should stabilize at 0.5VDC. If the voltage value is jumping and difficult to read, you can
put a large capacitor (for example, 1uF) across the two DMM probes to try to get a
sable reading.
Repeat the measurement for the following signals of the counter pins: 1, 2, 3, 4, 5, 6, 7,
9, 10, and 11. For pin 12 (Carry Out) of U4, the voltage reading should be about 2.5V.
If the counter is not working, check that U4 pin 14 is not constantly stuck at 0V or 5V.
Also check that pin 13 and pin 15 are near 0V to enable the counter.
Repeat the same test for the corresponding signal pins of U5.
The sync pulse at U4 pin 15 is too short and cannot be checked with a DMM. The DC
voltage level is near zero when measured with a DMM.
Test 4: Counters and LED drivers ULN2003
This test checks that the counting pulses at U4 and U5 are driving LED0 – LED19
properly.
Disconnect power supply and insert U6, U7, and U8 (ULN2003). U3 is the only IC not
installed in this test. Three bypass leads will be needed on the U3 socket between pin
pairs (2, 14), (5, 13) and (8, 12). The bypass (2, 14) turns on the counter oscillator. The
other two bypasses turn on both Q1 and Q2 so that the light spot of both counters are
visible simultaneously. The test setup is shown in Figure 7.
