QAMP-80 • 13
QRP power amplifier.
With the above all set up and handy, let's get testing!
1. Rotate Bias pot R4 fully CCW.
2. Connect a multimeter to TP1 ; set the meter to read up to 5 volts DC.
3. Connect a dummy load to J1, the amplifier RF output. In a pinch, a
light bulb may be used - see the section, "Verifying RF Power Output."
4. Temporarily install a jumper from the collector of Q3 to ground.
5. Apply power to the amplifier but do not turn on the transmitter.
Measure the current drawn by the amplifier and slowly rotate the bias
pot, R4, clockwise until you reach a reading of ¼ amp. Do not allow the
current to rise above ½ amp. If you cannot adjust or reduce the
current, disconnect the power supply and consult the troubleshooting
hints section. The voltage at TP1 should be about 3.2 to 3.5 volts.
6. Turn off power. Disconnect jumper from Q3 to ground.
7. Connect the QRP transmitter to the input of the amplifier. Key the
transmitter. You should hear the T-R relay click and see amplified
power output. Measure the DC current draw; it should be in the 1 to 3
amp range depending upon power output. Unkey the transmitter. The
relay should drop out and the current should drop back down to ¼ amp
This completes the testing of your QRP power amplifier. The PC board should
be mounted into a protective enclosure to guard against accidental contact.
The Ramsey CQAMP case set provides an ideal perfectly sized cabinet that
matches all other Ramsey kits.
Study the following sections on DC power supply and RF power
considerations. Operate your transmitter with good amateur practice.
YOUR POWER SUPPLY AND RF OUTPUT POWER
For optimum performance, one or two volts of extra DC supply power can
make quite a difference in any RF power amplifier. For example, two lantern
batteries in series, or 8 D cells, will obviously provide about 12 volts with
sufficient current capability for casual operating. For maximum RF output
power, use a supply of 13 to 14 volts DC. The easiest method is to place two
fresh D cells in series with your power source if a full 13.6 - 15 volts DC is not
available. Be aware that batteries are not the optimum power source,
especially if you are prone to long QSOs! A word of caution concerning wall
plug style AC adapter power supplies: They are not suitable for operation of
your amplifier due to their poor regulation, AC ripple content, and RFI
susceptibility.
With 1 watt of drive and a supply voltage in the 11-12 volt range, you can
expect a 1 to 2 amp current draw and about 10 watts of RF output power. With
a solid 13 to 14 volt supply, you can expect about 2 to 3 amps current draw
and up to 10 or 12 watts of RF output power. With 2 watts of RF drive, expect
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