Crystal Radio #72 Dave's Easy Tuner!
It was two days before the hamfest, and I had nothing to show off there! I know this would have disappointed a couple of people. I was thinking of not even showing my pretty face there! But I did have two days. Not enough time to put out a spectacular DX radio, but enough time for something fairly simple but good. So Dave's Easy Tuner is born and the hamfest is attended.
Not too much for this idea. It is a twist on my favorite single coil sets. Instead of the second variable capacitor, I used a switch and 5 fixed capacitors.
This is a simple circuit that gives good performance. The performance kickers are the litz coil and output matching transformer. I kept it simple by using a tapped coil and I left out the selectivity enhancement circuit. It is a project you can make with your kids or grand kids.
The tuning is accomplished with the 410 pF variable capacitor and the 5 position tuning range selector switch. The 188 µH coil resonates with the capacitors (and antenna) to the frequency desired.
A 5 position switch with strategically selected fixed capacitors will give you full range tuning, and will allow for some selectivity vs. sensitivity adjustment. As the selector knob is turned to the left , there is a higher fixed capacitance connected to the antenna. This can improve sensitivity at a selectivity cost. Except for the lowest part of the band, there is a 2 or even 3 switch position tuning overlap.
The 5 position switch sets the antenna series capacitance. The values of the 5 fixed capacitors are not critical. The capacitor that connects to the antenna should be over 330 pF. This is so the radio will tune down to 500 kHz when connected to my antenna. The other capacitors can be distributed over a range going down to around 22 to 30 pF. Use what you have in your junk box.
The diode is the familiar 1N34A germanium type signal diode. I tapped the litz coil at 36 turns from the grounded end at the coil hub. Tapping the coil gives a better diode match and loads the tank less. The signal is then routed through a "benny" consisting of a .1µF capacitor and a 39k ohm resistor. This keeps the tank from being loaded down during the reception of strong signals. The selectivity is improved during these periods as well as less audio distortion.
The headphones are a high impedance type. The Piezophone, regular high impedance magnetic headphones or sound powered phones can be used with this radio. If you are using only one set of headphones, you can leave the 3 position impedance selector switch off. I wrote a page describing the T725 transformer.
I wound a spider coil (my favorite) using a piece of 1/8 inch (3 mm) HDPE material. This High Density PolyEthylene has very low RF losses. The spider form must have an odd number of slits to operate properly. This gives the automatic spacing between windings.
The outside diameter is 4-¼ inches (11 cm) in diameter. The hub is 2 inches (50 mm) in diameter. 46 turns of 165/46 litz wire makes a coil around 188 µH, which will tune the MW band nicely with most antennas. If the top end of the band can not be tuned and there is room at the bottom, then take a turn off the coil and see how it works. Mike Peebles was kind enough to give me his insights as his antenna system is different than mine. I think this design will satisfy nearly every antenna configuration.
The Dave Easy Tuner is made with a piece of Garolite® 8x6 inches (19x14 mm) in size. I cut this based on the size of the oak wooden base. The chassis is a piece of HDPE, 1/8 inch thick and 8x5 inches in size. The 410 pF variable capacitor is mounted by using two screws in the bottom.
This is important: Make sure to place a washer between the bottom of the capacitor and chassis. The capacitor's phenolic insulator sticks slightly below the metal bottom of the capacitor. If you tighten the screws, without some kind of spacing, you can dislodge the insulator and ruin the capacitor.
The front panel and chassis are held with two metal brackets. I used four 2 inch
spacers to hold the chassis above the base. There is plenty of room to do the wiring underneath
the chassis, thus keeping the chassis top free of wiring clutter.
The dial pointer is a thin piece of styrene cut to a shape made by my compass and french curve. Those curves, plus the one across the top of the panel give the radio a nice look.
I have a metal numbered dial scale on this radio. Unfortunately these are rare to find and you will have to come up with your own way to dress up your radio.
Tuning is accomplished with the switch and variable capacitor. It is possible to make a tuning graph so you know what the dial numbers mean for frequency, depending on the switch position. Then if you want to tune to a specific frequency, you can look it up on the graph.
Circuit Adjustments to match your junk box.
This radio is very non critical to the selection of parts. Use what you have in your junk box to make up the values of the fixed capacitors. My unit uses slightly different values as stated in the schematic. I used what I had on hand.
The outcome turned out to be a happy one. I made a nice radio, had a nice, nearly rain free day at the hamfest, and was able to produce something that I am proudly showing everyone here. You may want to make one of these for yourself.
Thanks for stopping by for a visit. 73 and good crystal DX. Dave - N2DS