TM 9-4935-601-14-7&P
Model 8640B
8-32. If component was unsoldered, remove solder
from mounting holes, and position component as
original was positioned. DO NOT FORCE LEADS INTO
MOUNTING HOLES; sharp lead ends may damage
plated-through conductor.
8-33. Transistor Replacement. Transistors are
packaged in many physical forms. This sometimes
results in confusion as to which lead is the collector,
which is the emitter, and which is the base. Figure 8-2
shows typical epoxy and metal case transistors and the
means of identifying the leads.
8-34. To replace a transistor, proceed as follows:
a. Do not apply excessive heat; see 'able 8-1 for
recommended soldering tools.
b. If possible, use long-nose pliers between
transistor and hot soldering tools.
c. 1When installing replacement transistor, ensure
sufficient lead length to dissipate soldering heat by using
about the same length of exposed lead as used for
original transistor.
d. Integrated circuit replacement instructions are
the same as those for transistors.
8-35. Some transistors are mounted on heat sinks for
good heat dissipation. This requires good thermal
contact with mounting surfaces. To assure good
thermal contact for a replacement transistor, coat both
sides of the insulator with Dow Corning No. 5 silicone
compound or equivalent before fastening the transistor
to the chassis. Dow Corning No. 5 compound is
available in 8 oz. tubes from Hewlett-Packard; order HP
Part No. 8500-0059.
8-36. Diode Replacement. Solid state diodes have
many different physical forms. This sometimes results
in confusion as to which lead is the anode (positive),
since not all diodes are marked with the standard
symbols. Figure 8-2 shows examples of some diode
marking methods. If doubt exists as to polarity, an
ohmmeter may be used to determine the proper
connection. It is necessary to know the polarity of the
ohms lead with respect to the common lead for the
ohmmeter used. (For the HP Model 410B Vacuum
Tube Voltmeter, the ohms lead is negative with respect
to the common; for the HP Model ,412A DC Vacuum
Tube Voltmeter,
the ohms lead is positive with respect to the common).
When the ohmmeter indicates the least diode
resistance, the cathode of the diode is connected to the
ohmmeter lead which is negative with respect to the
other lead.
NOTE
Replacement instructions are the
same as those listed for transistor
replacement.
8-37. Illustrated Parts Breakdowns
8-38. Illustrated parts breakdowns for the generator's
major assemblies are given on Service Sheets A
through E. They are keyed to disassembly and
removed instructions (given on the alphabetical service
sheets) and to the replaceable parts list given in Section
VI.
8-39. BASIC CIRCUIT THEORY
8-40. Binary Circuits and Symbols
8-41. Introduction. The binary circuits and symbols
used in this manual are as shown in Figure 8-3. This
instrument uses three different families of logic circuits:
TTL, ECL, and EECL. Most of the logic devices used in
this instrument are TTL; there are notes on the Service
Sheets that contain non-TTL devices that indicate what
families the devices belong to. Table 8-2 indicates the
voltage levels that are associated with each family. The
table also shows the effect that an open and a ground
has on each family.
8-42. In general, binary signals that are active-low are
indicated with an L in parenthesis (e.g., CLOCK(L)
indicates a clock signal that is active-low). Active-high
signals are indicated with an H in parenthesis. A circle
at an input indicates that it is active-low or triggers on a
low-going edge; a circle at an output indicates inversion
or that the output is active-low. Active-high inputs,
inputs which trigger on a high-going edge, and active-
high ouputs are shown without the circle.
Complementary outputs are usually designated with a
not-bar (e.g., the complement of J/K flip-flop's Q output
is its Q output). Both Q and Q may be simultaneously
high in some instances (e.g., when both SET and
CLEAR are low on some D flip-flops).
8-5
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