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7970B/7970C
Theory of Operation
+40V (NOM)
+40V UNREG
-40V (NOM)
-40V UNREG
+20V UNREG
-20V UNREG
TRANSFORMER
+20V (NOM)
ASSEMBLY
~&
+12V REG
-20V (NOM)
~:9
-12V REG
+12V (NOM)
~&
+5V REG
J
/1-
~
+
' " ' "
REF
+5 VOLT
-12 VOLT
+12 VOLT
REGULATOR
REGULATOR
REGULATOR
~
~
7970·509
Figure 2-2. Power Supply Voltage Regulators, Simplified Diagram
and comparing the divided voltage with the internal refer·
ence. The output voltage of the +12 volt regulator is adjust·
able by variable resistor R4. Series pass transistor Q2 (lo-
cated on a heatsink external of the regulator) is protected
by current limiting. The current foldback knee is set to ap-
proximately 2.8 amperes by R1 and R2. Short circuit cur-
rent is set to approximately 1.3 amperes by R7 and R8
(located on heatsink external to the regulator).
2-21.
-12 VOLT REGULATOR. The -12 volt regulator
uses an integrated circuit voltage regulator. The reference
for the -12 volt regulator is derived from the regulated + 12
volt source. Current limiting of the -12 volt regulator is set
at 1.2 amperes as determined by R9 and R20. Diode CR14
protects the -12 volt regulator against excessive common
mode voltage. Diode CR17 limits the output voltage to -14.7
volts in the event that the + 12 volt reference is lost.
2-22.
+5 VOLT REGULATOR. The +5 volt regulator
uses an integrated circuit voltage regulator. The reference
for the +5 volt regulator is generated from the + 12 volt regu-
lator output using R23, R25, and R26. Current limit of the
+5 volt regulator is approximately 4.0 amperes controlled
Part 2
by current sense resistor R6, located external of the regu-
lator on a heatsink. In the event of an over-voltage, silicon
controlled rectifier CR1 conducts and shorts the +5 volt
supply. The 4 ampere short circuit current will blow fuse
F5 which is in series with the +10 volt unregulated supply.
2-23.
CONTROL AND STATUS CIRCUITS.
2-24.
The control and status circuits process commands
from the front panel controls and interface and generate
controlling signals for the tape transport and data circuits.
The control and status circuits also generate status signals
for the interface and front panel indicators.
2-25.
After power is applied, the tape is not ten-
sioned, limit switch W2S1 is open and the capstan and
reel servo circuits are disabled (figure 2-3). The capstan
and reel servo circuits are disabled because relay K1 is
deenergized and because ground voltage is supplied to
the Tape Tension signal line through the B contacts of
deenergized relay Kl. Ground voltage supplied to Q4,
on the control and status PCA, results in an inactive
output from the LDFD FF to the capstan servo circuit.
2-3
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