The Experimental Radio Project
 

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Now it was time to connect the 3rd coil to the RF amp and run a crystal diode to the audio amp. Then I would be done!

 I turned it on and It howled like a banshee, with horrendous squealings and "motorboating" as the dials were turned. The RF amp was oscillating.

I began to substitute components, but because they were soldered in they were being ruined every time I cut one out. Eventually I bought the
solderless breadboard seen at the lower left. This allows you to connect components without soldering them. In this photo the RF amp is gone.

 
A work-around to the RF amp problem:
 


The TA7642 "radio on a chip" has an RF amplifier built into it! Using this chip I could do away with the external RF amp and the crystal diode. I really wanted that diode in the circuit but it would have to go. The only problem I saw was that the chip ran on 1.5 volts but the radio used a 9 volt battery.

The completed detector circuit using the TA7642. I kept the component leads long in case I had to take it apart.
 
How do you get 1.5 volts from a 9 volt battery? With a voltage divider.
The voltage divider, which powers the TA7642
 
I couldn't get the TANDEM TUNER to work so it was chopped out.

Surprisingly, with the coil, detector chip and audio amp I had a perfectly functioning radio already. It didn't work very well though.

Notice another control has been added under the speaker. The TA7642 is easily overloaded by a strong signal. To make the chip behave you
reduce the voltage to the chip. (as opposed to reducing the input signal). A potentiometer was added in series with the voltage divider that
would reduce the voltage by .5 volts. It could be controlled from the front panel. Later this knob would be labeled "Detector Gain"

 
A new front end coil was made, using a design I knew worked in crystal radios.
75 turns tapped primary, 30 turns secondary. Radio waves would, of course,
travel better through GREEN wire. Sounds like common sense to me!
The coil was attached to the selector switch on the front panel.
This meant the front panel had to come off, which meant
that everything attached to it had to be removed first.
 
The new layout. All the controls on the front work, but the radio itself still isn't working as well as I'd like.
 
More experimenting was needed.
 

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