Heathkit SB-1000 - Troubleshooting Guide

Page 68
Heathkit
CIRCUIT DESCRIPTION
Refer to the Schematic (Illustration Booklet, Page
70) while you read the following paragraphs. The
component numbers are arranged in the following
groups to help you locate specific parts on the
Schematic, circuit boards, and chassis:
1-99 Parts mounted on the chassis.
101—199 Parts mounted on the ALC/power
circuit board.
201—299 Parts mounted on the meter circuit
board.
301—399 Parts mounted on the rectifier
circuit board.
401—499 Parts mounted on the filter circuit
board.
Power Supply
Power transformer T1 supplies the power required to
operate the Linear Amplifier. A dual-winding pri-
mary allows the Amplifier to be operated from 120
VAC or 240 VAC. These windings are connected in
parallel for 120 VAC operation or in series for 240
VAC. Two 10-ampere or two 15-ampere fuses protect
the transformer against overload depending upon the
primary voltage. Capacitors C32 and C33 are pro-
vide AC line bypassing.
This amplifier uses a special combination safety
interlock/switch-saver circuit. Interlock — switch
SW4, PWR switch SW3, relay K2, and paralleled
resistors R4A/R4B are connected in series across the
AC line. When either Interlock switch SW4 or PWR
switch SW3 are open, no power is applied to relay
K2. The contacts of this high-current relay do the
actual connection of the AC line voltage to the
windings of power transformer T1. Only the current
that is drawn by the relay coil passes through the
PWR switch.
Relay K2 has a 120 VAC coil. During 120 VAC
operation, resistor combination R4A and R4B is
shorted out to apply the full line voltage to the
relay coil. During 240 VAC operation, however, the
resistor combination provides current limiting for
the relay coil.
Fan motor B1 is connected across only one of the
primary windings so that it always operates on 120
VAC, regardless of which input voltage is used.
HIGH VOLTAGE
The larger of the secondary windings of transformer
T1 is connected to a full-wave, voltage doubling
circuit formed by the components on the rectifier and
filter circuit boards. Diodes D301 through D310
rectify the secondary AC voltage while capacitors
C401 through C408 filter this voltage. Capacitors
C301 through C309 and C311 are connected across the
diodes to protect them against transients. Resistors
R405 through R409 and R411 through R413 are
connected across the filter capacitors to equalize the
voltage drop across each capacitor. These resistors
also help discharge the filter capacitors when the
Amplifier is turned off.