,
the terminal farthest from the outside of the fuse holder where the cap is
installed. The terminal on the fuse holder nearer the chassis or panel
goes to the switch and should go nowhere else. (See info on bypass caps
below.) If you use an open clip-style fuse holder under the chassis, use
either end of it.
WHITE is "neutral" and goes UN-fused to the system, e.g. power transformer
primary. Do not put a fuse in both power cord lines. This can lead to a
dangerous situation, though it's less likely to be dangerous with a three
wire grounded line cord than with the deadly fused two-wire un-polarized
line cord plug.
On the line cord wall plug,
- the round, longer pin is green
- the larger flat pin is neutral
- the narrower flat pin is line or hot.
Note that European color coded line cords have different colors.. Here
they are:
LINE (US) Black = (EU) Brown
COMMON (US)White = (EU) Blue
GROUND (US) Green (EU) Green/Yellow.
(begin extract from message by Bob Nickels)
From: "Robert Nickels" <w9ran at oneradio.net>
Subject: [Johnson] Fused Plugs
...
A good source of replacement 3-wire power cords are those used by computers
and peripherals, but most of them use the European color designations For
years, the colors of individual conductors in cords for use in North America
have been black for line, white for neutral, and green for earth (ground).
But in order to harmonize worldwide standards, two major UL equipment
standards, UL 1950, Information Technology Equipment, and UL 2601, Medical
and Dental Equipment, started several years ago to require the more
traditional European conductor color coding: brown for line, light blue for
neutral, and a combination of green and yellow for ground.
The color conversion is as follows:
LINE (US) Black = (EU) Brown
COMMON (US)White = (EU) Blue
GROUND (US) Green (EU) Green/Yellow
(end extract from message by Bob Nickels)
Older HP and other test equipments were equipped with an oval line cord
connector and matching cord. The Belden/Volex 17280 power cords are
apparently the normally-connected cord.. There is a version with reversed
line and neutral. In all of them, the offset (center) pin is chassis
ground. When working with these equipments and line cords, do take time to
sort out hot from neutral so you retain the safety aspects of the fuse
connection.
RF Bypass caps should be installed as follows:
One from Line to Neutral, after the fuse.
One from neutral to chassis.
When our boatanchor equipment was made, it was common to use both a
two-wire, ungrounded line cord and two bypass caps, one from each side of
the line to the chassis. Do not re-create this situation when you install
the three wire grounded line cord. This causes a danger of fire should the
line to chassis cap short but not draw enough current to blow the circuit
breaker, and this arrangement also causes the chassis to be at half the
line voltage if the safety ground is not present (such as in a two-wire
outlet used with an adapter, or in an outlet improperly wired or faulty.)
There are currently available "Safety" capacitors meant for line bypass
applications. You can tell them from normal caps in the catalogs because
they cost about 5 times as much as normal caps. If you take apart computer
power supplies or junk TV sets, you will wind up with one or two from each
unit.
A reasonable review and discussion of caps used in older radios and line
bypassing is at:
http://www.justradios.com/safetytips.html
ABC's of SAFETY Capacitors for Tube Radios
They do fail, however, to discuss the method of installation above. The
authors are in Canada and offer a number of services for old radios,
including technical information, and capacitors.
Fuses:
The topic of fuse installation and choice of fuse type and rating is a
complicated one, but here are some points to consider:
1) For normal equipment, put one fuse only, in the Hot wire of the line
cord as near as practicable to the point where the line cord enters the
equipment. The black, Hot wire of the line cord should run directly to the
tip of the fuse holder - see part 2 below for more details.
In equipment such as the Valiant transmitters that are made with extensive
RFI filtering on all leads exiting the case, you may not want to add a rear
chassis skirt mounted fuse holder. In other radios that never did have a
fuse, such as the Hallicrafters S-20R, adding a fuse holder would ruin the
originality of the set. In these cases, mount an open fuse clip below the
chassis, possibly using an existing screw or transformer mounting bolt.
Some military equipment was built with a fuse and possibly a switch section
in both wires of the line. This was done where the equipment was to be
used aboard ship or in other places where the power circuits were floated
from ground for safety and reliability reasons. (This situation also
applies generally in Sweden and some other European countries.) If the
equipment is in good condition you can leave it as it was built but make
sure you have a good three wire grounded line cord and that your outlet
grounds are properly connected.
Note: the three-terminal MS series, "Amphenol" type connectors used by the
military on such equipment as the CV-591 and CV-89 were installed with (at
least) two different pin arrangements.. Be very careful that you dope out
what you have when working on equipment of this sort. Mis-matching the cord
to the equipment could lead to trouble.
2) The Hot or line wire of the power cord goes directly to the rear of the
fuse holder, the one farthest from the outside of the fuse holder where the
cap is inserted. The terminal on the fuse holder nearer the chassis or
panel goes as directly as possible to the switch. This reduces to the
absolute minimum the amount of conductor inside the equipment that is not
protected by the fuse. And it reduces the chance of a shock when you are
installing or removing the fuse if the line cord is still energized.
3) The "cold" or neutral side of the line cord should NOT be fused. The
Green, safety ground wire should never be fused. Also, the safety ground
should not pass through any connector other than the line cord connector
and should not pass through any printed circuit path. It should be
connected directly to the chassis.
4) Follow manufacturer's specs for the type and rating of the fuse. Beware
of low voltage fuses that may fit the fuse holder you have. Fuses rated at
32 volts may not properly protect you and the equipment in normal line
voltage situations. If you don't know the current rating needed, make a
guess at the rating and use smaller and smaller fuses until they blow from
time to time.. then increase the fuse current rating a bit.
5) "Inline" type fuse holders can be used under a chassis with no holes to
drill or screws needed to hold down an open style fuse holder. They come
in two sorts, apparently. One sort is just like a panel mount fuse holder
but has no mounting nut and no external solder connections. Wires enter the
cap on one end and the holder body on the other end. The other sort is
rounded and fastens together in the middle. This link shows both types and
has them for sale:
http://www.members.tripod.com/ralph_graves/littlefuse.htm
GFI and Hot Chassis Troubles:
The R-390 series of receivers, among others, was built with robust line
filters. The arrangement of the capacitors places half the line voltage on
the chassis if the chassis is not grounded. This is NOT because the line
filter capacitors are leaking or shorted, it is normal voltage division
behavior of the circuit as installed. If the chassis is grounded, enough
current flows in the safety ground wire to unbalance the two line currents
and trip many ground fault interrupter devices. Old style methods of
bypassing both wires of a two-wire line cord to the chassis can do the same
things. Steps to correct these troubles include:
1) Remove the line filters or line bypass capacitors connected in the old
style and either leave them out or install bypasses as above.
2) Use an isolation transformer or a "Sola" type constant
voltage transformer (most of which have isolated secondaries). Note: most
"Variacs" or variable voltage transformers do NOT provide any
isolation. Some that do apparently exist but they are rare. Further,
fuses in these things may be in the input wire only and not in the variable
voltage output wire. This can lead to overloading the low voltage turns of
your transformer. Both input hot and output hot should be fused, or at the
least only the output wire.
3) Use properly grounded three wire line cords with non-GFI protected power
circuits.
Outlet Safety Testers:
Most home stores and electrical supply houses can sell you a small gadget
to test your outlets with. They have a number of neon lamps that indicate
proper functioning or various fault conditions. The cost is less than $10
and is one of the best investments in your safety you can make. Even
"licensed professional electricians" can and have made mistakes wiring up
houses, and time and wear can open up safety grounds that were installed
properly in the beginning. You owe it to yourself, your family, and later
occupants of your house to buy and use one of these very helpful gadgets.
Think safety.
Install proper grounded line cords.
Live long.
(end of diatribe)
At 12:24 PM 8/21/00 -0700, Leslie Zwiebel <wb6orz at pacbell.net> wrote:
Hi, Roy, if you'd be so kind, could you please explain what you mean
by the
following. I have the GFI tripping problem, and have been hoping for some kind
of work-around. I don't think I understand what you mean by the "IEC
connector":
Leslie,
IEC stands for International Electrical Code or some such thing (the
correct name is likely in French.) The connector I mean is the one found
on all computers and monitors and such equipment nowadays. The common
three-wire grounded plug computer power cords plug into them. The chassis
connectors come in a wide variety of forms, features and ratings. Many are
rated at 3 or 5 amps. these are fine.. A few are rated at 10 or 15 amps
and are not needed for our receivers. You can find these connectors at
hamfests and at most parts supply companies. You can extract them from
defunct computers and printers, although these are normally meant for
mounting on PC boards and not for solid mounting in a hole with nuts and bolts.
Some have built in RFI (radio frequency interference) filters. These
connectors are usually bigger than the non-filter ones. They have modern
capacitors and inductors in them. The current passed by the capacitors is
MUCH lower than the current passed by the line filter capacitors in the
R-390 receivers, and I would expect that they will not trip GFI devices
(though I have not tested this with the connectors I have).
The problem with the R-390 line filters is that they were made with large
value bypass capacitors, and both lines are bypassed directly to ground. I
think that the R-390 non-A filter has more capacitors in it than the -A
version. In any case the capacitor from line to chassis allows enough
current to flow into the safety ground line that the ground fault
interrupter detects the imbalance in the hot and neutral wires and
disconnects your power. If you operate your radio with OUT a proper
chassis ground, the chassis will be at about half the line voltage above
ground.. that is about 60 volts. This is enough to give you a strong
tingle. The capacitors are acting as a voltage divider from the line hot
to the line neutral. This is normal electrical behavior of the capacitors
and not because the capacitors are "leaking" due to low insulation
resistance or because they have failed. They are simply acting as a
capacitive voltage divider and placing the chassis at half the line voltage.
One solution is to have a non-GFI-protected outlet in your shack, but that
defeats the purpose of the GFI protection, and in some locations may,
repeat *may*, be against the electrical code, especially for new installations.
Another solution is to remove the line filter from the radio, make a plate
to take its place and install an IEC line cord connector, preferably one
with RFI protection.
Another solution, is to bypass or remove the line filter and connect the
power cord to the set with no connector and no line filter. In this case,
IF you want to have rf bypassing, connect the bypass caps you provide, one
from line (black) to neutral (white) and one from neutral to chassis. This
is NOT the way most old radios were built, but is safer and just as
effective. A short of the first cap will blow your circuit breaker. A
failure of the second one will go unnoticed but will create no danger. It
may trip a GFI device, however, depending on the currents in the neutral
wire and the actual resistance of the ground return connections in your house.
Yet another solution is to use an isolation transformer, or a Sola-type
constant voltage regulation transformer. These both have isolated
secondaries. NOTE: Almost ALL "Variac" variable voltage transformer
devices do NOT provide any isolation.
I use computer line cords for my boatanchors by removing the female,
equipment-end connector. Note that the color coding in European line cords
is not black - white - green. Be wide awake if you run into one of
these. The one with the green color is the safety ground, but do take time
to carefully sort out the other two.
From: Morris Odell <MorrisO at vifp.monash.edu.au>
To: "'Roy Morgan'" <roy.morgan at nist.gov>
Subject: RE: This group
Date: Fri, 1 Sep 2000 08:13:37 +1100
X-Mailer: Internet Mail Service (5.5.2650.21)
Hi Roy,
Thanks for the message
> Glad you made out well.
> I'd appreciate those suppliers and part numbers.. I need some
> of those cables.
Here's what I received:
> From the Allied catalog, there are two part numbers:
>
> 612-3569 Belden 17952
> 612-3577 Belden 17280
Morris
Date: Thu, 31 Aug 2000 19:11:27 -0700
From: Arden Allen <gumbear at pacbell.net>
Subject: Re: Belden cords
To: Old Tube Radios <boatanchors at theporch.com>
I was asked for help with obtaining the catalog number for the Belden power
cords for vintage HP equipment, the one's Morris is looking for.
Everything you need to know is contained in Newark's page, URL is:
<http://www.newark.com/modules/117/6144.html>
Reversed Line and Neutral, that's what I call FREEDOM OF CHOICE! Good luck
all.
From: "L. Mark Pilant" <n1vqw at arrl.net>
MIME-Version: 1.0
To: Old Tube Radios <boatanchors at theporch.com>
CC: carruth at geo-thermal.com
Subject: Re: Need AC Cords
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Lenox, these cords are real popular on the older HP, Fluke, and
Wavetek test equipment. While some (many?) consider them rare,
it is really a case of knowing where to find them.
They are still made, and are available from Allied, Newark, and
I believe Mouser. It is a Belden/Volex 17280 power cord. They
go for about $5 each.
Date: Sun, 9 Sep 2001 23:34:17 -0500
To: Old Tube Radios <boatanchors at theporch.com>
From: Paul Nelson <drhydro at qwest.net>
Subject: Re: Need AC Cords
Cc: boatanchors at theporch.com
CAREFUL!
There are two versions of this old cord- Mark Pilant and A B Bonds both
referred to 'em in previous posts.
As I recall, in older equipment the line and neutral pins are reversed ...
( It appears that the safety ground wire is always the center, offset pin
in these oval-connector cords. RSM)
Anyway, the latest version is Belden/Volex 17280, the older one is 17952.
(should both be available from Newark.)
A search on Volexpowercords.com shows that for 17280, which is the newer
version, an end view of the connector sez ground is center, neutral is
left, and line is right.
I can't find an end view of the 17952 cord. But if I can find an old one
out at work, I'll check to see what it is.... should be able to find one in
the junkpile out there.
powercordsandbypassing.txt
From k1LKY
Last Updated: November 15, 2004
- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254, Fax: 301-948-6213
roy.morgan at nist.gov --
More information about the Hallicrafters
mailing list