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3.
Remove fuse
Fl.
(Remove shield for access on Mode!
2
display stations.)
4. Connect a meter lead on each end of fuse holder.
5. Tum power on.
6.
Check for a meter indication of 6.3V ac.
7. Turn power off, and remove meter leads from fuse
holder.
8. Reinstall fuse and shield (if removed).
5.1.3.5
High-Voltage Power Supply Check
1. Turn power off.
2. Remove HV power supply leads 3 (UV) and 4 (DC
retu rn). See Diagram 7 -14.
3. Set CE VOM selector
switch
to RX10
position.
4. Measure resistance between HV power supply pins 3 and
4.
5. Reverse meter leads and measure again.
6. Replace HV power supply (par. 5.3.2.10) if readings in
step 4 or 5 indicate a shorted or open condition.
5.1.4 Keyboard Checks
Two different style keyboards may be found on IBM 3277
Display Stations. Both are identical in operation and vary
only slightly in external appearance. The rows of keys on
Type
I I
A" keyboards have an even slope downward from
top to bottom. The rows of keys on Type "B" keyboards
are slightly concave. The two types can be distinguished
with the keyboard cover removed by observing the location
and type of cable connection and the location of the +5V
regulator (large transistor on heat sink). The Type A
keyboard cable plugs into the rear edge of the printed
circuit board, and the regulator is located in the left corner
of the circuit board. The Type B keyboard cable plugs into
a 24-pin SL T connector which is mounted in the left corner
of the circuit board, and the regulator is located in the right
corner of the circu it board.
The following paragraphs contain check procedures
which can
be
used to isolate keyboard problems. When the
check procedure differs for the different keyboard types (A
and B), the differences are noted. Ensure that the correct
procedure is used. Keyboard problems can be isolated to
one of three possible areas:
1. External to the keyboard.
2. A FRU in the keyboard.
3. The keyboard assembly. (Type A assembly consists of
circuit board with key modules; Type B assembly
consists of circuit board and electronics assembly with
key modules in an all-keys assembly.)
Problems external to the keyboard should be diagnosed as
outlined in the Troubleshooting Diagrams. Keyboard FRUs
I
include keybuttons, switch modules, spacebar assembly,
support modules (Type A only). circuit board and elec-
tronics assemblv (Type B only), and the audible response
device. Do not attempt to repair any other keyboard units.
While performing the following checks, the keyboard
assembly' should be visually inspected for shorts or grounds
that could cause erratic operation or erroneous results.
A faulty keyboard assembly should be replaced with a
good one. A faulty Type A assembly should be returned to
the factory if it is less than one year old. The date code is
marked on Type A keyboard assemblies by week (01-52)
and year.
5.1.4.1
Voltages
Type A Keyboards (See Diagrams 7-7 and 7-8 for Type A
keyboard locations):
1. Check for -12V dc between KB 1 pins Z(-) and X
(ground).
2. Check for +8V dc between KB1 pins V(+) and X
(ground).
3. Check for +5V dc across keyboard capacitor C1. This
voltage is developed (by the voltage regulator on the
keyboard assembly) from the display station +8V dc
power supply. Replace the keyboard assembly if the
+8V dc is correct and the +5V dc is not correct.
Type
8
Keyboards (See Diagram 7-19 for Type B keyboard
locations) :
1. Check for -12V dc between KB 1 pin 002 (-) and 008
(ground).
2. Check for +8V dc on one of the voltage regulator
terminals (under the circuit board and electronics
assembly) and +5V dc on the other terminal. Reference
the meter negative (-) lead to test pin 008. Input to the
regulator is +8V dc, and the output is +5V dc. Replace
circuit board and electronics assembly if the +8V dc is
correct and the +5V dc is incorrect. Check KB 1
connector and display station power supply if +8V and
+5V are incorrect.
5.1.4.2 Key Module and Encoding - Type A Keyboards
Only (Diagram 6-11)
Correctly functioning key modules present two active
(2.5V dc minimum) input lines to the keyboard assembly
encoder. With two and only two active inputs present, the
encoder generates the strobe signal and the encoded
character output. This procedure checks that the strobe
s;gnal is generated properly, and the key modules activate
only two inputs to the encoder. Replace the key module if
it is found to be defective. Replace the keyboard assembly
if the encoder or the printed circuit board is defective.
Strobe check:
1. Set selector switch on vol t-ohmmeter to a position that
will accurately measure 5V dc.
2. Place minus (-) meter lead on KB 1 tab X (ground).
3. Place plus (+) meter lead on KB 1 tab R (strobe signal).
Checks, Adjustments. and Removals
5,),)
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