Tektronix 422, Instruction Manual

Page: 79 / 213

Maintenance—Type 422 AC-DC
boards can be replaced w ith o u t removing the boards from
the instrument. Observe the soldering precautions given
under Soldering Techniques in this section. However, if the
bottom side of the board must be reached or if the board
must be moved to gain access to other areas of the instru­
ment, only the mounting screws need to be removed. The
interconnecting wires on most of the boards are long
enough to allow the board to be moved out of the way or
turned over w ithout disconnecting the pin connectors.
GENERAL:
Most of the connections to the circuit boards in the
indicator are made w ith pin connectors. However, some
connections are soldered to the board. Also, the connec­
tions between the Attenuator and the Input Am plifier
boards are soldered. See the special removal instructions fo r
the Input Am plifier boards.
Use the following procedure to remove a circuit board:
1. Disconnect all pin connectors from the board and
unsolder any soldered connections.
2. Remove all screws holding the board to the chassis.
3. L ift the circuit board out o f the instrument Do not
force or bend the board.
4. To replace the board, reverse the order of removal.
Correct location of the pin connectors is shown in Figs. 4-8
through 4-17. Replace the pin connectors carefully so they
mat e correctly w ith the pins. If forced into place
incorrectly positioned the pin connectors may be damaged.
INPUT AM PLIFIER U N IT REMOVAL:
The Input Am plifier boards and the attenuators can be
removed from the Type 422 as a unit, or the circuit boards
can be removed separately. To remove the board only, fo l­
low the procedure described under GENERAL. To remove
as a unit, proceed as follows:
1. Unsolder the wire from the Input Coupling switch
and the ground lead from the BNC connector.
2. Remove the front-panel VO LTS /D IV and V A R I­
ABLE knobs.
3. Remove the securing nuts on the V O LTS/D IV switch
and the GAIN control.
4. Remove the tw o screws located at the rear o f the
Attenuator which secure the assembly to the chassis.
5. Disconnect all pin connectors on the board. Do not
unsolder the soldered connections.
6. L ift up the rear o f the assembly and slide it out of
the instrument. The support rod on the lower left side of
the instrument must be removed to allow clearance fo r the
Channel 1 Attenuator. A nutdriver or socket wrench should
be used to remove this rod, as an open-end wrench may
damage the rod and prevent the power supply from being
secured to the indicator properly.
7. The Input A m plifier board can now be removed from
the assembly as follows:
a. Unsolder the remaining connections between the
Attenuator and the circuit board.
b. Remove the three screws which hold the circuit
board to the Attenuator.
8. To replace the unit, reverse the order o f removal.
When replacing the Channel 2 Attenuator assembly, be sure
the INVERT and X I 0 GAIN AC switches f i t properly in
the actuating assemblies.
Transistor Replacement. Transistors should not be
replaced unless actually defective. If removed from their
sockets during routine maintenance, return them to their
original sockets. Unnecessary replacement of transistors
may affect the calibration o f this instrument. When transis­
tors are replaced, check the operation of that part o f the
instrument which may be affected.
CAUTION
POWER switch must be turned off before removing
or replacing transistors.
Replacement transistors should be o f the original type or
a direct replacement. Fig. 4-3 shows the lead configuration
of the transistors used in this instrument. Some plastic case
transistors have lead configurations which do not agree w ith
those shown here. If a transistor is replaced by a transistor
which is made by a different manufacturer than the origi­
nal, check the manufacturer's basing diagram fo r correct
basing. A ll transistor sockets in this instrument are wired
fo r the basing used fo r metal-case transistors. Transistors
which have heat radiators or are mounted on the chassis use
silicone grease to increase heat transfer. Replace the silicone
grease when replacing these transistors.
WARNING
Handle silicone grease with care. Avoid getting sili­
cone grease in the eyes. Wash hands thoroughly after
use.
C ath o de-R ay Tube Replacement. Use care when
handling a CRT. Protective clothing and safety glasses
should be worn. Avoid striking it on any object which
might cause it to crack or implode. When storing a CRT,
place it face down on a smooth surface w ith a protective
cover or soft mat under the faceplate to protect it from
scratches.
The CRT shield should also be handled carefully. This
shield protects the CRT display from distortion due to
magnetic interference. If the shield is dropped or struck
sharply, it may lose its shielding ability.
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