[Hammarlund] 55 VAC on the Chassis

[Roy Morgan]

At 09:09 AM 4/3/2004 -0500, John M. Trindle wrote:
>Now onto the 55VAC on the chassis problem,

(Was: " LSB Reception on 129-X Fixed!")

John,

You will find two capacitors, one from each side of the power cord to the 
chassis.  This is bad.

Please replace the two-wire power cord with a three wire grounded one and 
if you must include bypass caps as described below:


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 off to the switch and should go nowhere else.  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@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.  In all of them, the offser 
(center) pin is chassis ground.  There is a version with reversed line and 
neutral. 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.

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 off 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 ratings needed, make a guess at 
the rating and use smaller and smaller fuses until they blow from time to 
time.. then increase the 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 external connections. The 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


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --