Replacing The Power Module U338 (U339); Front Panel Removal - Agilent Technologies 6622A Service Manual

Before you can remove output 3, located in the main chassis,
you must remove the G PIB board.
a. Disconnect all cables. Note the routing of cables if they
must be removed since cable position may affect the
output's ripple performance.
b. Remove one ground screw located at rear of board near
terminal block. Retain the lock or flat washers for
reassembly.
c. Remove one ground screw located at extreme front of
board. Retain the lock or flat washers for reassembly.
d. Remove two screws which hold the heat sink assembly
to the chassis. These must be removed completely.
e. Slide the board slightly forward to disengage the keyed
standoffs.
f. Lift board out of chassis.
4-8 80 Watt Output boards
Models 6621A and 6622A each have two 80 Watt Output
boards; one mounted in the upper chassis and one in the
lower chassis. Model 6623A has one 80 Watt Output board
which is mounted in the upper chassis assembly. To remove
an 80 Watt Output Board, proceed as follows:
Before you can remove the output board located in the main chassis (output
1), you must remove the PIB board.
G
a. Disconnect all cables. Note the routing of cables if they
must be removed since cable position may affect the
supply's ripple performance.
b. Remove two screws on rear of board.
c. Remove two screws located near the front of the board.
Retain the lock or flat washers for reassembly.
d. Remove four screws which hold the heat sink assembly
to the chassis. These screws must be removed
completely.
If an insulator is present beneath the heat sink assembly, be sure to align
all four holes in the insulator with the heat sink holes when replacing the
four screws.
e. Slide the board forward slightly to disengage the keyed
standoffs.
f. Lift the board out of the chassis.
4-9

Replacing the Power Module U338 (U339)

Use the following procedure when it has been determined
that the power module (U338 on 40 W boards; U338 and
U339 on 80 W boards) is defective and must be replaced:
NOTE
NOTE
NOTE
40 Watt Output Boards
a. Remove the output board as described in paragraph 4-
7.
b. Carefully unsolder all 14 power module pins.
c. Remove the power module and heat sink from the
board.
d. Remove four power module screws and retain all
hardware.
e. Wipe the heat sink surface to remove the old layer of
heat sink compound. Apply a thin layer of new
compound to the back surface of the replacement
power module.
f. Reassemble using the reverse of the above procedure.
Before soldering the new power module/heat sink assembly to
the board, be sure that the heat sink lies flat and securely
against the board. Use the long heat sink screws and M4
nuts to temporarily hold the assembly to the board while
soldering.
80 Watt Output Boards
a. Remove the output board as described in paragraph 4-
8.
b. Use the same procedure outlined above but note that
for 80 Watt boards, U338 and U339 are a matched pair.
Therefore, both power modules must be replaced if
either module fails.
c. Note that if an insulator is present under the heat sink
assembly, be sure to align it properly as outlined in
paragraph 4-8.

4-10 Front Panel Removal

The front panel contains the keypad assembly, LCD display
assembly, and the line switch. You must remove the front
panel in order to gain access to these components. The LCD
display and line switch are replaceable components. On
some units you may have to replace the front panel if the
keypad is defective. To remove the front panel assembly,
proceed as follows:
a. Disconnect the keypad and display ribbon cables from
P202 and P203 on the GPIB board. Ensure that these
cables are not pinched between the front panel and the
chassis when reassembling.
b. Remove the grounding screw nut located behind the
front panel (upper right corner or below display). This
nut is accessible through cut outs in the chassis.
Do not overtighten the grounding screw nut when
replacing the front panel.
4-4