Tektronix 2445, Instruction Manual

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Theory of Operation—2445 Service
+42-V supply is maintained, preventing component damage
in the load circuitry. Should the +42-V supply be pulled
below +15 V (excessive loading or supply failure), the
voltage at pin 3 of U 1371A falls below the voltage at pin 2
and the amplifier output voltage goes low. This forward
biases CR1262 and lowers the reference voltage fo r U1260,
reducing the output voltage.
Current lim iting fo r the +15-V supply is provided by
the internal circuitry of the three-terminal regulator.
+5 V Regulator
Regulation o f the -+5-V supply is provided by a circuit
similar to those of the -+87 V and the +42 V Regulators.
As long as the relative polarity between the +-15-V and the
+-5-V supplies is maintained, base drive to Q1281 is supplied
through R1283. The current through Q1281 provides base
drive fo r series-pass transistor Q1280.
When voltage-sense amplifier U1300B detects that the
output voltage has reached +5 V, it begins shunting base-
drive current away from Q1281 via CR1281 and holds the
output voltage constant.
Current lim iting for the +5-V supply is done by U1300A
and associated components. Under normal current-demand
conditions, the output of U1300A is high and diode
CR1282 is reverse biased. However, should the current
through current-sense resistor R1281 reach approximately
2 A, the voltage developed across R1281 w ill raise the
voltage at pin 2 o f U1300A (via divider R1282 and R1286)
to a level equal to that at pin 3. This causes the output of
U1300A to go low, forward biasing CR1282. This sinks
base drive current away from Q1281 and lowers the output
current in series-pass transistor Q1280.
—15 V Regulator
Operation of the —15 V Regulator, composed of three-
terminal regulator U1330, operational amplifier U1270C,
and their associated components, is similar to that o f the
+15 V Regulator w ith the following major changes. The
control voltage at the three-terminal regulator's reference
pin (pin 1) is established by the current through series-
resistors R1333 and R1334. The reference pin is clamped
by CR1332 at about —5.6 V should a failure in the sensing
network occur. (Clamping also prevents latchup of the
operational amplifier during startup o f the power supply.)
Finally, the sensing divider formed by R1331 and R1332 is
referenced to the +10-V reference instead of ground to
enable sensing of negative voltage.
—8 V Regulator
Operation of the —8 V Regulator is similar to that of the
+-87 V and +42 V Regulators. Due to the lower operating
voltages o f the —8 V Regulator, the common-base transistor
present in both the +87 V and the +42 V Regulators is
not required. Current lim iting in the —8-V supply occurs
at about 480 mA.
—5 V Regulator
Operation of the —5 V Regulator is similar to that of the
+5 V Regulator. Current lim iting in the —5-V supply occurs
at about 2 A.
+5 V Inverter Feedback
Operational amplifier U 1371C and associated com­
ponents are configured as a frequency-compensated voltage­
sensing network. The circuit monitors the +-5-V digital
power supply line from the rectifiers and provides feed­
back to the Preregulator Control 1C (U1030) via opto-
isolator U1040 (both on diagram 9). The feedback is used
to slightly vary the voltage-sensing characteristics of the
Preregulator Control circuitry. The feedback (FB) signal
slightly varies the voltage to the Inverter output trans­
former and holds the output of the 5-V secondary windings
at an optimum level. Output levels o f the other secondary
windings are related to the +5-VD level and are also held
at their optimum values. This technique minimizes power
losses in the series-pass transistors and increases regulator
reliability.
The Power-Up Delay circuit, composed of Q1370,
Q1376, U 137ID , and the associated components, ensures
that the various regulated power supplies have time to
reach their proper operating voltages before signaling the
Microprocessor that the supplies are up.
When power is first applied, the LINE UP signal from
the Preregulator Control circuit goes HI, indicating that
the power switch has been closed and that ample supply
voltage is available for driving the Inverter transformer.
The HI is applied to the base o f Q1370, but since the
collector is not properly biased yet, no transistor current
w ill flow. As the Inverter begins to run, the various voltages
from the secondary rectifiers begin coming up to their
proper levels. A +2.5-V reference voltage is applied to
operational amplifier U1371D pin 12 and forces the output
high, biasing Q1376 on. The resulting LO at the transistor's
collector signals the Microprocessor that the power supplies
are not yet stable.
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