Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
TRIGGER IN (PIN 2)
CONTROL VOLTAGE “ 6.8 V
START UP OR AFTER CURRENT LIMIT
J ·— NORMAL OPERATION - » |
2 7 2 7 - 1 6 1
Fig. 5*42. Timing waveforms (stylized) for soft-start circuit.
Rectifier-Filter Circuits
Transformer T4071 has three secondary windings: The
first furnishes current to the +300 V and +100 V supplies;
the second furnishes current to the —7 V, +7 V, and +9 V
supplies; and the third furnishes current to the +17 V and
-1 7 V supplies. The regulated supplies ( + 5 V reference,
+5 V, - 5 V, +15 V, and —15 V) derive their current from
the rectifier-filter circuits.
The ac voltage from pins 7 and
8
of J4071 is applied to a
bridge rectifier composed of CR3053, CR3056, CR3055,
and CR3054. The output of this rectifier is filtered, then ap
plied to the remainder of the instrument as the +100 V
supply.
The +300 V supply is derived by stacking a 2X multiplier
on the +100 V supply. CR3052, CR1042, CR1034, and
CR1022 compose this circuit.
and — 7V supplies. The + 7 V supply is only used on the
power supply board; the —7 V supply is used by other cir
cuits in the 492/492P.
The third winding of T4071 is pins 13, 14, and 15, which
furnish current to full-wave bridge rectifier CR5052,
CR5062, CR5065, and CR5055. The output of this rectifier
is also divided to become the +17 V and —17 V supplies.
The —17 V supply is used only on the power supply board;
the +17 V supply is used both on the board and elsewhere
in the 492/492P.
Voltage Reference Supply
The +17 V is fed through R6021 and R6020 to the volt
age divider that feeds zener diode VR6026. The 6.2 V from
the zener diode is divided across R6029, R6028, and
R6023. +5 V REF adjustment R6028, is set for precisely
+ 5 V at TP6027.
The ac voltage from pins 9 and
1
0 supply current to full-
wave rectifier CR4061/CR4062; its output is filtered and
sent to the remainder of the instrument as the +9 V supply.
Two other taps off the same winding (pins 11 and 12) supply
current to the bridge rectifier that consists of CR4063,
CR4057, CR4053, and CR4065. The output divides across
filter capacitors C3051 and C4051, to become the +7 V
Regulator Circuits
Four of the available voltages from the power supply are
regulated: +15 V, -1 5 V, + 5 V, and - 5 V. In function, all
four regulators are the same. The circuit differences are
minimal so only the +5 V regulator is discussed here. Sig
nificant differences are discussed following this description.
REV FEB 1983
5-97
