Ramsey Electronics QRP-80 Bedienerhandbuch PDF herunterladen (Seite 23)

HR-80• 23

modification will achieve any particular ideal signal-to-noise ratio or

audio frequency response, but is does significantly boost the output

of the NE602 reaching the LM386 audio amplifier IC. Instead of the

traditional transistor audio preamp circuit, an additional LM386 is

used in its minimum gain configuration (gain of 80), plus one

electrolytic capacitor (1 to 10µF) to achieve minimum parts and PC-

board drilling. This use of two LM386 ICs amplifies the audio signal

of the NE602 by a factor of 1000.

This modification requires cutting the circuit trace leading from C8 to

the volume control, R3.

Additional LM386 used as AF preamplifier between U1 and U2:

 Pins 1,7,8: not used

 Pins 2,4: to ground

 Pin 3 to ‘-’ side of c*(2.2pF)

 Pin 5 to ‘+’ side of new 1 to 10µF capacitor

 Pin 6 to ‘+’ DC supply board trace

The ‘-’ side of the new capacitor is connected to the circuit trace

leading to the volume control. Mount this capacitor as close to the

body of the new LM386 as possible. Keep all leads as short as

possible. This modification results in very good speaker volume but

will also require closer attention to battery quality and condition or

else the audio circuits will cause oscillator instability. Two alkaline

cells in parallel or a 12 volt supply are recommended. If the gain is

too much, use a resistor to reduce the voltage supplied to pin 6 of

preamp LM386

Finally, we remind you again that modification of any PC board puts

you on your own, voiding your factory warranty, making your unit

ineligible for factory repair. Make sure the receiver is working

properly and that you understand how to use and repair it BEFORE

making any changes in the original design. Enjoy it!

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Ramsey Electronics QRP-80 Bedienerhandbuch Seite 23