August 1965 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Restoring and/or upgrading
vintage radio receivers is still a very popular pastime for hobbyists, and for that matter
for some professional servicemen who preform maintenance on established equipment installations.
Three of the most significant changes that can be made to older receivers to improve
sensitivity are to clean up the power supply DC output, replace noisy components like
vacuum tubes and leaky capacitors, and tune / modify / replace RF and IF filters.
This article discusses a method of replacing a stock LC filter with a high
selectivity mechanical filter. The nice thing about an analog receiver is that
narrowband, steep-skirt filters can be substituted without concern for group delay at
the band edges that can (and will) wreak havoc on digital signals.
Super Selectivity for Your Receiver
By Charles Caringella W6NJV
Mechanical filter sharpens bandwidth for optimum
reception of AM, CW, and SSB
If your receiver or transceiver employs a 455-kc. Lf. strip, sharp selectivity can
be achieved by substituting a recently introduced mechanical filter (Lafayette 99 K 0123)
for the first i.f. transformer to help you cope with today's crowded radio bands. Several
important advantages make this installation highly desirable.
Steep skirt selectivity makes it possible to overcome the masking effects of strong
or local signals as little as 5 kc. away. Once the filter is installed, it doesn't need
to be adjusted while the receiver is in operation. No objectionable effects such as ringing
or hollow sounds commonly associated with crystal filters are present. The filter can
be installed in most vacuum-tube-type amateur, commercial, or CB equipment. Finally,
it works well in AM, CW, and SSB receivers.
How It Works. The mechanical filter is basically an electromechanical device. It consists
of an input transducer, a resonant mechanical section having several metal discs, and
an output transducers, as shown above. Both transducers are crystal types. An electrical
signal applied to the input transducer is converted into mechanical vibrations which
travel through the resonant mechanical section to the output transducer, where they are
reconverted to electrical signals.
Fig. 1 - Bandwidth of 2 kc. at 6 db expands slowly to 6 kc. at 60
db. Steep skirt characteristic makes it possible to separate closely spaced stations.
Fig. 2 - Typical receiver first i.f. stage before modification. Internal
circuitry of transformer can be ignored. However, the frequency of the mechanical filter
should be the same as the transformer to be replaced.
Fig. 3 - In addition to the mechanical filter, only two capacitors
and two resistors are added (within the dotted lines). Once the filter is installed and
the remaining i.f. transformers have been peaked, no further adjustments need be made.
Fig. 4 - Mechanical filter and added components are grouped together
into a subassembly and mounted in the same manner as the original i.f. transformer.
Fig. 5 - Bottom view of receiver before the first i.f. transformer
(T1 ) is removed. It is not necessary to disturb any other part of the receiver.
The selectivity characteristics of the filter are determined by the resonant metal
discs. Each disc is carefully machined to extremely close tolerances to make it vibrate
at a desired frequency, such as 455 kc. The discs are made of a ferro-nickel chromium
alloy for extreme hardness and resistance to corrosion. Each is supported by - and coupled
to the others with - a thin rod. The rod runs the entire length of the filter, and is
attached to the transducer at each end. Only those signals within the filter's passband
can get through.
Nominal bandpass characteristics of the filter used in this project are shown in Fig.
1. At 6 db down on the response curve, the bandwidth is approximately 2 kc.; and at 60
db down, the bandwidth is approximately 6 kc.
It is natural for mechanically resonant elements, such as metal discs, to have multiple
resonances which allow spurious transmissions through the filter at frequencies other
than those in the primary passband. By employing conventional type i.f. transformers
at the input and output ends of the filter, these spurious signals are attenuated. Signal
frequencies of plus or minus 20 kc. from the i.f. (435 kc. and 475 kc.) are cut by a
minimum of 40 db. Frequencies above 475 kc. and below 435 kc. are far enough away from
the rest of the receiver's passband to be blocked, and thus be of no consequence.
Input and output impedance is 10,000 ohms. Capacitive coupling is required to prevent
B+ on the input side from getting to the output side, which is in the grid circuit of
the next stage, and to prevent B+ from shorting to ground. In order to minimize the number
of connections to the filter, the bottom leads of the windings in both transformers are
already connected to the ground foil on the filter's printed circuit board. Only three
connections are needed: plate, grid, and ground.
Construction. The only parts you will need, in addition to the mechanical
filter, are two 10,000-ohm, 1/2-watt resistors, CR1 and R2), two 0.001-μf. ceramic
disc capacitors (C1 and C2), a 1" x 1" piece of Vectorbord or other suitable material,
six push-in terminals, and an L-shaped mounting bracket.
Except for the removal of the first i.f. transformer, all components and connections
in your receiver or transceiver remain the same. A typical circuit before modifications
is shown in Fig. 2. Variations in component values or in i.f. transformer design in different
receivers are not critical and will not adversely affect the installation of the filter.
Figure 3 shows the same portion of the receiver after the filter has been installed.
The actual filter and additional components are mounted on a subassembly as shown
in Fig. 4. While it is not necessary to shield the filter - its components are already
housed in metal cans which have been grounded to the printed circuit board - it is necessary
to have a good ground connection between the board and the receiver's chassis.
The one-inch-square piece of Vectorbord is bolted to the bottom of the L-shaped bracket.
Resistors R1 and R2 and capacitors C1 and C2 are mounted on the board. The push-in terminals
serve to hold the components and the connections to the receiver. Before and after photos
show how the subassembly is mounted on the chassis. Check to see that the board fits
in the chassis opening, to fully seat the bracket.
Alignment. Generally, once the filter assembly has been installed,
no further alignment is necessary. However, you might try to peak the remaining i.f.
transformers in the receiver. Just in case the two transformers on the filter have been
diddled with, they too should be aligned for maximum output at the designated intermediate
Fig. 6 - After the first i.f. transformer is removed, the mechanical
filter subassembly is installed, and held in place by two screws. The board should be
made small enough to pass through chassis opening.
Fig. 7 - Above-the-chassis view of mechanical filter mounted in place
of T1. Insertion loss is on the order of 1.5 to 3 db.
Posted May 25, 2018