Am3 Radio Rev. E - Page 2

A simple 1-transistor amp centers around a MPSA18, 2N5962, or 2N5089 NPN. The base-emitter junction of the transistor serves as the detector, eliminating the need for a diode. The 10uF cap in the emitter is a 6.3 Volt Tantalum. NOTE the polarity of this capacitor. It MUST be oriented this way, else the radio is silent! The Xtal earphones have tinned leads, just twist the pair of lefts, and the pair of rights together forming a parallel pair. Overall gain in the collector is estimated as 46.5db: at the 'phones 44.5db using the MPSA18 NPN transistor. The signal current gain is estimated as 690 and the Voltage gain 270.

Better performance tips: turn off all the televisions, flourescent lights, and computers. Computers need to be COMPLETELY off including the UPS device. Make the ground connection to the telephone extension block (or finger-stop) using solid wire. Check the phone line itself. The use of solid wire (round cable) is better than stranded wire (flat cable).

DISCONNECT THE GROUND WIRE
AT THE TELEPHONE OR THE TELEPHONE JACK
DURING THUNDERSTORMS.

Reception can be over 2000km (Cuba has been received here at distances over 2200 km) and the high powered stations come in nicely at night. Inexpensive breadboards with metal ground-plane can be found at electronix-express for about $8. The ground plane is needed for the return line of the circuit, and thats why the wiring diagram is supplied. The use of the power bus lines makes the ground-plane availible.

So there's a beginning radio that can work rather well. Most of the parts are availible at mouser dot com (ALL caps, transistor, crystal earphones, and a battery holder if wanted). The coil form can be a pair of cake frosting cans which are readily availible around birthdays and holidays. All thats left to find is the wire and resistors. I hope you enjoy building and listening to the Am3 Radio, its come a long way in 55 years, and even managed to lose the external antenna.

Advanced Crystal Earphone Tweak

Getting more signal to the ears can be accomplished by very carefully cutting the earphone solder connections off and stripping the 4 leads. The 7 strands of wire are literally hair thin and delicate requiring skill and patience, even with the right tool. I tried this with a pair of sharp scissors, and I did get this to work. Its not easy, but the reward is a good solid COPPER contact for the phones. Since this is stranded wire, simply plugging in the connection is not possible. For a good method, find a European connector block, saw off two adjacent connections, and connect Left and Right earphone wires. The other side of the block connects to the circuit board by solid magnet wire. After making the new left and right connections, DISCONNECT THE GROUND, THEN THE BATTERY, and REMOVE THE TRANSISTOR. Place the board-side connections over the proper breadboard contact, and push the wire into the board. When both connections are installed and secure, REPLACE THE TRANSISTOR, then RECONNECT the BATTERY, and replace the ground last. The volume increase is quite notable!

Solderless Breadboard diagram

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