Tektronix 485, Instruction Manual

Page: 23 / 197

Circuit Description—4 8 5 /R 4 8 5 Service
TO
p in
3
U1910
TP1918
LS
O
I---------------------- 1
| JO OFF |
O ON
I______ ---------- 1
r t \
(A)
Fig. 3-4. (A) Schematic showing auxiliary switch for disabling line
stop circuit, (B) power plug polarity for DC operation.
+59.4 V and +120 V Supplies
The 59.4 and 120 V supplies are similar to the 15 V
supplies except for the output connection, which ties the
negative rectified voltage to ground. This elevates the
transformer center tap to the peak AC voltage on one side
of the winding, which is 59.4 V. The 120 V supply is
generated by the full rectified voltage on the transformer.
+180 V Supply
The 180 V supply is formed by a voltage doubler which
rectifies the 59.4 V peak AC swing and adds it on the
120 V supply.
+25 V Supply
The 25 V supply is formed by a voltage doubler, which
rectifies the 12.4 V AC peak-to-peak swing on the 5.5 V
winding and adds it on to the 15 V supply.
+50 V Supply
The 50 V supply is regulated from 59.4 V by Q2046. A
temperature-compensated reference is provided by
VR2042. The reference voltage is compared to a sample of
the 50 V output at U2042 which acts as an inverting
amplifier. If the +50 V goes negative, the output of U2042
goes positive. CR2042 is short circuit protection for
U2042.
+9 V Supply
The 9 V supply is re-regulated from the +15 V supply by
Q2056. A 9 V reference is provided by a divider from the
+50 V supply. U2052 is an inverting amplifier which drives
Q2056 directly.
+5 V Supply
The +5 V supply is essentially identical to the +9 V
supply except for the value of the output voltage.
—9 V and —5 V Supplies
The —9 V and —5 V supplies are essentially identical to
+9 V and +5 V supplies except that the driving amplifiers
are ground referenced.
POWER SUPPLY PRO TECTIO N C IR C U IT
The power supply protection circuit provides fault
protection for the inverter, low voltage, and high voltage
power circuits. Fig. 3-5 shows a simplified block diagram of
the protection circuit. A schematic of this circuit is shown
on diagram 14 at the rear of this manual.
Power supply protection is accomplished by operating
the Inverter in a pulse mode. When a fault is present, the
Inverter will come on for a short period of time, then shut
o ff for a longer period of time. The cycle repeats until the
fault is removed. Approximate timing of the duty cycle is
shown in Fig. 3-6.
Stop Monostable
Q1900 and Q1902 form a monostable multivibrator
which acts to stop the Inverter when a fault is detected by
U1910. During the start period T1831 supplies current to
charge C1901 and C1904 thru CR1901 and CR1904. At
the same time T1848 supplies power to U1910. As U1910
becomes active, pin 8 acts as a current sink, holding Q1902
off. Pin 8 of U1910 will remain at ground during the start
period and sampling period. If a fault exists at the end of
the sampling period, pin 8 of U1910 stops conducting,
allowing Q1901 and Q1902 to turn on. When these
transistors turn on, the voltage on all secondaries of T1831
will go to zero, stopping the Inverter. Once triggered on,
the stop monostable will stay on while Cl 901 discharges
through R1902 into the base of Q1902. If U1910 is
removed from its socket or is otherwise nonfunctional, the
stop monostable will stop the Inverter after the first few
cycles of operation. In this mode, the duty cycle will be
much shorter because Cl 901 will not have sufficient time
to charge.
3-10