Tektronix 422, Instruction Manual

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Circuit Description—Type 422 AC-DC
longer. Since D1155 remains in its high-voltage state longer,
the Steering Switch and Power Control stages conduct fo r a
longer period. This allows more o f the input current to flow
in the primary circuit o f T1201. With an increase in current
in the primary o f T1201, more voltage is induced into its
secondary to compensate for the original decrease in output
level The action is similar but opposite for an increase in
input voltage level.
Isolated Supply. The Isolated Supply provides positive
and negative voltage levels for the operation o f the DC-DC
Regulator circuit. The voltage induced into the secondary
of T 1201 is rectified by center-tapped, full-wave rectifiers
D1242-D1243 and D1244-D1245 to produce positive and
negative outputs of about 12 volts. The output at the
cathodes o f D1242-D1243 is filtered by C l 195 and Cl 199
to maintain a stable +12-volt output level. Filter network
C1245-C1246-L1246 maintains a stable -1 2 -v o lt output
level from the anodes o f D1244 and D1245. The center-tap
of this winding o f T1201 is connected to chassis ground
through C l242 and to the zero-reference voltage level.
Low-Voltage Recitifier/Filter. The output voltage from
the secondary o f T1201 is rectified and filtered to provide
the regulated DC output voltages fo r the indicator. A ll of
the o utput voltages are produced by center-tapped full-
wave rectifiers except the -1 1 0 -v o lt output. Capacitive
filtering can be used to provide stable output voltage be­
cause o f the regulation of the primary voltage applied to
T1201 and also due to the frequency of operation (approx­
imately seven kilohertz). The rectifiers for each voltage and
their associated filte r networks are as follows: +95 volt,
D1202-D1223 and C1202-C1203; +55 volt, D1203-D1222
and C 1 2 0 4 -C 1 2 0 5 -L 1 2 0 4 ; g r a tic u le - lig h t voltage,
D1214-D1215 and C1214; +12 volt, D1216-D1217 and
C 1216-C 1217-C 1218-C 1219-L1217-L1219; - 1 2 volt,
D1212-D1213
and C1210-C1211-C1212-C1213-L1212-
L1213. Diodes D1224 and D1225 along w ith capacitors
Cl 224 and Cl 225 form a voltage doubler network to
provide a -1 1 0 -v o lt output from the same winding that
provides the unrectified voltage fo r the +55-volt output.
High-Voltage Driver. The unrectified secondary voltage
which provides power to the - 1 2 - and +12-volt outputs is
also used for high-voltage driver power. This voltage is con­
nected to the high-voltage transformer in the indicator
through terminals 1—4 and 3—6 o f J1201.
Ground Connections
Capacitors C l 173, Cl 181, C1231 and C1242 provide
short high-frequency current paths between the various
ground planes in this supply and the zero-reference voltage
line. Although these capacitors appear schematically to be
connected directly between tw o areas which are both at
ground potential, this configuration eliminates ground
loops which might otherwise result due to the method o f
construction necessary. Also, the component location on
the circuit board requires that the collector o f Q1163 be
returned across the board in parallel w ith the + 12-volt lead
and connected to the zero-reference voltage level at C l 199.
Resistor R1165 damps ringing at the collector o f Q 1163 by
absorbing reflections along the short-circuited transmission
line formed by the above lead configuration.
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