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AA7 • 13
rather than only a receiver or scanner, make sure it is not possible to transmit
by accidentally pressing a mike button or CW keyer. Transmitted RF into the
AA7 is likely to ruin one or both of the transistors.
Transistor Placement Information:
Both transistors may be ordered from Ramsey Electronics, but they also may
be found at a local electronics parts distributor. You may use either "SK" or
"ECG" replacements:
3N201 = ECG454 or SK3991
2SC2498 = ECG10 or SK9139
LED Power On Indicator
A visual power on indicator can be an enjoyable and practical enhancement
of your completed kit. LEDs (Light Emitting Diodes) are available at low cost
and are ideal for this purpose, providing you don't mind the extra current drain.
Voltage for a standard LED is easily taken from the appropriate top terminal of
S2. The switched +9V terminal is the middle of three terminals in the row
nearest the center of the PC board. This positive DC must be connected to the
anode (longer wire) of the LED. The cathode lead is connected through a
proper current limiting resistor of about 1K ohm to the closest PC-board ground
point. (Note: This can be locations such as the black wire of the battery, the
mounting case of either jack, but it CANNOT be the antenna.) Mount the LED
at any convenient point near the switch.
TROUBLESHOOTING GUIDE
The fact that there are two independent preamplifiers makes the AA7 easier
to diagnose than many other devices. If a problem occurs only when S1 is
pressed inward, troubleshooting can be concentrated on that part of the circuit.
If the problem is common to both settings, then the components and
connections common to both preamps should be checked. Make sure you
have not omitted any of the jumper wires required.
There are other characteristics or phenomena associated with preamplifiers
and active antennas which do NOT mean that your AA7 is malfunctioning:
Strong AC hum in the HF setting:
The antenna is too close to an AC cord or
power line, and/or the RF gain is set too high.
HF signals stronger at VHF than at HF setting:
As explained in the AA7
circuit description, either preamp may be used for HF. Signal strength will be
greater with the VHF preamp, but the HF signal-to-noise ratio should be better
with the dual gate MOSFET HF section.