DEC DECtape 551 Manual page 71

open circuit in the base-emitter path. A good transistor indicates an open circuit in both di-
rections between collector and emitter. Normally 50 to 100 ohms exists between the emitter
and base or between the collector and the base in the forward direction, and open-circuit con-
ditions exist in the reverse direction. To determine forward and reverse directions, a transistor
can be considered as two diodes connected back-to-back. In this analogy, PNP transistors are
considered to have both cathodes connected together to form the base, and both the emitter and
collector assume the function of an anode. In NPN transistors, the base is assumed to be a
common-anode connection, and both the emitter and collector assume the function of a cathode.
Multimeter polarity must be checked before measuring resistances since many meters (including
the Triplett 630) apply a positive voltage to the common lead in the resistance mode. Note
that although incorrect resistance readings are a sure indication that a transistor is defective,
correct readings provide no guarantee that the transistor is functioning properly. A more rei iable
indication of diode or transistor malfunction is obtained by using one of the many inexpensive
in-circuit testers commercially available.
Damaged or cold-solder connections can also be located using the multimeter. Set the multi-
meter to the lowest resistance range, and connect it across the suspected connection. Probe
the wires or components around the connection, or rap the module lightly on a wooden surface,
and observe the multimeter for open-circuit indications.
Often the response time of the multimeter is too slow to detect the rapid transients produced by
intermittent connections. Current interruptions of very short durations, caused by an intermittent
connection, can be detected by connecting a 1.5-volt flashlight battery in series with a 1500-
ohm resistor across the suspected connection. Observe the voltage across the 1500-ohm resistor
with an oscilloscope while probing the connection.
Dynam ic Bench Tests
Dynamic bench testing of modules can be performed through the use of special equipment. A
Type 922 Test Power Cable and either a Type 722 or Type 765 Power Supply can be used to
energize a system module. These supplies provide both the +10-vdc and -15-vdc operating
le~els
for the module in addition to ground and - 3-volt levels which may be used to simulate
signal inputs. The signal input potentials can be connected to any terminal normally wired to
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