Isolating Trouble In Transistor; Replacement And Repair - HP 608E Operating And Service Manual

Model 608E/F
5-53. Numerous test points have been estabHshed in
the signal generators and are Identified by a circle-
star designator on Figures 7-
4"
and 7- 5. The dc volt-
ages listed with the test-point designators were ob-
tained with the instrument In an INT 1000 - mode
of operation.
5-54. ISOLATING TROUBLE
IN
TRANSISTOR
CmCUITs.
5-55. The following procedures and data are given to
aid indeterminingwhether a transistor is operational.
Tests are given for both in-circuit and out-of-clrcuit
transistors.
5-56. IN-CIRCUIT TESTING. The common causes
of transistor failures are internal short- and open-
circuits. In transistor circuit testing the most Im-
portant consideration Is the transistor base-emitter
junction. Llkethe control grid of a vacuum tube, this
is
the operational control point in the transistor.
This junction
is
essentially a solid-state diode. For
the transistor to conduct, the diooe must conduct; that
Is, the diode must be forward biased. As with simple
diodes, the forward-bias polarity is determined by the
materials forming the function. Use the transistor
symbol on the schematic diagram to determine the
bias polarity required to forward-bias the base-emitter
junction. The A part of Figure 5-22 shows transistor
symbols with terminals labeled. Notice that the emit-
ter arrow conventionally points toward the type N ma-
terial. The other two columns of the illustration com-
parethebiasing required to cause conduction ana cut-
off in transistors and vacuum tubes.
If
the transistor
base-emitter diode (junction) is forward-biased the
transistor conducts.
If
the diode is heaVily forward-
biased, the transistor saturates. However,
if
the
base-emitter diode is reverse-biased the transistor
is cut-off. The voltage drop across a forward biased
emitter-base diode varies with transistor collector
current. For example, a germanium transistor has
a typical forward-bias, base-emitter voltage of O. 2 -
0.3 volt when collector current is I - 10 rnA, and 0.4-
0.5 volt when collector current is 10 - 100 rnA. In
contrast, forward bias voltage for silicon transistors
is about twice that for germanium types: about O. 5 -
0.6 volt when collector current is low, and about Q.8 -
0.9 volt when collector current is high.
5-57. Figure 5-22, part B, shows simplified versions
of the three basic transistor circuits and gives the
operating characteristics of each. When examining a
transistor stage, first determine if the emitter-base
diode is biased for conduction (forward-biased) by
measuring the voltage difference between emitter and
base. When using an electronic voltmeter, do not
measure directly between emitter and base: there may
be sufficient loop current between the voltmeter leads
to damage the transistor. Instead, measure to a com-
mon point (e. g., chassis).
If
the emitter-base diode
is forward-biased, check for amplifier action by
Section V
Short-circuiting base to emitter while observing col-
lector voltage. The short-circuit eliminates base-
emitter bias and should cause the transistor to stop
conducting (cut off). Collector voltage should then
shift to near the supply voltage. Any difference is
due to leakage current through the transistor and, in
general, the smaller this current, the better the
transistor.
If
collector voltage does not change, the
transistor may have either an emitter-collector short
circuit of emitter-base open circuit.
5-5&. TESTING TRANSISTORS WITH AN OHMME-
TER. The two common causes of transistor failure are
internal short- and open-circuits. Remove thetransis-
tor from the circuit and
USi!
an ohmmeter to measure
Internal resistance. See Table 5-3 for measurement
data.
CAUTION
Most ohmmeters can supply enough current
or voltage to damage a transistor. Before
using an ohmmeter to measure transistor
forward or reverse resistance, check
open-circuit voltage and short-circuit cur-
rent output ONTHE RANGE TO BE USED.
Open-circuit voltage must not exceed 1. 5
volts and short-circuit current must be
less than 3 rna. See Table 5-4 for safe
resistance ranges for some common ohm-
meters.
5-59.
REPLACEMENT AND REPAIR.
5-60. The following procedures contain instructions
for work on etched circuit boards, replacement of
electron tubeS, atlenuator probe replacement and re-
pair, replacement of Lamp 081, and diagrams for
locating parts in the signal generator.
5-61. ETCHED cmCUIT BOARD REPAm.
5-62. The etched circuit boards in the Signal Genera-
torareof the plated-through type consisting of metal-
lic conductors bonded to both sides of insulating ma-
terial. The metall1c conductors are extended through
the component mounting holes by a plating process.
Soldering can be done from either side of the board
with equally good results. Table 5-5 lists recom-
mended tools and materials. Following are recom-
mendations and precautions pertinenttoetched circuit
repair work.
a. Avoid unnecessary component substitution; it
can result in damage to the circuit board and/or adja-
cent componentS.
b. Do not use a high-power soldering iron on etched
circuit boards. Excessive heat may lift a conductor
or damage the board.
5-23