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segment digital display character 8. Internal blanking sig-
nals form the plus or minus sign and the decimal point. To
form each of the digital display characters 0 through 9,
the
Character Generator converts
the
serial BCD data from pins
10, 11, 12, and 13 of U260 into Z-Axis (or blanking) Out-
put signals. The Z-Axis signals blank one or more of
the
segments of the character 8
to
form the other characters.
The current
through
0278, which is gated by 0275 and
set by S120 contacts 5 and 6, causes the decimal point to
be positioned properly on the crt.
Table 9-2 shows the
relationship
between one displayed
character and the BCD output of U260. The BCD data
consists of four groups of BCD data (one group for each
character); therefore, this relationship can be measured
(with a voltmeter) only when one of the four characters
is the number to be measured and the other three charac-
ters are O's.
Figure 3-5 shows the relationship between one displayed
character and the Z-Axis Output from U270. This output,
like the BCD data, is repeated four times (once for each
character) and can be measured (with an oscilloscope) only
when one of the four characters is the number to be mea-
sured and the other three characters are O's.
Polarity Sign Control
The voltage at pin 14 of U270 controls the sign on the dis-
play. Approximately 0.7 volts negative causes a positive
sign, while about 0.7 volts positive causes a negative sign.
A positive 6 volts will turn the sign off.
In therms mode of operation when RMS pushbutton S185
is selected, a positive 6.5 volts is applied through R267 and
turns the sign off.
In the ohms mode of operation, S160 applies a current to
the base of 0182 which causes it to saturate and apply a
positive 6.5 volts through R267, again turning the sign off.
In the Gm Converter a positive input causes
a
negative 0.7
volts to appear at pin 6 of U 186, which causes a positive
sign.
A negative input causes a positive 0.7 volts
to appear
at pin 6, which causes a negative
sign.
Overrange Circuitry
For the following description, refer to the Overrange
Cir-
cuitry
Simplified Schematic Diagram,
Fig.
3-6,
and the
Overrange Timing Diagram, Fig.
3-7.
The overrange indication, that appears on the readout dis-
play as scrambled character segments or 8's, is caused by a
negative unblanking signal at the
input
to the Z-Axis Am-
plifier 0390, which prevents blanking of the unused char-
@
Theory of Operation-213 Service
DISPLAYED
CHARACTER
2
3
4
5
Z-AXIS
OUTPUT
I
I
6
________ h . . l ___ _
7
8
I
I
I
9
n
I
I
I
I
I
0
n
I
t
+
REFERENCE
REFERENCE
(BLANKING
(BLANKING
SPIKE
SPIKE
BETWEEN
BETWEEN
CHARACTERS)
CHARACTERS)
j
..
7
DIVISIONS
. .
,
HORIZONTAL
DEFLECTION
Set INPUT COUPLING to GND for O character verification
(display is all zeros). For pattern verification, sync test
oscilloscope on pin 2, U270 and insure all characters except
verification character
is
a 0.
1481-31
Fig. 3-5. Character Generator 2-axis output.
3-7
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