Ramsey Electronics HR40 - Manual (Page 23)

HR40
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frequency response, but is does significantly boost the output of the SA602
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 40), plus one electrolytic capacitor (1 to 10 uF) to
achieve minimum parts and PC-board drilling. This use of two LM386 ICs
amplifies the audio signal of the SA602 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:
ˆ Pins 1,7,8: not used
ˆ Pins 2,4: to ground
ˆ Pin 3 to ‘-’ side of c*(2.2 pF)
ˆ Pin 5 to ‘+’ side of new 1 to 10 uF capacitor
ˆ 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!
1
MEGOH
M
NE60
2
PIN
7
COUNT
ER
TO
27
pF