The WAYSALOT Crystal Radio #75
Don't drop this on your toe! It will hurt! This radio sure weighs a lot (a total of nearly 12 pounds (5,5 kg). Now that I got all the legal warnings out of the way..... How about this for a radio? Chunky, just like in the old days.
My #75 radio came about when I received a half dozen of those big variable capacitors. I see that the seller has run out, just as I am showing you this set! They are of high quality, but the values are not useful to most. The size and weight are also negatives. But they got my interest when I got one as a gift a couple of years ago. If you have one of these capacitors, here is the project for you. Of course, please feel to tweak it and make it to your tastes.
The Waysalot crystal radio is a two piece set, just as all my dx sets are built. The antenna tuning unit (ATU) uses one of those big capacitors, a double pole 3 position switch (I used a 4p5t) and not much else.
The three switch positions select the tuning range. I will start with a discussion about the capacitor. It is a 7 section capacitor. However, 4 of the sections are low capacitance which can be ignored for this discussion as they don't play much of a part here. The 3 main sections are around 300 pF maximum and 25 to 35 pF minimum capacitance. The minimum capacitance especially causes a problem for tuning in a single range. By selecting capacitor sections, I can make a versatile antenna tuner. By switching to a lower range, a more selective tuning can be achieved, so long as you aren't out of the resonant tuning range.
In the high position, the capacitor in parallel with the coil is removed. Only the series capacitor to the antenna is connected. This series LC will properly tune the highest frequencies.
The mid position is the standard ATU that is in many of my radios. One section in series with the antenna and one across the coil. This will tune most of the band.
The low position is for tuning the 530 to 1200 kHz range. It places another section across the coil.
The tuning is silky smooth with the addition of a 6:1 vernier drive. The large knob gives it a nice feeling. The dial is calibrated with the three tuning ranges. This was a lot of work and maybe a little confusing, but I sure like calibrated dials in my sets.
The detector board uses a contra coil and one section of the big capacitor. Here is where this capacitor really shines. The 300 pF section, while not the perfect value, works well with a contra coil. Since there is capacitor added to reduce the dial spread and make the inductance reasonable, it is perfect. There is an additional ceramic trimmer capacitor to adjust the exact tuning range.
An interesting feature of the contra coils are, that the high range inductance is ¼ of the low range inductance. When you quarter the inductance, you double the frequency! Taking advantage of this, I made a set of labels with double numbers. This made calibration easy and less confusing. Due to the distributed capacitance of the coils, this is not exactly a doubling of frequency, low to high. But it isn't too far off. Make sure you calibrate only the high range.
I used a selectivity enhancement circuit or SEC in this radio. Some like this circuit and some don't. Other ways to sharpen up the radio selectivity is to use a tapped coil, such as in my #74 radio. After that is the famous FO-215 diode. The signal then proceeds through the RC network (called a benny) and then to a 200k:1k ohm transformer. This will match reasonable to many headphones. For better matching a multi-tap transformer such as the Überformer. That's about it.
The ATU coil is a simple 145µH value that is popular with my sets. It is made with a spider form that is 4¾ inches (120 mm) outside diameter and a 2¼ inch (57 mm) inside diameter. The form is 0.125 inches thick (3 mm) and made of HDPE material. I wound 38 turns on this coil, later trimming to 37 turns. If you have a shorter antenna, you may want to start at 39 turns.
The contra detector coil is wound on a 4¾ inch (120 mm) form, with the same 1¾ inch (45 mm) center. There are two windings. The second winding is wound opposite from the first winding. This is important! Please read my contra coils page and understand before winding.
For this coil, after using Jeff Welty's Contra Coil Calculator I found a series inductance of 266 µH was the right size. The inside coil is wound with 34 turns on the inside coil and 24.5 turns on the outside coil. This will get you close. The 5-50 pF trimmer will iron out any difficulties.
The base of the Waysalot radio is made from ¼ inch (6 mm) HDPE material. Since some of the rf connections are made directly to the board, it is important to use a low loss material. Otherwise you can use standoffs at the rf sensitive points. The front panel is 0.125 inch (3 mm) Garolite®. You can use what you want here. Garolite® is my favorite panel material. My chassis base is 11½ inches (29,25 cm) by 6 inches (15 cm). They were precut from my stock. It would have been better to have a slightly long board because of the length of the capacitor.
After cutting the chassis base and panel, the first thing to do is prepare the capacitor for mounting. The circuit board that is on the front needs to be removed. Also take off the two mounting brackets from the bottom and turn them around. I used some metal standoffs with 6-32 threads and a nut to attach them. If you have metric hardware, you can attach the standoffs with a screw threaded in. The standoffs are ½ inch (12 mm) long. The capacitor and vernier has to be aligned to work. The capacitor shaft is 6 mm, so I used a piece of tape around the shaft to help keep it centered.
The coil can be mounted on a 4 to 6 inch post. (12-15 cm) When winding your coils, make sure the lead lengths are long enough to reach where they have to go.
When moving the radio around, it is best to grab it by the capacitor to pick it up. I don't worry about the finger prints. If I spent all day polishing my capacitor, I would get nothing else done. The covers can be removed if you want.
The ATU coil may need trimming. Check with a signal generator and radio to make sure the full frequency range is achieved. If you don't have a signal generator, use stations heard on a radio with the ATU near it. The antenna must be connected for the calibration to be correct.
There are two adjustments on the detector section. The SEC capacitor going to the diode should be set at mid value. The 5-50 pF capacitor directly across the big capacitor can be preset to maximum. Using a signal generator, set that capacitor to receive 530 kHz when the big capacitor is at maximum. Check to make sure that the low range tunes higher than the high band tunes low. The overlap should be a minimum of 20 kHz, 30 or more is ok too. If you can't reach these values, check your wiring, (It can be confusing. That is why I put up a pictorial with the switch that I used) Make sure your coil is wound properly!
The SEC capacitor should work well at mid position. The settings are most critical at the high end of the band. But too much capacitance shouldn't be taken away or the low end gets weaker. If you like, this capacitor can have a knob for adjustment.
The detector board operates in two ranges. Select the range you want and tune the dial to where you want to be. Then by placing the coils about 6 inches apart (15 cm), tune in your station. Adjust the distance of the coils (usually further apart) until the best reception is achieved.
I feel good about making an ugly duckling but high quality capacitor work as a dx crystal set. The price and quality offsets the negatives.
Thanks for stopping by for a visit. May your static levels always be low. 73 and good crystal DX. Dave - N2DS