Instruction manual
DDF1 31
TESTING
Basic Testing
It is wise to review your workmanship, looking for circuit shorts or opens prior
to applying power. Use an ohmmeter to measure the resistance between
power and ground; the value should be greater than 2k ohms. Apply 12
VDC to the RDF unit with the power supply current limited to 150mA to pre-
vent damage in the event of a problem (normal current drain is 100mA).
Verify the presence of the following supply voltages: = 5V on the output of
VR1, pin 16 of U8, pin 8 of U4, pin 8 of U6, pin 16 of U7, pin 24 of U11, pin
16 of U5. Verify +2.5 V on pin 14 of U3, verify +12 V on pin 4 of U1 and U2.
This completes the basic testing of the Doppler RDF circuit.
The following tests are recommended to identify and troubleshoot
problems, but don't worry if you don't have all the necessary test
equipment. Satisfactory performance can usually be achieved without
performing these tests.
Functional testing
Before connecting the antenna switcher, verify the operation of clock oscilla-
tor U4 by connecting an oscilloscope to pin 3. The output should be a square
wave with an amplitude of 5 V and a period of 125 ms (8 kHz). Use a fre-
quency counter to verify the clock frequency is 8 kHz +/- 250 Hz if you de-
sire. Getting the clock frequency to exactly 8 kHz is not necessary for proper
operation, however, the values of R27 and R28 can be changed to adjust
the clock frequency. Verify that closing switch S2 disables the clock. Open
S2 (scan stop) and verify the operation of BCD counter U7 by connecting the
oscilloscope sequentially to pins 14,13,12, and 11. The signal frequency on
these outputs should be approximately 4, 2, 1 and 0.5 kHz, respectively. Ver-
ify the presence of a square wave signal on pins 2,4,6 and 8 of buffer U12.
Signal Level Indicators
The following test uses an audio signal generator to simulate the presence of
the Doppler tone. Disconnect the speaker from audio level control potenti-
ometer R50 to prevent loading the signal generator. Connect an audio signal
generator to the receiver audio input terminal. Set the generator to apply a
500 Hz sine wave with amplitude 1V P-P. Rotate R50 until the audio over-
load LED D3 illuminates. Then, adjust R50 until low signal level LED D4
lights. Adjust R50 so that LEDs D3 and D4 are off, set calibrate control R36
to the center of its range and adjust damping control R19 for minimum damp-
ing (fully CCW).