Photo and Text information
CR-3 CRYSTAL RADIO
Crystal Radio Building Contest
CRYSTAL RADIO DESCRIPTION
The CR-3 Crystal Radio was built
to illustrate a simple receiver could be fabricated from junk box parts
and give good performance with minimum antenna requirements. Another
important aspect was the desire to realize the best workmanship and
layout neatness consistent with the selected components.
As this is a crystal radio construction
project after 40+ years since two units (CR-1 and CR-2) were built as
a teenager with limited success, it was decided a simple single tuned
circuit using modern components ( germanium diode and good quality
tuning capacitor) would be used. It was also thought this simple design
could be easily duplicated by others as all the parts are readily available.
With reference to the schematic,
coil L1 reduces foreign broadcast bleed through when the radio is used
with a relatively short antenna. Switch S1 gives the user the choice
of feeding the antenna input directly to L2B and C1 for best sensitivity,
or using a link coupled input for improved selectivity. Coil L2A can
be moved back and forth over L2B to change the degree of coupling. High
impedance ( 2K ohm or higher) headphones are used. For the non-purist,
a battery operated audio amplifier can be used to produce loudspeaker
The chassis and front panel are
fabricated from double sided PC board and soldered together at a 90
degree angle after mounting holes are drilled for the various components.
After the tuning capacitor, coil and terminal strips are attached with
screws, then the chassis plate can be mounted to the wood base. The
coils are hand wound on two different size cardboard tubes, where the
outer coil’s (L2A) inside diameter is chosen to be a smooth “slide-fit”
over the outside diameter of coil L2B after the wire has been wound
on and coated with clear lacquer. Both coils have close wound turns
using # 28 enamel wire. The tap for the diode is made by scraping the
enamel off at 25 turns from the hot end and soldering on a short length
of #28E for connection to the terminal strip. Wiring is point-to-point
using neat 90 degree bends. A home made tuning knob and dial are used.
The knob was fabricated from a jar top with a tin can lid as the knob
reception has been limited to local (within 30 mile radius) Baltimore
AM stations. Nightime reception has netted stations out to 500+ miles.
Selectivity is good considering only a single tuned circuit is used.