This is only a preview of the March 1990 issue of Silicon Chip. You can view 44 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "6/12V Charger For SLA Batteries":
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
|
VINTAGE RADIO
By JOHN HILL
Wave traps - a useful vintage
radio accessory
A "wave trap" was a common radio
accessory in the 1920s and can still he of
use today for those who dabble in vintage
radio. This month's column delves into the
mysteries of this long forgotten device.
Back in the days when most
domestic receivers were of the
"reaction type", selectivity (the
ability of a radio to separate out adjoining stations) was something that
left much to be desired. Early radio
sets were not very selective.
A simple reaction type receiver is
reasonably selective on distant stations but nearby stations will
literally swamp the dial, being audi-
ble from one end to the other.
Selectivity can be improved by
altering the aerial tappings in order
to1tighten the coupling to the tuning
coil but any improved selectivity by
this method will be at the expense
of distant station volume. Selectivity can also be increased by removing the earth wire from the receiver
or by using a shorter aerial. Again,
both methods will reduce the long
distance performance of the set.
The regenerative receiver's lack
of selectivity was just one of many
reasons why superhets rapidly took
over from reaction types.
In my childhood days I had a
1-valve·reaction set in my bedroom.
In those days I was living approximately 6km from 3BO's transmitter
at Bendigo and from memory, I'd
say that 3BO completely obliterated
about one third of the dial. Stations
within that particular zone were
unlistenable because of local station interference. It is this type of
situation that lends itself to a wave
trap for such a device can almost
completely tune out a local station,
thus making more stations available for listening.
A wave trap is about the most uncomplicated electronic device one
can make. It consists of nothing
more than a coil of copper wire and
a tuning capacitor (see Fig.1}.
VC1
15-415pF
L1
TO ANTENNA
~--TERMINAL OF
RECEIVER
L1 : 60T, 0.5mm ENAMELLED COPPER WIRE
CLOSEWOUNO ON 40mm DIA FORMER
Fig.I: a wave trap consists of a tuned
circuit in series with the aerial input.
A basic wave trap is nothing more than a tuned circuit made up of a coil and
a variable capacitor. The capacitor need not be a single gang unit. Instead,
you can use a 2 or 3-gang unit salvaged from a derelict radio.
14
SILICON CHIP
A quick glance at Fig.1 shows it
to be nothing more than a simple
tuned circuit which is identioal to
the tuning coil and tuning capacitor
of a radio receiver. Ideally, it
This is the author's wave trap. It need not have been so large but the box was
available - it previously housed a crystal set. A much smaller unit could be
built using more modern components.
This superhet receiver has an intermediate frequency of 175kHz. A wave trap
helps to reduce the number and intensity of the heterodyne whistles that occur
when such a set is operated in close proximity to a powerful local station.
should cover the same frequency
range. Adding a diode and a set of
earphones to a wave trap would
convert it to a crystal set.
When using a wave trap it must
be placed in series between the
aerial and the aerial input on the
receiver. If the wave trap is tuned
to the local station, the tuned circuit of the trap resonates at the station frequency, allowing only a
reduced signal to pass through to
the receiver. All other radio frequencies pass through unhindered.
Perhaps that last statement is not
quite correct. Almost unhindered
would be a more accurate statement for there is a slight drop in
volume on other stations when a
wave trap is in use. Also, frequencies close to the one being trapped
are diminished to some extent. Unfortunately, every convenience has
its price.
TRF receivers
We will digress for a moment and
discuss a particular type of early
radio, for it may help to make the
operation of a wave trap more
clear.
One way that early receivers
were made more selective was to
introduce more tuned circuits. This
was the way the first TRF (tuned
radio frequency) receivers operated and it was common to see
radios in the mid to late 1920s with
two or three tuning dials on the control panel.
In the case of a 3-dial set, there
were two stages of radio frequency
amplification prior to the detector.
This involved three separately tuned circuits and such a set up increased selectivity by a considerable degree.
Incidentally, those old TRF
receivers with independent dials
were quite difficult to tune for it
was most unlikely that a radio station could be received with all the
dials set at the same numerical
position. In order to listen to a particular station, each dial had to be
set to a position where each tuned
circuit was correctly aligned with
the others. If the circuits were not
properly synchronised, the operator would hear nothing.
A special log chart was usually
supplied with multi-dial radios so
that station positions could be
recorded for future reference.
It is little wonder that single knob
tuning became popular. However,
this required far greater accuracy
in the manufacture of coils and tuning capacitors.
Now the reason for the foregoing
paragraphs on TRF receivers is
this: installing a wave trap is, in effect, the same as adding another
tuned circuit to the radio being
used.
When using a wave trap, the tuned radio frequency circuit of the
trap does not track with the rest of
the receiver and is only effective at
one particular frequency - the frequency of the local station that it is
designed to suppress. Under these
circumstances, the wave trap
makes the set more selective
because it reduces the signal
strength of the station it is tuned to
restrict.
MARCH 1990
15
Wave traps were common when this old Martin receiver was the latest thing.
Reaction type receivers, such as the Martin, often needed an additional tuned
circuit to minimise the swamping effect of a strong local station.
A 3-dial TRF receiver such as this unit was quite tricky to tune so it was little
wonder that single-knob tuning eventually took over. Multiple tuned circuits
helped these old receivers to be more selective.
Unlike the first tuned circuit of a
TRF receiver, the resonant frequency in a wave trap is not passed on to
other stages for further amplification. It works in a similar way but
no use is made of the resonant
energy, although enough signal
passes through so that the trapped
frequency can still be heard on the
receiver at a much more restrained
level.
The term "wave trap" has been
around for a long time and although
the name explains the function on
the device, I think that it should
have a more scientific name. In actual fact, I believe a wave trap
16
SILICON CHIP
should be called a "series connected, variable resonance, radio
frequency choke". Now doesn't
that sound a whole lot better than
wave trap? However, for the convenience of readers, I shall continue
to use the term wave trap.
Early superhet problems
Readers can be excused for
thinking that a wave trap is only of
use when using an ancient reaction
type receiver in close proximity to a
local station. While this is the most
obvious application for a trap, it
can also be used to advantage with
early superhets which had low in-
termediate frequencies.
Although the standard IF for
domestic receivers has been
around 455kHz for about 50 years,
this was not always the case. Many
of the sets from the early to mid
1930s operated on much lower intermediate frequencies. One such
frequency was 175kHz and this IF
was quite common in early superhets. The peculiarities of these particular receivers becomes obvious
when they are used close to a
powerful station.
In my own situation, I live approximately 4km from 3CV's 5kW
transmitter and this causes con~
siderable local station problems
with all of my vintage radio listening, including my early superhets.
There are several types of
receiver malfunctions caused by
such strong interference. Firstly,
heterodyne whistles appear at
numerous points on the dial there are not just a few whistles but
dozens of them.
Secondly, harmonics of the local
station frequency also appear at
various places around the dial. One
of my old superhets tunes in about
three 3CV's and two 3BA's (a more
distant local station).
It is under these annoying circumstances that the old wave trap
can be of assistance for it can help
control the previously mentioned
problems. Using a wave trap will
eliminate many of the whistles and
harmonics because it reduces the
local station signal strength to more
manageable levels.
AGC problems
Another situation where a wave
trap can be an advantage is with a
radio that lacks automatic gain control. If such a receiver is used
carelessly and is tuned onto a local
station without backing off the
volume, the speaker can be severely overloaded and possibly even
damaged.
Again, a wave trap will help
reduce the signal strength to more
normal levels.
A practical wave trap
Although a wave trap is basically
just a coil and a variable capacitor,
it is far more convenient to use if
properly housed. The one shown in
FRONT PANEL
VINTAGE RADIO
REPAIRS
1
=~
REAR
PANEL
EARTH _ _,_
TUNING
CAPACITOR
(VC1) 15•415pF
-
~
-
LAWSON RADIO has been in the
business of repairing radios for 40 years.
If John Hill"s column has rekindled the
memories, but you are not confident
enough to carry out your own repairs then
call us now, for an obligation free
quotation .
Should you want to sell or purchase a
vintage radio contact Michael today.
EARTH
ANTENNA
L1 : 60T, 0.5mm ENAMELLED COPPER
WIRE CLOSE WOUND ON 40mm DIA FORMER
Fig.2: here's how to wire up a practical wave trap with IN/OUT
switching. Use one section of a 2 or 3-gang tuning capacitor if
that's what you have available.
298 GREAT WESTERN HWY
LAWSON, NSW 2783
(047) 59 1254
the accompanying photographs has
been placed in a wooden box (an
earthed metal box may be even better). It has both aerial and earth
connections, plus a double pole double throw (DPDT) switch which
either switches the aerial straight
Although this old 4-valve Airzone is a
superhet, it lacks AGC. Using a wave
trap on this and similar receivers
helps to throttle back the
overwhelming effect of a local
station.
through to the set or to the set via
the wave trap.
The earth connection just goes
straight through the box from the
terminal on the back panel to the
terminal on the rear panel - it is
more convenient to have both the
aerial and earth leads coming from
the same position on the work
bench. It is also very convenient to
be able to switch the trap in or out
at the flick of a switch rather than
having to connect the trap into the
aerial lead when it is required.
Fig.2 shows a wiring diagram
that completely cuts out the trap
when the aerial is switched straight
through.
The number of turns on the coil
will vary according to the gauge of
the wire, the diameter of the former
and the capacitance of the tuning
capacitor. 60 turns on a cardboard
tube from a toilet roll may be a
reasonable starting point.
If that local radio station is spoiling your vintage radio fun, then
trap it out with an old fashioned
wave trap. If you decide to build
one, the convenience of the switchable model is well worth the ex~ e~ct.
~
~esurrection
!Jadio
The original vintage
wireless specialists
Our skilled technicians offer QUALITY
repairs & restoration.
We have a large stock of bakelite and
timber radios fully restored and for sale.
Valves, high voltage capacitors and
other specialised parts available.
SPECIAL - this month only:
1O & 15µF 450 volt electrolytic caps
$3.00 plus p&p
WANTED - valves, radios, etc. ,
purchased for cash.
Call in to our showroom at:
51 Chapel Street, Windsor, Vic 3181
PO Box 1116
Telephone: 529 5639
MARCH 1990
17
|