Tektronix 2440, User Manual

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Theory of Operation— 2440 Service
going to the main board and a small series resistor in the
line (R121 on the Main board) causes the actual output
level from the supply regulator to be closer to - 8 . 4 V.
(This is the voltage actually required by some of the - 8 V
load circuits.) Resistor R388 allows the supply to maintain
regulation with the remote sense line disconnected.
Current limiting of the combined - 8 V and - 8 . 3 V sup­
plies occurs at about 3 amperes.
- 5 V R egulator
Operation of the - 5 V Regulator is similar to that of
the + 5 V Regulator. Current limiting of the - 5 V supply
occurs at about 3.1 amperes.
+ 5 V Inverter F eed b ack
Operational amplifier U189 and associated components
are configured as a frequency-compensated voltage­
sensing network. The circuit monitors the + 5 V digital
power supply line from the rectifiers and provides feedback
to the Preregulator Control 1C (U233) via optoisolator
U155 (both on diagram 22). The feedback is used to trim
the + 5 V D level by controlling the Preregulator. The
FEEDBACK signal slightly varies the voltage to the
Inverter output transformer and holds the output of the
5 V secondary windings at an optimum level. Output levels
of the other secondary windings are related by turns ratio
to the + 5 V D level and are also held at their optimum lev­
els. This technique minimizes power losses in the series-
pass transistors and increases regulator reliability.
Pow er-U p
The Power-Up circuit, composed of U189A, Q295 and
the associated components, provides buffering and level
shifting of the LINE UP signal to the System Processor.
Operational amplifier U189A is configured as a com­
parator referenced to + 1 0 V REF . When adequate power­
line input voltage is available, the LINE UP signal will be
HI. The output of the comparator will be LO, turning off
transistor Q295. This results in a HI PW RUP signal to the
System
n
P, indicating that the power supplies are stable.
When adequate power-line voltage is not available, the
LINE UP signal from the Preregulator circuit goes LO, the
output level of U189A goes HI and turns Q295 on, result­
ing in a LO PWRUP signal to the System
ixP.
This indi­
cates that the various supply voltages may go out of regu­
lation in about 10 ms.
Capacitor C l 95 provides a negative-feedback path for
high-frequency signals and stabilizes operation of U189A.
DC-OK Sense
The output of the DC-OK Sense circuit is checked by
the System Processor after it receives the PWR UP signal
to verify that power supply voltages are within tolerance.
By itself, the resistive summing network made up of
R794, R795, R797, R686, R688 and R796 would produce
a voltage near zero volts if all supplies were within toler­
ance. This voltage may vary ± 0 .1 9 V, depending on slight
variations in the individual supply output levels. The
current in resistor R396 is, however, added into the sum­
ming node and shifts its operating point approximately
0.19 V positive.
The resulting voltage is compared to ground by com­
parator U395B and to + 0 .3 7 V by comparator U395A,
establishing the tolerance window. Both open-collector
outputs of the comparator are off, and the DCOK signal is
HI, as long as the summing-node voltage falls within this
window. Should the summing-node voltage exceed either
limit, the associated comparator turns on its output
transistor and pulls the DCOK signal LO, indicating that at
least one of the power supplies is not operating properly.
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