Page 34 - Troubleshooting Analog Circuits

Choosing the Right Equipment 21

Figure 2.9. You can use this short-circuit detector to find PC board shorts. Simply slide the test probe
along the various busses and listen for changes in pitch.

15. Access to any engineering or production test equipment, if possible. Use this equip-
ment to be sure that when you fix one part of the circuit, you aren’t adversely
affecting another part. Other pieces of equipment and testers also fall under the cate-
gory of specialized test equipment; their usefulness will depend on your circuit.
Three examples are a short-circuit-detector circuit, an AM transistor radio, and a
grid-dip meter.
A short-circuit-detector circuit. This tool comes in handy when you have to
repair a lot of large PC boards: It can help you find a short circuit between the ground
bus and the power or signal busses. It’s true that a sensitive DVM can also perform
this function, but a short-circuit detector is much faster and more efficient. Also, this
circuit turns itself off and draws no current when the probe is not connected. In the
short-circuit-detector circuit shown in Figure 2.9, the LMlO op amp amplifies the
voltage drop and feeds it to the LM331 voltage-to-frequency converter, which you
set up to emit its highest pitch when Vi“ = 0 mV. When using this circuit, use a 50- to
100-mA current-limited power supply. To calibrate the circuit, first ground the de-
tector’s two probes and trim the OFFSET ADJUST for a high pitch. Then, move the
positive test probe to Vs at A and trim the GAIN ADJUST for a low pitch. Figure 2.9
illustrates a case in which one of the five major power supply busses of the circuit-in-
trouble has a solder short to ground.
To find the exact location of the short, you simply slide the positive input probe
along the busses. In this example, if you slide it from A toward B or D, the pitch won’t
change because there is no change in voltage at these points-no current flowing
along those busses. But, if you slide the probe along the path from A to C or from K
to M, the pitch will change because the voltage drop is changing along those paths.
It’s an easy and natural technique to learn to follow the shifting frequency signals.
An AM radio. What do you do when trouble is everywhere? A typical scenario
starts out like this: You make a minor improvement on a linear circuit, and when you
fire it up you notice a temble oscillation riding on the circuit’s output. You check
everything about the circuit, but the oscillation remains. In fact, the oscillation is
riding on the output, the inputs, on several internal nodes. and even on ground. You