Crystal Radio #77 — A Budget DX Set
Ok folks, here comes another crystal radio. I don't build them so often anymore. Only when I have a new design to offer do I present you with another article. This is such a time.
My #77 crystal set is designed so it is reasonably easy for most builders to make. To build, all you need are sawing, drilling and measuring tools. Every component in this radio has been selected to give the "biggest bang for the buck" by matching their relative performance to the other components. Consideration has been given to the parts being easy to obtain. A signal generator is desirable for dial calibration. The coils can be purchased pre-wound so no additional test equipment is needed.
Some of your might have wanted to build high performance crystal set but were soon discouraged with how difficult it is to come up with the parts. Then many times what you end up with are parts that just don't fit the job. As of the original publication of this article, all parts are available.
How much does this cost? I won't tell you it is cheap. The major electronic components (vernier drives included) will be around $150. This includes the pre-wound litz wire coils. You may find some of the stuff in your junk box, which will cut the cost. The additional mechanical parts and small electronic parts aren't real expensive. A parts list is shown near the bottom of this page.
It is possible that something like this radio could end up as a kit. I am in talks with a well known kit producer to see if this can come tru
A Quick Circuit Description
Much of the circuit descriptions can already be found around my website. But I'll give it a quick run through here too.
This radio is built in two sections. An antenna tuner/coupling device and a detector unit. I do my dx sets this way so that the coupling distance can be easily adjusted. Normal coupling for this radio is from 6 to 10 inches (15 - 25 cm). To get it right, I built the sections each on a separate chassis.
The antenna tuner is a design that is popular with dx radio builders. The T-Tuner has been found to do an efficient job in bringing the rf energy from the antenna to the detector.
The detector uses a parallel coil/capacitor as the tuning element. There
is a selectivity enhancement circuit
to reduce the loading on the tank coil. This is a lot better than tapping
a coil. The trimmer capacitor adjusts the match to the detector.
A diode or a 27mH rf choke going to ground provides the dc return so the detector will work. The rf choke does a better job, but they aren't always available. So the diode will work ok.
The detector output is fed to the famous Bogen T725 matching transformer. This very reasonably priced transformer gives the big bang for the buck. It will match any type of headphones, wether it be a crystal earphone, piezophone all the way to magnetic and sound powered (balanced armature)headphones. A 9 position rotary switch selects the tap just right for the headphones that you are using. The range is from 8 to 20,000 ohms.
So there you have it!
One of the major design elements in my budget dx radio are the coils. Again, the selection of the coil design came down to how well the quality is matched to the rest of the radio and the cost. The use of the large litz did not fit any of the criteria for this radio, although 660 litz coils will work very well.
These coils were designed with this radio in mind from the beginning. We begin with the coil form itself. If the wrong material is selected, much of the litz wire performance gets soaked up in the form itself. I chose a form with a 3.5 inch outside diameter (89mm). This is an ABS styrene drain pipe coupler made by Genova Products. The part number is S40130. ABS styrene has low losses at MW frequencies.
Please check my cylinder coils page and the schematic below for winding details.
The selectivity is reasonably good, even at the high end of the band. A wave trap can be used to null nearby stations.
Why So Many Trimmer Capacitors?
I knew someday you would ask. As I look over designs, no one uses trimmers in their crystal sets. Part of this comes from me using the frequency calibrated dials. This has encouraged me to design my radios to use as much as the dial to tune the band as I can.
The dials conveniently attach to the front of the vernier drives. Having a calibrated dial makes crystal radio operation so much nicer.
To achieve this dial spread, I use trimmer capacitors to get the exact adjustment that I want. In the case of the 40pF trimmer on the ATU, this allows final calibration of the dial to different antennas. This should be adjusted near the top end of the dial, with a signal generator or a known station. At the time of adjustment, the antenna and ground must be connected to the connection points on the ATU.
The detector tuning capacitor also has the trimmer capacitor across it. This trimmer should be adjusted to the top end of the band (1700 kHz) is tuned when the tuning capacitor is nearly in the open position. The main tuning capacitor has a capacitance taper that allows for a wider frequency spread at the top end of the band. This spread is important to the tuning of the radio as well as the dial calibration spread. See how much nicer that the dial looks when the numbers aren't so squeezed at the top end of the band?
As a note, the end plates of this capacitor are adjustable. I bent the smaller fins out somewhat so that the dial would spread better. A picture is shown below. Only one section is being used so only those plates need to be bent. The smallest section gets bent out a lot and then gradually less as the plates become larger.
After you do that, adjust the capacitor near the lowest value and adjust the trimmer for 1700 kHz.
I found that the security enhancement trimmer has a definite peak in signal level. This is where I set it. I am surprised that this peak remains constant through out the band. I hope you have that same luck.
Another question that is sure to be asked is why I am using a dual gang capacitor when only one gang is being used? There are a couple of reasons. First the offset shaft provides a way of making the capacitor more linear to frequency and less to capacitance. This is not found on the single gang run of the mill crystal set caps. Another thing not found on the cheaper capacitors is the dual wiper conductivity. This is that copper piece in the center of the capacitor shaft. With the metal folded over, this provides two wipers. The small capacitors usually sold only have one wiper. These features add cost to the radio, but I felt it is money well spent.
Below the pictures is a quick list of the order of construction. Another first here at makearadio.com. The critical step (for looks) is the mounting of the vernier drive. Try to get it centered. Also try to get the height of the capacitor matched with the vernier shaft. You don't want to have any binding. For this reason, the holes that mount the capacitor through the stand offs are drilled last.
You will be disconnecting the panel from the chassis many times during construction. Don't try to take shortcuts here. Just go through the routine when needed. For final assembly, tighten the brackets first and work your way back to the capacitor. Tighten a little before the final tightening. Kind of like mounting a wheel and tire on a car.
This is a two hander radio. One hand is on each knob while you slide the sections at various distances. You will get the feel of how this set operates after a couple of hours.
Thank you for visiting my website and viewing this project. I hope that you will encourage you to build a crystal set.
Front view of the detector unit.
Rear view of the detector unit.
Front view of the AT Unit.
Rear view of the AT Unit.
Close-up of the ATU insulated coupling.
Close-up of the detector unit bent capacitor blades.
Chassis and panel before drilling.
Device I used to find the hole height.
A pencil can be used here too.
Building Sequence Outline
Cover your Garolite® (12x14-½ inch area, 30x36,9 cm) with wide masking tape.
Mark the 4 sizes and cut. Add edge decoration as desired.
Place masking tape on the back of the panels where the brackets and verniers will mount.
Mark, drill and countersink bracket holes (8). The panel should hide the chassis edge.
Mark the center, vertically on the back of each panel.
Screw standoffs on variable caps. Find shaft height on panel. (see picture of tool above).
Drill vernier pilot hole, as well as the two vernier mounting screw pilot holes.
Drill vernier main hole with 7/8 inch (22-24 mm) forstner bit. Drill 1/8 inch (3 mm) vernier mounting holes.
Mount vernier and capacitor. Mark and drill the three capacitor mounting holes on the chassis
Drill and countersink the rest of the holes on chassis (5 holes on detector and 4 on ATU)
Drill for switch and phone jack on detector panel.
Mount parts and wire as needed.
Mount the 4 inch dial scale faces and place bumper feet on the bottom of the chassis.
Check for proper operation and do all the alignments.
Add calibration markings to dials after the radio is aligned correctly
1 - Garolite® panel 7 x 5-½ x 1/8 inch, (all cut from a 12x24 inch sheet)
1 - Garolite® chassis 7 x 9 x 1/8 inch McMaster Carr
1 - Garolite® panel 5 x 5-½ x 1/8 inch (my original was 6 inches tall)
1 - Garolite® chassis 5 x 9 x 1/8 inch (original chassis length was 8-½ inches)
8 - Feet
4 - Keystone 618 angle brackets
21 - Flat-head screw 6-32-3/8 McMaster Carr
10 - Lock washers #6 internal tooth McMaster Carr
16 - Nuts 6-32 McMaster Carr
10 - Hex standoff, 1-¼ inch long, plastic 6-32 male and female threads McMaster Carr
4 - Binder head screw 4-40x ½ McMaster Carr
4 - Nut 4-40 McMaster Carr
1 - Solder lug #4 Mouser
5 - Solder lug #6 Mouser
5 - Binder head screw 6-32x ¼ inch McMaster Carr
2 - Vernier drive, 6:1
2 - Large knob, ¼ shaft
1 - Small knob, 6 mm shaft hole
2 - HDPE 4 inch dial discs DIY or Peebles Originals (special order)
2 - Flat-head screw 6-32x¾ McMaster Carr
2 - Washer #6 McMaster Carr
2 - Thumb nut #6
1 - Bogen T725 Transformer
1 - Rotary switch 1P9T
1 - Terminal Strip, 3 to 5 point
1 - Spacer 1 inch 6-32 threaded each end DIY
1 - Shaft coupler 1/4 inch metal Mouser
1 - Dowel rod, plastic 1 inch McMaster Carr
1 - Trimmer capacitor 40pF eBay
2 - Trimmer capacitor 20pF eBay
1 - Variable Capacitor, dual 400pF S plate shape
1 - Variable Capacitor, (dual) 330pF O plate shape
1 - Coil, on 3½ styrene form. 150µH 36 turns 165/46 litz
1 - Coil, on 3½ styrene form. 275µH 53 turns 165/46 litz
2 - Germanium diode 1n34a type
1 - Resistor 33k low wattage Radio Shack
1 - Capacitor, disc 0.1µF, low voltage Radio Shack
1 - Capacitor,fixed 220pF, low voltage Radio Shack
1 - Phone jack, ¼ inch Radio Shack
Crystal Radio #77 Schematic
73 and good crystal DX. Dave - N2DS