Tektronix 2445, Instruction Manual

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Theory of Operation—2445 Service
Before any of the Low Voltage Regulators may function
properly, the +10-V reference voltage must be established
as previously described. When the +15 V Regulator turns
on, current flows through Q1370, and pin 2 o f U1371D is
pulled above the +2.5-V reference through divider R1370
and R1372. The output o f U 137ID goes low, turning o ff
Q1376. The PWR UP signal at the collector goes HI,
signaling the Microprocessor that the power supplies should
now be operating properly.
When power to the instrument is turned off, the LINE
UP signal goes LO (as explained in the Line Up Signal
description). The falling LINE UP signal turns Q1370 o ff
and drives the output o f U1371D high. The output level
from U1371D turns on Q1376 and pulls the PWR UP signal
to the Microprocessor LO. This LO initiates the power­
down sequence used to store the current front-panel setup
conditions in EAROM and to shut down the instrument in
an orderly fashion. The delay between the time that the
PWR UP signal goes LO and when the regulated power
supplies fall below their normal operating levels provides
ample time fo r the Microprocessor to complete the power­
down sequence.
Fan Circuit
The fan motor used in this instrument is a brushless,
dc motor that uses Hall-effect devices to control its rotation
speed. The two Hall-effect devices sequentially drive the
four field-control transistors (U1690A, B, C, and D) which
in turn control field current to the fan m otor windings. The
fan's speed is determined by the amount of drive current
supplied by Q1698 and varies w ith ambient temperature.
As the ambient temperature in the cabinet increases,
the resistance of RT1696 decreases, and additional base
drive is provided to Q1698. The transistor conducts harder,
and the fan's motor speed is increased to provide more
cooling capacity.
The back EMF produced by the m otor field windings
is also proportional to m otor speed. This back EMF is
rectified by diodes CR1691, CR1692, CR1694, and
CR1696 and is applied to the base node o f Q1698 via
R1697. This current opposes the normal bias current of
the transistor and acts as a form o f negative feedback to
stablilize the m otor speed from cycle to cycle.
POWER DISTRIBUTION
Schematic diagrams 11 and 12 illustrate the power
distribution of the 2445. The connections to the labeled
boxes (representing the hybrids and ICs) show the power
connections to each device, while connections to non­
power lines are shown by the component and schematic
number. Power supply decoupling is done w ith traditional
LRC networks as shown on the diagrams.
Several intermediate supply voltages are generated by
devices shown on diagrams 11 and 12. An approximate
+32-volt supply for the A and B Sweeps is developed by
emitter-follower Q700 and its associated components.
Zener diodes VR125 and VR225 develop approximate
+6.2-volt supplies for the CH 1 and CH 2 Preamps respec­
tively, and zener diode VR2805 establishes an approximate
—6.8-volt supply fo r U2800 and U2805.
INTERCONNECTIONS
Schematic diagram 13 illustrates the circuit board
interconnections of the 2445. Connector numbers and
cabling types are shown.
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