Heathkit SB-1000 - Equipment Interconnections

Page 62
Heathkit
GENERAL INFORMATION
SAFETY INTERLOCK
While the Amplifier's top cover is in place, the
interlock switch closes to allow AC line voltage to
reach the power transformer. When the top cover is
removed, the interlock opens and disconnects the line
voltage. This does not discharge the bank of power
supply filter capacitors. Be sure to allow the filter
capacitors to discharge before you touch anything
inside the Amplifier. You can select the High
Voltage function of the Multimeter to check the high
voltage potential.
TUBE
Tt is not abnormal for the tube to show a dull red
color. But if the plate shows a bright orange or
yellow color, immediately investigate the tuning
and drive conditions and make any necessary cor-
rections.
After prolonged operation, let the Amplifier
run for several minutes without excitation so
the fan will cool the tubes before you tum the
Amplifier off.
DRIVING POWER
This Amplifier is designed to operate at full ratings
(refer to "Specifications") when it is driven by an
exciter that has approximately 100 watts of RF
output. You can use an exciter that has lower output
power, but the Amplifier's output will be less. If you
use an exciter that delivers more than 100 watts,
carefully adjust the driving power to avoid
“overdrive” and the creation of spurious signals,
which create needless interference to other
operators. We highly recommend that you use a
monitor scope for continuous output monitoring. The
display on an oscilloscope is the best readily
available way of determining the amplitude of the
voice peaks which, if excessive, can cause “flat-
topping” and radiation of distortion products.
IMPORTANT: In no case should you advance the
power output control of your exciter beyond the point
where the Amplifier's Power Output indication
ceases to increase. If you turn the control past this
point, nonlinear operation may occur.
ALC (Automatic Level Control)
Whenever the Amplifier is overdriven, the ALC
circuitry creates a negative voltage that is fed back
to the exciter to reduce its gain and help prevent
“flat-topping.”
Protective circuitry of this nature is a valuable
circuit element, but it is not a substitute for proper
adjustment of the exciter drive.
Setting The ALC ADJ Control
Before the ALC circuitry will function properly, the
ALC AD} control on the rear of the Amplifier must be
adjusted to work with your particular exciter. The
following steps outline the procedure:
NOTE: The Amplifier must be properly tuned on CW
before you adjust the ALC.
1. Set the MULTIMETER switch to POWER
OUTPUT.
2. Set the transmitter audio control to
approximately 20% higher than normal.
3. Speak into the microphone in a normal
tone of voice and observe the indication on
the 0 to 2000 RF watts meter scale.
4, Adjust the ALC ADJ control on the rear of
the Amplifier until the Amplifier shows
no clipping on an RF monitor scope. NOTE:
If an RF monitor scope is not available,
adjust the ALC ADJ control so the audio
peaks do not excced the single tone output
of 1000 watts.
AMPLIFIER
The OPR/STBY switch may be used to remove the
Amplifier from the RF line while you tune the
exciter (the Amplifier is in a bypass mode). It does
not, however, switch off the AC or DC power
supplies.