October 1935 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Before there was radio, it really didn't matter much how much
electromagnetic energy at any frequency was spewed into the
air and into electric wires as long as the amplitude was not
great enough to physically damage affected equipment. There
was no need for an FCC or unintentional radiation limit regulations.
It was not long after radio came along that the presence of
electromagnetic interference (EMI) made itself painfully obvious
due to its presence on audio as static. Motor brush arcing,
electrical atmospheric phenomena (lightning, meteors), switching
on and off of circuits, intermittent connections, nearby radio
spurious emissions, high voltage transformers, and in this case,
neon lighting were among the sources quickly identified. Depending
on the size of the sign and length / diameter of the tubes,
the voltage needed to trigger the neon gas into electroluminescence
could be 12 kV or even more. At that level, especially
with outdoor applications subject to high humidity and water
condensation, even minor breeches in insulation can permit arcs
to form between conductors and metal frames.
I clearly recall having received my first high voltage burn
many moons ago while servicing an outdoor neon sign for a store
in a strip plaza in Annapolis, Maryland. The next one occurred
while troubleshooting a CRT power supply for a Precision Approach
Radar (PAR) display in my USAF shop's MPN-13 mobile radar unit.
It must have been that second one that finally taught me to
be more careful since I don't think I've gotten whacked by anything
more than 120 V since then ;-)
The "Neon" Interference Problem
The interference set up by neon signs involves problems which
the Service Man does not ordinarily encounter.
J. Albert Lynch
A growing source of radio interference is the increasingly
popular neon sign. In many cases the interference is so bad
as to entirely ruin radio reception in the immediate vicinity
while the sign is in operation.
Fig. A - A typical neon sign display - indoor
The neon sign, from a practical point of view, is extremely
simple. In order to service these signs for radio interference
we need simply a very elementary idea of the construction and
contents of the completed tube, a fair idea of the construction
of the entire assembly, and a thoroughly sound idea of the wiring
and sources of trouble.
Our first example of a neon tube (Fig. 1A) will be a length
of glass tubing about 1/2-in. in diameter, and approximately
13 ins. long. We will now insert a wire into each end of the
tube, seal off the ends, apply a vacuum pump and then after
pumping, inject the required amount of neon gas and seal off
the glass. We now have a simple neon tube. If we now connect
a transformer, rated at several thousand volts, to these wire
terminals (Fig. 1B) we will find that we have the usual red-glowing
neon tube. We would now notice that the glass, except at the
extreme ends, remained cool when operating. The ends would get
quite warm and the wires extending inside the tube would, because
of the electronic bombardment going on inside the tube, wear
down and be short lived. For that reason instead of inserting
a wire inside the tube we will now build up an electrode which
will consist (Fig. 1C) of a metal plate surrounded by a mica
sleeve inserted inside a glass envelope. A wire is now attached
to the copper plate, brought out one end of the glass envelope
and the envelope sealed off at that end, the other end being
left open for attachment to the glass tubing used for the desired
outline or letter. We now have arranged a terminal sufficiently
large to give a reasonable life and so arranged as to keep cool.
Fig. 1 - Some details in the design of neon
As a summary, the neon letter is simply a glass tube, shaped
as desired, with an electrode at each end, from which the air
has been removed and the required amount of gas inserted. This
is all that the average Service Man needs to know about the
tubes themselves, as the actual building of the tubes is a separate
trade in itself.
We will now take a typical window sign (Fig. 2) and follow its
construction and installation. In this sign (Figs. A and 2)
the letters in the word Logans are approximately 4 1/2 ins.
high, and in the word bar approximately 8 ins. high. The border
outline is approximately 8 ins. larger than the letter assembly.
The layout man in the sign shop, usually the sign painter, lays
out a full sized pencil sketch of the sign, including a notation
of the color, or colors desired, i.e., wording red, border blue,
all red, - red and gold or whatever combination is desired.
The sketch is then sent to the glass blower; or, more frequently,
to an outside plant specializing in neon tube work. The glass
blower then shapes his glass to the pattern, sending back the
units complete and ready for lighting. In this case, the assembly
and outline is approximately as in Fig. 2. We receive from the
glass blower three separate and complete units, border - Logan's
- bar, anyone of which can be used alone, but which we now wish
to shape into one assembly. Each unit is a continuous length
of tubing with an electrode and wire terminals at each end.
These units must be connected electrically in series and we
will find the electrodes so shaped as to each be headed toward
the electrode to which it is to connect. The circuit is as follows,
one end of transformer secondary to one end of border unit,
other end of border unit to one end of unit Logans, other end
of unit Logans to one end of unit bar, other end of bar unit
to second terminal of transformer secondary, making a complete
Fig. 2 - The construction of the sign in
We now come to radio interference. This may be roughly divided
into primary and secondary. Primary interference is caused usually
by pick-up in the primary of the transformer, by the lines feeding
into transformer, or, by a defective transformer. If the sign
is on a separate circuit., at least a different circuit from
that of the radio, and the sign is fed by a circuit run in a
well-grounded metallic conduit, you will rarely have trouble
from the primary source unless the transformer is defective.
A line filter connected near the transformer and rated at 5
A. per transformer will usually be a positive check against
primary interference provided that we have checked as to separate
circuit and grounding.
The secondary source of interference is usually from either
a defective secondary coil, pick-up from secondary wiring, pick-up
from tubes, partial ground or short in secondary wiring, pick-up
from back bends in tubing caused by difference in potential
in these sections of glass, pick-up from suspension wires from
which window signs are hung, or pick-up from metallic objects,
such as metal ceiling, pipes, and so forth, in the immediate
vicinity of the sign. The solution here is to ground out, or
neutralize all points of possible pick-up. I suggest that the
Service Man proceed as follows: always remembering that he is
working on a simple electrical system that simply happens to
be of high voltage, and with his theory of radio in mind, always
looking for the most logical point in the particular installation
from which he can expect radiation. We will first check both
types of trouble on a window sign. The procedure is as follows;
Turn the sign on to see if it is operating properly. If the
sign runs fairly steady and without any serious sign of fluttering
of the tubing, you can assume that as far as the tubing and
transformer go you are O.K. If the sign only lights dimly, or
in parts, you can assume a broken unit. The units can be tested
with a neon tube tester that can be obtained, complete with
directions, from any tube supply house, or by the following
method: take a piece of high tension cable and jump across each
unit in turn. When the defective unit is jumped the balance
of the sign will light. The jumper must of course be connected
from the wire of one of the electrodes at one end of the unit
to the wire on the other end of the unit. If the entire assembly
flutters badly, does not light at all or only in part and we
find no defective unit, then it must be the transformer. Disconnect
the secondary leads and with a high tension wire for a jumper
touch the two secondary terminals. A good hot spark should result.
If no spark results, or a weak one, get a new transformer. Most
transformers above 7,000 V. are center tapped. In that case
they should be tested from the case to each terminal in turn.
This will test the two secondary coils. assuming that the sign
is operating properly. We will now proceed to clear the radio
interference. As we proceed, if any one of these clear the trouble,
you, of course. go no further. First, make sure that the sign
is running off a different circuit than the radio; second, make
sure that the case of the transformer is grounded; third, tie
several fine bare wires, (about number twenty-five) across the
face of the glass; fourth, cross secondary leads coming from
transformer to unit; fifth, break wire suspending sign in window
and insert about a 6 in. section of waxed cord or ordinary twine;
sixth, cut a 5 A. line filter in the primary circuit of the
transformer as near as possible to the transformer. There is
one type of installation that is liable to be quite troublesome,
that is, where a sign is hung on a leaded glass window. In this
case I would first solder fine wires to the leaded sections
at four or five points near the sign, tie these wires together
and run them to the nearest water pipe for a good ground, then
see that the tubing is kept as far as is reasonably possible
from the leaded glass.
With outside signs we proceed about the same as with the
window sign. If the sign is on a flasher, filter the motor circuit
of the flasher, and if necessary filter each primary circuit
across the breaker points in the conventional manner. In
general check all installation for these possible points of
1 - Separate line circuit (at least separate from radio).
2 - Transformer and sign cases grounded. 3-Flashers filtered.
4 - Keep aerial away from sign.
5 - Supply line to sign in metallic conduit and grounded.
6- Fine wire ties across face of units.
7 - Loose secondary connections, or arcing to case of sign
or other wire,
In special reference to window displays.
8 - Crossing secondary feeders.
9 - Cutting in insulators in suspension wires to stop further
radiation from this point.
Posted November 23, 2015