below $100 in today’s dollars. Plus, they all still meet
factory specs.
Types of Boxes
Resistor and capacitor boxes come in two basic styles
— one being a hit-and-miss selection of standard values,
which are the cheapest and least useful. The other is in
decade values which will cover four, five, or six orders of
magnitude in range, with a resolution of the lowest
decade range (i.e., lowest range of 100 pF — steps in 100
pF to the largest value of the box — and lowest range of
10 ohms — steps of 10 ohms to the largest value of the
box).
Of these types, the only one I would consider is the
decade style due to its wide range and almost infinite
resolution (based on the lowest decade range). Just
imagine that a seven decade box can cough up 10 million
values of resistance in one ohm steps!
Substitution boxes are still being manufactured and
are available through most retailers. They come in a
variety of electrical switching schemes such as digital,
rotary, and slide switches. The thumbwheel switches are
of modest cost, and will be either in straight decimal or
BCD format. (The decimal type requires nine component
values per switch, while the BCD type will only require
four component values.) Each switch covers one decade
of values (0-9) that are stacked side by side. They will read
out in numerical values for each segment. When all are
set, they will give a direct readout of the total value such
as 0.241200 µF or 520,970 ohms.
However, their voltage ratings are 50 VDC
or at best maybe 100 VDC, and low
wattage.
This limitation may be of no concern
to you, but if it is and you desire higher
voltage and/or wattage, you definitely
have to go with the rotary style switching
or slide switches.
When using the rotary switch type to
accommodate higher wattage/voltage
components, it can become very pricey as
the voltage and value ratings increase.
Each switch will cover 11 positions: a full
decade plus zero (0-10). This type offers
good ergonomics and speed of switching.
Then, comes the slide switch style; these
are the lowest cost version of the
switching schemes.
This configuration will have four
individual switches per decade: either in a
1-2-3-4 sequence, or a BCD format with a
1-2-4-8 sequence. The panel is usually
arranged as four vertical switches which
covers one decade; the value is
numerically labeled as the numbers just explained. Then,
each decade just repeats in a horizontal direction.
So, if it has six decades of range, it will require 24
slide switches (four values times six ranges) with each
decade labeled in the same sequence, but with higher
multipliers. By adding up all the switch values that are
engaged, you come up with a total value.
These switches have the advantage of high voltage
and a cheap selling price. However, after using one of
these on several occasions, it almost drove me nuts trying
to keep track of adding and subtracting numbers
constantly (1-4 per decade) to obtain the actual value.
They also require a lot of panel machining for rectangular
slots and mounting holes.
The Internet has a lot of info on constructing these
boxes but — as usual — it tends to be hit-skip information
without much mention of advantages/disadvantages about
their operation and usefulness. What I want to describe in
this article is various types of DIY construction of all the
various switching modes, along with the benefits and
caveats of each type. I will only cover the basics of
construction and leave it up to the builder as to
enclosures, knobs, etc., to suit his/her fancy.
The Resistance Decade Box
Let’s begin with resistor sub boxes since these will be
by far the simplest and cheapest. We will look at this in
three different switching schemes: digital, rotary, and slide
switches.
FIGURE 2.
FIGURE 1.
Digital
switch
schematic.
42 September 2016
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