Dial VR1 to about half turn position and apply power. Measure the voltage at pin 14 of
U4 socket. It should be about 2.5V. If you reduce the resistance of VR1, the oscillator
should run at a higher frequency and higher duty cycle. This will increase the voltage
reading. If the voltage reading is constantly at a certain voltage and does not change
with VR1, the oscillator is not working.
Check VR1, R15, C4 and C5 if the oscillator is not working. Also check the voltage at
U2 pin 4. It should be near 5.0V for the oscillator to work. If it is less than 3.0V, check
the bypass lead. You should get identical reading at pin 14 of U5 socket since the two
pins are connected together.
Test 3: CD4017 counters
In this test, we want to verify the operations of the CD4017 counters U4 and U5.
Disconnect power supply and insert U4 and U5. Do not install U3, U6, U7 and U8.
Install a bypass lead between pin 2 and pin 14 of U3 socket.
Apply power and observe the waveforms at pin 14 and pin 1 of U4 with a two-channel
oscilloscope. You should see a 5Vp-p pulse at pin 1 for every ten clock cycles of pin 14,
as shown in Figure 5. Repeat the measurement for the following signals of the counter
pins: 1, 2, 3, 4, 5, 6, 7, 9, 10, and 11. Pin 12 of U4 is the Carry Out pin. It is at logic high
for the first half of the courting cycle and goes to low at the second half. The test setup
and the observed waveform are shown in Figure 5.
If the counter is not working, check if pin 14 is receiving the proper clock signal. Also
check that pin 13 and pin 15 is near 0V to enable the counter.
Repeat the same test for the corresponding signal pins of U5.
