Crystal Radio #76
Hi Gang. Remember me? It's been a long time since I built my last radio. I have been gathering ideas with my associates at the Secret Radio Ranch over the summer. Now I'm back, fresh as a daisy (so long as you don't stand too close), ready to go again. Enough of the Uncle Miltie comedy routines. Now on with the show!
I recalled back to my #16 Miller Time, as well as the great looking #52 radio that I had built in previous years. I thought it might be time to do another set. But this time, the set was to be built with dx radio features. These are: A high quality variable capacitor with ceramic insulators.
A high quality variable capacitor with ceramic insulators.
Dual in line verniers for a s-l-o-w tuning rate.
Ferrite toroidal coils with 165/46 litz wire.
A variable "Benny" control.
The famous Überformer output transformer.
Any of these features can be downsized if the budget or desire doesn't allow. Don't be expected to put litz on the coil if it is difficult to put food on the table. A Bogen T725 transformer will work ok too, with reduced performance.
The Antena Input Circuit Description
The circuit is shown below. Starting with the antenna input on the left, there is a 5 position switch to select how far up the primary coil the antenna will connect. This is a new feature, and perhaps useless to some. Since I haven't built many projects with toroids, I felt this was a good way to judge the best tap on the transformer to use. I also felt it is possible to get the best match for the antenna that I or others have available. Come to find out, this is a useful feature. It is like moving the coils in my dual spider coil sets. As it turns out, having the tapped antenna input allows for the best sensitivity on the low end of the band by moving the antenna up the coil, and at the high end, crank the switch back down.
The Tracking Circuit
The 4 capacitors,two coils and rf transformer are part of a tracking tuning circuit. This is also called band-pass tuning.
The main coils are wound on Amidon FT-114A-61 ferrite toroidal forms. The resonant tuned sections are wound with 165/46 litz wire. Both tuned sections have 39 turns of litz on them, having an inductance of 220 µH (with a 440pF dual gang capacitor).
One coil (L1)has 3 turns of small wire with a switch tap at every half turn. This adjusts how the antenna is coupled to the radio. Adjust the turns and taps to your liking. (You don't really need 5 taps, but it went with the 5 tap switch that I used.)
The secondary of this coil (L2) resonates with the two capacitors across it for tuning. L3 just has one winding as the detector is capacitive coupled.
RF transformer T1 is an Amidon FT50B-61 with three turns on both the primary and secondary. A smaller toroid can be used here, but you may want to add more turns. The .025uF can be a .022 or .033uF without difficulty. I found the capacitor helped the mid band sensitivity. Thanks to the members on the The RadioBoard for this hint.
Update Jun 24, 2013 I had a question regarding the winding of T1 as to winding polarities and directions. There is no special way to wind this transformer as the two windings go to separate, non coupled circuits. Just wind as you wish.
Of course it seems for every positive, there is a negative. The sensitivity of this type of crystal set is poorer than with other radio designs. But you get a single turning control radio with fairly good selectivity.
The important requirement is to have good frequency tracking throughout the tuning range. If you don't get this right, the radio will fail to satisfy. When two or more tuned circuits are ganged with a single spindle capacitor, tracking is always an issue. The tracking is the most critical at the top of the band. Therefore I installed two small trimmers to align the top of the band tuning.
To adjust the bottom of the band tracking, turns of wire can be removed from one coil or the other. This adjustment isn't as critical as the top end of the dial. More on the alignment will be covered below.
After the Track-Tuning Circuit
Beyond the track-tuning circuit is another 7-45pF trimmer, 27 mH choke as well as the diode and rf bypass capacitor. The trimmer and choke are part of a Selectivity Enhancement Circuit (SEC). I have used this on many of my dx radios.
Then comes the "Benny". The 250k variable resistor is set approximately to the impedance of the transformer (200k ohms). This reduces coil loading and distortion on louder signals. The .1uF capacitor allows for easy audio flow to the Überformer. (If you use a Bogen T725 instead, please lower the 250k pot to 100k and increase the capacitor to .22uF.)
Starting from the bottom up, the box is a bamboo storage container sold by Bed, Bath and Beyond. They are cheap. I buy them when I have a discount coupon. The box is 12x6x2.5 inches (30x15x6,3 cm). There is a ridge on the inside that is for stacking boxes, but I use it to fit the black Garolite® chassis into. I do a rough cut and then with my small bench sander, I sand until the Garolite® fits nicely. The Garolite® is 1/8 inch or 3mm thick.
Then I start positioning the pieces on the top to make sure I have room for everything and that it will look nice. I used more pieces of Garolite® to make the front stand up panels. Four metal brackets hold the front panels to the chassis. Two verniers are included in all this. The capacitor was placed on standoffs so the height would be good so that I can use a 4 inch (10cm) plastic tuning indicator dial.
Mounting the capacitor and verniers is the main task in this project. All the other switches, jacks and other parts can be fit where they will work well. Of course, this should be planned before the first hole is drilled!
Now a couple of words how the Garolite® chassis is mounted to the wood box. Since my early projects I have used a small pieces of pine wood attached to the sides of the wood box with a pair of screws. Then I measure and drill the holes to mount the chassis. (Allow for the box edge when measuring the Garolite®.) Then a T-Nut would be placed under the pine wood and screwed into place.
The only change is that this time I used wood glue to attach these two pieces of pine wood to the box. That worked out very well! I will do this in the future. It only took me 8 years to figure it out.
Before winding the toroidal coils, I wrapped the toroids with plumber's pipe tape (Teflon). This is meant to improve the performance of the coils. To keep the toroids away from the chassis, I used a scrap piece of HDPE. Not sure if that is really necessary, but I'm not going to throw away Q on sloppy construction!
After that, the wiring is completed. The wiring is easy on this set. The tracking trimmers were placed on the bottom of the chassis and held in place with thick bare wires. All the wiring is out of sight when the project is finished. If I say so myself, this set looks elegant. The lady of your house may disagree. Work it out, don't contact me about this.
You should have an inductance meter for making sure the coils are the same inductance. I recommend the one from AADE for a hundred bucks as a kit. You won't regret it. Also a signal generator is very helpful, unless you have strong signals in strategic positions on the band.
Start by setting all trimmers at the mid value. Tune your signal generator to 1600 kHz and tune the radio to about 90% open on the variable capacitor. Peak the trimmers. Then move to 600 kHz and check the position of the capacitor (Do you think it will do down to 530?). You can also adjust one of the trimmers and rock the variable tuning capacitor to see if the tuned circuit is still peaked.
If it seems to be off, then you will have to adjust the windings on one of the coils. Take a couple of turns off and resolder the wire on the one toroid. Check and see if the tracking seems to be working, by first going back up to the top of the band.
If the tracking is further off, then put the turns back on, and take two turns off the other coil.
This may take hours to do, so please be patient. If you have aligned superhet receivers before, you know the routine here and it should be easier.
Ok, connect your antenna and ground. Pick out the best pair of your high impedance sound powered phones, regular magnetic headphones, or a set of Piezophones. Then start tuning around. When you hear a station, then adjust the switches. If you get a real loud signal, adjust the Benny until you hear the lowest amount of distortion.
At first, I wasn't so sure that this was going to be that good of a radio. These band-pass track tuning systems have a way of losing signal. This radio in a short time has exceeded my modest expectations. This was my best effort attempt to make a simple to use but good quality crystal radio. I have no regrets.
While first testing this radio, I was pleased to hear a new station. It was a 237 watt station about 500 km from this location. This is a good omen! Here is my entry in the 2009 Sprint Contest. I heard that station again and was able to log it.
Thanks for stopping by for a visit. 73 and good crystal DX. Dave - N2DS