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Dave's Crystal Radio #71

Diode Dave's Duo Dynamic DeLuxe DX Delicious Delight!

Dave Schmarder's #71 Crystal Radio

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Hi Friends. After my quick dabble with space wound cylinder coils in my #70 set, I am firmly back in the familiar territory of the spider coils. Would you expect anything different from me? While this set is much like my #66 , 68 and 69 radios in looks, there are some developments that occurred after the earlier sets were built. Some were based on the components available to me at the time of construction, some were little improvements that makes this radio even nicer to operate. Since I'm in a hurry to show you this set, I'll move right along.

The Idea

This radio came to being because of a request by a friend for one of my sets. He wanted it "just like your #66 set". This one turned out to be about like my #66. The difference is due to the parts availability, improvements thought of since the last sets and the fact that since these are built with the plans in my head, There are no measurements around to make another. I just hope each time that everything is placed right and something doesn't bump into something else on the opposite side of the chassis. You can see my layout drawings on the masking tape covered HDPE and Garolite®.

I'm in love with my spider contra coils! However, not all variable capacitors are in love with those coils. That is, the capacity isn't optimum for the best coil size, as well as the tuning ranges and dial spread. If the capacitor range is less than about 15-300 pF, you won't be able to tune the entire band very well with a contra coil. This is taking into account having a reasonable amount of dial spread. Dial spread is the ability to have the frequency ranges you desire to cover as much of the capacitor tuning range as possible.

Once the variable is much over 400 pF, then it can be a problem getting a good dial spread. However by placing a higher fixed or variable padder capacitor in series with your main variable, then you can end up with an effective capacitance in the range that is best for contra coil operation. The perfect value is about 350 pF.

Circuit Description

The Antenna Tuning Unit (ATU) provides a way to transfer RF energy from the antenna to the detector, and at the same time provide some selectivity. Dual tuned circuit sets are always more selective than single coil radios. I won't describe this circuit much as it is well known by DX crystal radio builders and is shown and described elsewhere on my site. You will see an extra padder capacitor in the pictures but not described on the circuit. This is because I used an unusual 3 gang variable. The values were such that I put in a padder to adjust the dial spread of the ATU. This isn't that important, but just something that makes my radios just a little nicer.

The detector section is somewhat like some of my other dx sets shown on my pages. The detector board starts with a contra spider coil. This is a specially wound coil with two windings. They are wound in such a way that they can be series or parallel connected and still maintain a lower distributed capacitance than if the coil was wound in a conventional manner. There are other nice features of this coil that you will find on my contra coil pages.

Tuning this radio is achieved by the main 475 Holy Grail type high quality variable capacitor. Since this capacitor value is too high for use in a good contra coil radio, I have reduced the value of this capacitor by use of a 400-1200 pF padder type mica capacitor. Since this value wasn't high enough, a 470 pF fixed mica capacitor was added in parallel. You may want to use a 680 pF fixed capacitor for a more middle of the range adjustment of the padder. Since I was able to adjust the padder the way I wanted, I left the 470 in there.

Over the years I collected a lot of screwdriver adjust small value trimmer capacitors. I would get them as part of eBay deals or whatever. I was wondering what I would do with these and contemplated selling the bunch on eBay. I never thought that these would be useful in crystal sets. Oh how I was wrong. This set ended up with three of them.

The biggest one is a 100 pf trimmer. This is used to adjust the dial spread range. If your coil is calculated and constructed correctly, this gives the final little adjustment to put your tuning range right on target.

After the main tuning system is a type of selectivity enhancement circuit (SEC). This is known as the trademarked Hobbydyne circuit. This is a circuit consisting of a differential capacitor, an optional trimmer, and a 27 mH rf choke. The idea is to reduce the tank loading and provide some impedance matching. More information can be found on my DX Notes page.

The differential provides a "capacitance divider" circuit that feeds the diode. But there is some capacitance from the detector that cause the main tank to be detuned. A small trimmer capacitor is added to one leg of the differential capacitor to compensate for this. This is called "tracking". Unfortunately it isn't perfect but works across about 2/3 of the differential tuning range. This cuts the need to adjust the main tuning so much when the selectivity adjustment is made.

There are some cases where you don't want to use the SEC. If the interference is low and you want more volume, then bypassing this circuit is advisable. The lower in the band you listen, the less important the SEC is. I built in a feature do bypass the SEC, if desired. This is built in to the band switch as two extra positions on each band. They are named "Wide" and "Narrow" (how creative of me!). You can see on the schematic how the signal is routed through from the tank over to the detector diode, or is directed through the SEC.

There is one downfall. That is the capacitance differences between the SEC and without it. I used another small air trimmer to compensate for this. So one could say that the two smaller trimmer capacitors only increase the pleasure factor in using this radio, but does not pull in more stations.

After the diode is a fairly simple audio output system. I use a parallel combination of a 180k resistor and .1 uF capacitor to equalize the dc resistance of the transformer winding and the it's impedance. This improves the selectivity during times of strong signal reception by not loading the tank due to the high conductivity through the diode. I use this RC network as a place to connect a milli-voltmeter to for measuring the relative signal strength. More information on this, and the contra coil can be found on Ben Tongue's web site. Spending a lot of hours there is very worth while if you want to learn how to design your own high performance crystal radios.

The Coils

There are two large coils in this radio. The ATU coil is approximately 155µH on a 6 inch (15 cm) HDPE spider coil form. There are 39 turns of 660/46 litz wire to make this inductance. The hub is 2 inches (50 mm) in diameter.

The detector coil is a contra wound style. This coil must be designed to work with the variable capacitors that you will be using. But in my case (using a 350 pf variable equivalent as the base), the coil is about 220 µH in series and 55µH in parallel. I used a 7 inch (17,75 cm) HDPE form, also with a 2 inch hub. The thickness of the HDPE is 1/8 inch (3 mm). The inside windings is 27 turns of 660,46 litz. The outer winding is 17.5 turns of the same litz. Please read the contra coil section concerning the mechanics of winding this type of coil and their advantages. These coils are available on a custom wound basis.


Below I have included some pictures of the construction phase of this radio. They include the panels and chassis before they are cut and drilled. Masking tape attached to these surfaces is a good way to mark your holes. After you are done drilling, just pull off the tape. I used a scroll saw to cut the pieces. A small one inch wide sanding belt finished the fine work on this radio. It took a long time to get it right but the pieces fit very well.

There are two pictures of a sub panel I built. The main purpose of this sub panel is to allow for a set back from the front panel of the differential capacitor and band switch. If they were mounted close to the panel, your hand capacity would detune the radio and make it difficult to use. The capacitor and band switch with their corresponding styrene shaft extenders keep this from happening. It was handy to mount the three air trimmers also on this panel, as two of them were connected to the differential capacitor and band switch anyway. I pre-wired this little sub panel as it was much easier to do without the main chassis getting in the way. If you use switches like I used, then the pictorial diagram will be very helpful.

To cut losses, two of the contra coil leads are soldered directly to the main capacitor ground and the top of the padder. Smaller wires then connect to the band switch. It isn't too difficult to wire the switch, but sections C and D, which are closest to the panel should be wired first.

I used 3 pair of thumb nuts with brass screws as the connection points for the antenna and ground, the test point and the diode. I like this type of connection and there is a bonus of a nice brass look. It is easy to experiment with the diode since the connections are on top of the chassis.

The chassis is made from HDPE 12 x 7-1/2 inches (30,5 cm x 18,2 cm and is 1/4 inch (6 mm) thick. There is a problem with mounting the impedance tap switch as well as the 1/4 inch phone jack (and the mica padder too). The panel is too thick for a nut to be put on the end of the bushings. I solved that by cutting a circle on the bottom side using a large forstner bit. Start by drilling a pilot hole and then drill about half way in with the forstner bit. Be careful so as not to drill too far. Then drill the 3/8 inch holes for the bushings to go through.

When making these chassis sets, you have to be aware of what is on each side when you are deciding where to put the components. For instance make sure the placement of the posts are far enough in and back from the edges of the chassis, in particular at the front. I placed the holes 1 inch (24 mm) back from the front and 1-1/4 (30 mm) in from the sides. This gives the shafts that go to the differential and band switch enough from to miss each other. (Don't you just hate it when two parts interfere with each other. It just ruins your entire day)

The oak wood base is 14 x 8 inches (35,5 cm x 20,3 cm) and is 3/4 inch (18 mm) thick. There are stick on rubber feet on the bottom. I also like to cut a 1/2 inch diameter recesses so that I can use shorter screws and the screw heads are not sticking the bottom. I prefer this method to a simple countersink as sometimes the holes have to be "adjusted" to fit.

Both of the main tuning capacitors are connected to the knob through a 6:1 vernier drives. A vernier is essential for operating high selectivity sets like this one! The variable capacitors have their shafts at equal heights from their mounting base. If yours aren't, then a shim could be made to bring up the height of the low one. The shaft of the ATU has to be isolated from the vernier. I used a metal coupler and a short piece of styrene rod to make the mechanical connection to the front panel. The vernier has to be grounded to avoid the unwanted hand capacitance effects.

Those are the main building pointers I can think of. Much of what needs to be done is covered by looking at the pictures and using that good ole' common sense. (Builders in Washington D.C. may need some help there). :) If you can't get a handle on something, send a short note and I will try to help.


On the detector board there are 4 adjustment trimmers to be dealt with. Fortunately some are not completely interlocked. Start by setting the differential and all the air trimmers at their mid point. Adjust the mica trimmer a couple of turns from being fully tight. Make sure you use the "Narrow" selectivity positions on the band switch when starting this alignment.

Using a signal generator, see how the tuning is. On the low range, the goal is to tune from 520 khz to 1050-1080 kHz. The high range should turn down to 1000 to 1030 kHz. You don't really have much control about where the high end of the high band actually will tune. Don't worry about that.

It is important that the high range tuning is able to tune lower than the low range tunes high. I like about 30 khz overlap here. If you need more dial spread, you can reduce the value of the 100 pf air trimmer. If you can't reach 520 kHz (gives you a 10 Khz cushion over the North American MW band edge), then adjust the mica trimmer a little higher.

I do this kind of adjustment in my sleep. Make sure to take the time and get it so it is right.

Now that this is done, tune the receiver to 1700 kHz and try to adjust the SEC tracking capacitor (on the leg of the differential) so as to keep the frequency shift as low as possible while turning the differential capacitor.

The last adjustment is to set the SEC compensation control so there is no shift between wide and narrow switch positions. Make sure your differential is set to mid position. Make this adjustment at 1700 kHz. Adjust the SEC compensation trimmer so that there is no tuning shift between wide and narrow.

Since there is a little bit on interaction between the capacitors, adjust the dial spread capacitor as well as the mica padder so the ranges are just as you want them to be.


This is a two handed radio. Both the ATU tuning as well as the detector tuning adjustments have to be made in tandem. A signal generator can help you spot the frequencies if you have trouble. I tend to rock the controls one station at a time and work my way up and down the band.

A good pair of sensitive sound powered (balanced armature) headphones will be just what you need.

Circuit Adjustments to match your junk box.

There can be a lot of substitutions to this circuit. You can leave out the two small trimmers if you don't mind the tuning shift. You will need something like the 100 pF trimmer for the dial spread adjustment. Try to use an air trimmer here.

Of course the main variable capacitors can be substituted to a certain extent. Make sure that the coil you make will tune the entire band.

The ATU needs a minimum of a dual 365 pf capacitor. You might miss the bottom channel or two (adjust for 530 kHz instead of 520 and hope). A dual 400 pf capacitor will give you leeway. You may need to adjust the coil size on the ATU so, add a turn or two extra when you are winding. The end value is also dependant on your antenna.


This radio marks a milestone in dx crystal set development, in my mind anyway. :) Although some of the things I jotted down may have been shown elsewhere before, I haven't seen it. So please don't e-mail me if you had thought of it first. :) This radio is truly a Schmarder design. Here are the crystal set developments included in this set:

Contra spider coil (I was first for this design shape)

An air trimmer to enhance the dial spread of the radio with the contra coil.

A main tuning padder capacitor to further enhance dial spread and allow for a wider variety of tuning capacitor and contra coil values to be used.

Selectivity enhancement circuit can be switched in or out (4 position band switch)

Compensation added so that there was no tuning frequency shift when the selectivity enhancement is out.

The Überformer

A padder in the ATU to further adjust the tuning range for improvement in dial spread but allow for compensation due to different antennas.

The special look of a wooden base, with the chassis held up by posts.

Thanks for stopping by for a visit. 73 and good crystal DX. Dave - N2DS

Dave's #71 Crystal Radio, Detector Chassis Under View   Dave's #71 Crystal Radio, ATU Chassis Under View

Dave's #71 Crystal Radio, Dial View ATU

Dave's #71 Crystal Radio, Dial View Detector

Contra Detector Coil Used in the Schmarder #71 Crystal Radio

Connections and Switches on the Detector Board on Crystal Radio #71

ATU Antenna - Ground Connections on Dave's #71 Crystal Set

Pictorial View of the Schmarder #71 Crystal Radio

Crystal Radio #71 Pictorial Schematic

Wiring Schematic of the Schmarder #71 Crystal Radio

Crystal Radio #71 Schematics

Pictures During Construction
Construction of my crystal radio

Construction of my crystal radio

Construction of my crystal radio

Construction of my crystal radio

Construction of my crystal radio

Construction of the #71

Closeout Radio Parts Available