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Dave's Crystal Radio #70, A real hoot!

Dave's Crystal Set #70

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Introduction

I've been busting to tell you the story of my #70 radio! What? No spider coils, you say? Yep, this time I decided to go a less traveled route on the way to the 100 th radio milestone. I figured that perhaps some of you were spinning with the constant barrage of spider coil radios.

Taking pictures of this set was difficult as it sure doesn't fit in my photo tent, or even on my couch. I received this nice personalized beach blanket as a gift a little while ago, and since I haven't made it to the beach yet, it makes a good backdrop for my big radio. So to keep you all abreast of my latest projects, I proudly present to you my #70 dx crystal set on this web page..

The Idea

This project first flashed in my head about a year and a half ago. I wanted to make something that was really big. Even bigger than my #48 dx crystal radio. The wood was prepared and waiting for the staining and finishing. I don't like to stain my wood until I have an idea of what I am going to build.

Tom Kipgen made me these really neat coil forms. They were made from ABS styrene 4 inch (10cm) pipe. He turned down the outside on his lathe to make the material as thin as practical. Then he grooved it so I could build a space wound coil. I then had the idea of suspending the coils above the rest of the radio and make them so they could be slid back and forth.

But how was I going to make all this come together in a nice looking radio? Mike Peebles was kind enough to make me some beautiful standoffs for my #68 and #69 radios. That, along with the 1/4 inch thick (6 mm) HDPE material I used on the chassis, this project was starting to materialize.

I still wasn't sure how the front panel was going to look. Then while walking around the neighborhood on a hot and sunny summer day, I saw the idea for this layout. I even had a lot of fun cutting and shaping the front panel.

The idea was for a dx radio, using the ideas learned from earlier sets. Of course each of my sets are different, so a good idea on one might not be good for another. I did like the switchable selectivity of my previous two sets.

The distances of all my dx sets coils must be adjustable. This is important from a selectivity vs. sensitivity standpoint. I found adjusting the spacing just a couple of inches made all the difference in the reception. It appears that with this radio the differences are even greater.

Circuit Description

My radio is a dual coil set. There is a tuned coil for the antenna tuning network as well as a tuned circuit for the detector. The antenna circuit is a circuit popularized by Mike Tuggle for dx crystal radio use. I have to admit that in my earlier experimentation days, I didn't find this tuner to work too well. It is lucky that I gave it a second try. I have found nothing better since.

The detector section is standard, more or less. There is the tuned circuit made up by the large coil and 475 pf holy grail capacitor. Nothing but the very best goes into my dx sets.

The output uses a special transformer, called the XTAL200k Überformer. This transformer is now a must in my best crystal sets.

Between these two sections is where it gets a little special. In the cleavage area there is a 3 position selector switch. This is a selectivity adjustment switch. There is a network consisting of a small air trimmer capacitor and a 27 mh rf choke. This provides a way to reduce the loading on the tank circuit caused by the detector diode and output circuits. By reducing the loading, the circuit efficiency (Q) is raised. But sometimes this is at the expense of selectivity. So having a switch to decide if this is in the circuit if a good idea. The third position (sharp) places an additional fixed capacitor in series with the variable trimmer. This reduces the loading even further on the tank circuit. But this is at the expense of sensitivity. I do believe this position might be handy at the very top end of the band.

A trimmer was added to compensate for the extra capacitance that appears across the tank as the selectivity switch position is changed. It was found that some added capacitance is the result of the selectivity enhancement circuit. Therefore, some capacitance was added when this circuit is bypassed. A 4-30 pf ceramic trimmer fills this bill nicely.

As mentioned earlier, I am using a space wound coil in this radio. The spacing is as small as possible as a wider spacing takes more wire, and the resistive losses could outweigh the advantages of the space winding. Space winding is said to reduce the proximity effect of the wire next on the neighboring windings. There has been some discussion that a space wound litz wire coil has no advantage over a close wound coil. I will leave this for others to figure out as the scope of this web site is building projects more than highly technical discussions.

Construction

The base is made from 3/4 inch (18 mm) thick oak lumber. It measures 24 inches wide and 8 inches wide (61 x 20,3 cm). After being cut and shaped, I applied a coat of ebony stain. This sure brings out that oak wood grain! After the staining, I applied several coats of polyurethane. The end pieces were also from oak. The ends were cut with my scroll saw and then sanded. These two wood columns are held vertical by two wood screws from the bottom of the base.

Working our way up, there are four standoffs that are made from Delrin. Just about any material can be used here. They are 2 inches (48 mm) tall.

The chassis is made from a 12 inch by 7 inch (30,5 x 17,75 cm), 1/4 inch thick (6 mm) High Density Polyethylene or HDPE. It is the low material of choice too.

The front panel is made out of thicker Garolite® than I usually use. I use the more expensive thicker material on my big projects. The front panel is built like a brick restroom. Special and detailed attention was paid to the roundness of the top. It had to be pleasing. Both sides are nearly the same, but as in life, there are always some differences. That is the way the scroll saw blade cuts.

As in many projects that depend on items found in the junk box, the two variable capacitors I found did not have he same shaft height from the bases. The detector was 1-1/2 inches (36 mm), while the antenna tuning capacitor was 2-1/4 inches tall. I ended up building a 3/4 inch (18 mm) thick piece to put under the detector capacitor so the shafts would be the same height. Easy enough.

The dials are made from 1/8 inch thick (3 mm) HDPE, made expertly 4 inches in diameter (10 cm) by our friend Mike Peebles. Ain't he a sweetheart?

Since the panel and chassis are so thick, I had to do a back cut into the materials so the switch and phone jack shafts would fit all the way through with space for the nut to fit. This is best done by drilling a small hole first. Then using a forstner bit, drill a partial hole in the back of the material. I used 1-1/4 inch (30 mm) for the switches. Then from the front side, I drilled the usual 3/8 inch (9 mm) holes. The switches are actually 4 pole, 5 position units that are available. Two poles, and three positions are used on the front panel switch and 1 pole, 5 positions for the transformer tap switch.

When I am building radios like this, there is a certain order that I do my chassis and panel drilling. I drill the holes for the position sensitive points first. In this case, the position sensitive points are the three angle brackets at the bottom that connect the panel and chassis together. After that, the two variable capacitors are next. They have to be right on the front panel (centered) and on the chassis. Nothing can be in their way. Also, I should mention that the shaft that goes to the antenna tuning capacitor has to be isolated from the vernier. I used a ceramic shaft coupler. I still noticed a little hand capacitance effect when tuning that capacitor, so I grounded the vernier. That fixed that minor problem.

Then, I measure hole positrons for front panel controls. Finally I drill the rest of the holes where they will work the best. I do all this in my head but you may want to make a list first.

After all the holes are drilled (don't forget all the wiring pass through holes), then it is time to wire the radio. The wiring isn't critical, but try to make it look fairly nice. After all you may want to show it off too.

Alignment

There is not too much alignment to this radio, except that you should check the tuning range (place the alignment capacitor in the middle and the selectivity capacitor at near maximum first).

Set your radio near 1700 khz and with your signal generator, tune a spot with the selectivity switch in the medium position. Then switch the selectivity switch to the broad position and adjust the compensation trimmer so the signal is tuned in again.

After you do that alignment, then marking the dial frequency settings can be done. You should have a signal generator to do this accurately. Using a calibrated receiver at night can work too. Both dials should be marked.

Operation

Operating this radio is easy (easier than building it). The selectivity switch should be set at broad for the bottom of the band and then reduced at the top end until you get used to how this radio works.

The coils can start at about 4 to 6 inches apart (10 - 15 cm) to start and then adjusted according to loudness and selectivity desired. Retouching the tuning dials might be necessary.

You will notice that the tone is the best at the broad setting. Switching to medium and sharp will reduce the high frequency response. This radio can be very selective.

Circuit Adjustments to match your junk box.

As most of us, the radio you build depends on what is in your junk box and that worldwide junk box called eBay. Variable capacitors down to 400 pf, and perhaps slightly less will work good here. You may want to add about 2 turns to the coil for a lower value capacitor. The nominal value of the coils for 400 pf capacitors is about 240 µh for the detector and 150 µh for the antenna tuning network.

If you are really budget strapped, you can substitute a lower stand count litz such as 330/46 or 165/46. Instead of the Überformer, you can use a Bogen T725. Going directly to a pair of high impedance headphones would cause very severe loading on the tank during the reception or stronger signals. But generally a very bad mismatch.

You can remove the 30 pf selectivity enhancement capacitor and replace it with a fixed capacitor. 22-27 pf is recommended. It should be a low loss type capacitor.

Conclusion

What a fun radio this is. I just get great job grabbing on those two knobs and giving them a gentle twist and the stations come right in. Sitting in front of this massive radio is a real kick. I hope you will think about making a big one too!

73 and good crystal DX. Dave - N2DS

Dave's Homemade Crystal Radio #70, Clouse-Up Switch View

Dave's Homemade Crystal Radio #70, Front View

Dave's Homemade Crystal Radio #70, Back View

Dave's Homemade Crystal Radio #70, Top View

Dave's Homemade Crystal Radio, #70. Bottom View

Dave's Homemade Crystal Radio #70, Space wound coil view.

Crystal Radio #70 Schematic

Crystal Radio #70 Schematic


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