Section VIII
TM 11-6625-2781-14&P-2
transistor operation and some transistor circuits are
8-7. TEST EQUIPMENT AND ACCESSORIES
shown in the following pages. Also included is basic
REQUIRED
information concerning the operation of Silicon
Controlled Rectifiers, Zener Diodes, and Varactors.
8-8. Test equipment and accessory requirements are
listed in the System Test and Troubleshooting
8-17. Transistor In-Circuit Testing . The common
Procedure, the indiv idual Serv ice Sheets, and in the test
causes of transistor failure are internal short circuits and
Equipments and Accessories list, Table 8-1. test
open circuits. In transistor circuit testing, the most
instruments other than those listed may be used if their
important consideration is the transistor base-to-emitter
performance equals or exceeds that of the equipment
junction. The base emitter junction in a transistor is
listed.
comparable to the control grid-cathode relationship in a
vacuum tube. The base emitter junction is essentially a
8-9. Circuit Board Extender. A circuit board extender
solid-state diode; for the transistor to conduct, this diode
is supplied with the HP 11592A Serv ice Kit. The
must be forward biased. As with simple diodes, the
extender board may be used to extend the A5 Voltage
forward-bias polarity is determined by the materials
Control assembly in the 8553B, clear of the housing
forming the junction. Transistor symbols on schematic
(see Figure 8-1) to prov ide easy access to test points
diagrams reveal the bias polarity required to forward-
and components. The rest of the assemblies in the
bias the base-emitter junction. The B part of Figure 8-3
8553B may be removed and reinstalled in an inverted
shows transistor symbols with the terminals labeled.
position by using the fasteners prov ided in the HP
The other two columns compare the biasing required to
11592A Serv ice Kit (see Figure 8-2) to prov ide access
cause conduction and cut-off in NPN and PNP
to test points and components.
transistors. If the transistor base-emitter junction is
forward biased, the transistor conducts. However, if the
8-10. ADJUSTMENTS
base-emitter junction is reverse-biased, the transistor is
cut off (open). The voltage drop across a forward-
8-11. The procedures contained in these sections do
biased, emitter-base junction varies with transistor
not include calibration or adjustment. Serv ice Sheets
collector current. For example, forward-bias voltage for
which contain adjustable components refer procedures
silicon transistors is about 0.5--0.6 volt when collector
in the Performance and Adjustment sections which
current is low, and about 0.8-0.9 volt when collector
should be performed after repairs are accomplished.
current is high.
8-12. GENERAL PROCEDURES
the three basic transistor 'circuits and gives the
8-13. The troubleshooting procedure is div ided into two
characteristics of each. When examining a transistor
maintenance levels.
The first, System Test and
stage, first determine if the emitter-base junction is
Troubleshooting Procedure, is designed to quickly
biased for conduction (forward-biased) by measuring the
isolate the cause of a malfunction to a circuit or
voltage difference between emitter and base. When
assembly. The second prov ides circuit analysis and test
using an electronic voltmeter, do not measure directly
procedures to aid in isolating faults to a defective
between emitter and base; there may be sufficient loop
component. Circuit descriptions and test procedures for
current between the voltmeter leads to damage the
the second maintenance level are located on the page
transistor. Instead, measure each voltage separately
facing the schematic diagram of the circuit to be
with respect to a common point (e.g., chassis). If the
repaired.
emitter-base junction is forward-biased, check for
amplifier action by short-circuiting base to emitter while
8-14. After the cause of a malfunction has been located
observ ing collector voltage. The short circuit eliminates
and remedied in any circuit containing adjustable
base-emitter bias and should cause the transistor to stop
components, the applicable procedure specified in the
conducting (cut off). Collector voltage should then
Performance and Adjustment action should be
change and approach the supply voltage.
Any
performed.
difference is due to leakage current through the
transistor and, in general, the smaller this current, the
8-15. GENERAL SERVICE INFORMATION
better the transistor. If the collector voltage does not
change, the transistor has either an emitter-collector
8-16. Transistors and diodes are used throughout he
short circuit or emitter-base open circuit.
Spectrum Analyzer in circuit configurations such as flip-
flops,
multiv ibrators, trigger circuits, switches,
oscillators, and various types of amplifiers. Basic
8-3
