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I congratulate Miss Liz for taking the time and patience that was needed, in order for her Radio Shack Special to come to life! It is with great pleasure that I share with you...words and pictures of her maiden voyage!
...your friend, Patrick
I came across the Radio Shack Special when looking for designs for an FM receiver. Since I had never made a PCB before, I was hesitant to build the RSS and decided to keep looking. However, after unsuccessfully trying several other PCB designs, I went back to the RSS.
On the tutorial site, I quickly realized that I didn't even have to design the PCB layout; I just had to transfer the one Patrick had created, which made my life a lot easier. I was able to get all of the parts without much trouble, although my Radio Shack was sold out of the kit containing the 5pF and 10pF capacitors. Luckily, I had some lying around from my previous FM receiver endeavors. Using the kit from Radio Shack and some carbon paper, I transferred the template to the PCB. I found that with the double-sided PCB in the kit, I had to leave the PCB in the solution for too long causing the traces to degrade in some areas. I then used a single-sided PCB and had no problems.
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Following the instructions on the site, I made my
drain and source coils. I didn't have a 5/16 of an inch standard
threaded bolt for the drain coil, so I used a Bic ballpoint pen
of the same size, which worked well. I then soldered all of my
components into place and performed my first turn-on.
Unfortunately, there was absolutely no sound. Even more
unfortunately, my 1K potentiometer caught on fire; impressive,
but definitely not good. I consulted the instructions and found a
discrepancy in Chart II and the picture in Chart III (step #22)
in how the potentiometer was connected. After consulting Patrick,
I realized I had connected the battery incorrectly. I immediately
fixed my connections and turned the receiver on again. Now I had
sound, but it was just fuzz like at the beginning or end of an
old recording. I thought this might have been because I was using
a tuning capacitor I had purchased and not one of the recommended
ones salvaged from a store-bought radio. So I swapped the tuning
capacitors, but still had no luck. With Patrick's help, I was
able to determine that the audio section of the receiver was
operating correctly by grabbing hold of the 0.1uF capacitor
(adjacent to the 10k potentiometer) and hearing a humming noise.
I decided to take the receiver to my school's lab and connect it to an oscilloscope to check the waveforms at various points of the circuit. In doing this, I inadvertently moved a component whose bottom was making contact with a wire stub from one of my components below and my receiver worked! I was then able to adjust my potentiometers so that performance was optimum. In the end, my receiver was able to pick up 4 stations clearly even in a lab full on interference. Note: I was only able to pick up 4 stations (from 88.1MHz to 93.7MHz) because of how I connected the tuning capacitor. To receive more, I would have to use the opposite side of the capacitor.
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I then performed various tests on my receiver to
measure its performance. From these tests, I determined that the
Radio Shack Special has incredible sensitivity and selectivity,
despite its relatively simple design. Overall I did not have too
many problems, and those I did were due to my own error.
Following this experience, I can safely say that the Radio Shack
Special is one of the best designs on the internet. The
components are easy to procure/make and the layout is spaced out
enough that soldering isn't too much trouble. After trying
designs that contained no explanation, I found the detailed
instructions truly invaluable, especially for someone who is
relatively unfamiliar with FM receiver design like me.
Additionally, when I had a question, Patrick was extremely
helpful and quick to respond.
All in all, I am extremely happy with my receiver and highly recommend using the Radio Shack Special as an FM receiver design.
Liz
