HP 8901B

Model 8901B
Service
A1 Keyboard and Display Assembly Removal
1. Remove the front-panel assembly from the instrument (refer to Front-Panel Assembly Removal
Procedure).
2. To separate the A1 Keyboard and Display Assembly from the front-dress panel MP14 and sub-
panel MP15,
first remove the Keyboard and Display insulator MP48 by unscrewing the four
pan-head screws, and removing the washers
and spacers which hold it in place.
3. Remove the six remaining screws and washers which fasten the A1 assembly to the sub-panel.
4. Disconnect the front panel LINE switch S3 jumpers at the A1 assembly.
5. Separate the A1 assembly from the front-dress panel and sub-panel.
Front-Dress Panel and Display Window Removal
1. Remove the front-panel assembly from the instrument (refer to Front-Panel Assembly Removal
Procedure).
2. To remove the front display window MP40, remove the three retaining clips and slide the screen
straight up (towards the top edge of the front sub-panel).
3. To remove the front-dress panel MP14, remove the front-panel jacks (if present) and slide the
dress panel downward (toward the bottom edge of the front panel). The bottom edge of the front
dress panel will have to be pulled out slightly to allow for clearance of the LINE switch.
REPLACEMENT OF PUSHBUTTON SWITCHES AND ANNUNCIATOR LEDs
Key Cap Replacement
1. To replace a front-panel pushbutton key cap, pull it off and snap on a new one. You will have to
either remove the Keyboard from the Front Panel Assembly (refer
to Front Panel Disassembly
Procedure)
or carefully use a pair of pliers to remove the keycap.
NOTE
Watch the angular position of the key cap as you snap it in place, since
eight different positions for installation are possible.
Key Cap Led Replacement
Many of the front-panel pushbutton key caps have molded-in clear lenses which are illuminated by
miniature LEDs located in the center portion of the switch at the circuit board. During production of
the instrument, the LEDs are first soldered in place and then the switch
is slid down around them and
heat staked in place. If replacement of the LED becomes necessary (due to burnout),
it can be replaced
without having to remove the switch.
To replace a key cap LED, use the following procedure:
1. Remove the pushbutton key cap (refer to Key Cap Replacement Procedure).
2. Place the Modulation Analyzer on a table top. Lower the front panel so that it is facing downward
(refer to the Fkont-Panel Disassembly Procedure). Unsolder the LED leads on the circuit side of
the printed circuit board as you pull the LED down through the middle
of the switch stem with
a pair of small tweezers.
3. Insert a new LED (one with long leads). Make sure the polarity is right. Pull the leads through
the circuit board and solder.
4. Clip off the excess LED lead length on the circuit side of the keyboard.
5. Put the front panel in place. Snap on the key cap. With the instrument power on, test the switch
function to make sure that the LED works.
8C-2 Disassembly Procedures