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 breaches 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
Fig. A - A typical neon sign display - indoor and outdoor.
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.
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
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
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.
Fig. 1 - Some details in the design of neon signs.
Fig. 2 - The construction of the sign in Fig. A.
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 series circuit.
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 interference:
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
Posted May 31, 2022
(updated from original post on 11/23/2015)