SBE SB2-LA - Power Circuits; Control Circuits; Troubleshooting and Maintenance

Cy sPOWER CIRCUITS
Anode power for the amplifier is obtained from two voltage-doubler rectifier
circuits, series-connected. Output voltage under load is 800 volts nominal at
1.2 amperes. Rectification is by parallel-connected silicon diodes and filter-
ring is accomplished by large electrolytic capacitors,
Also obtained from the main power transformer is the bias voltage. This is
a voltage-doubler rectifier circuit.
Heater voltage is taken from a separate transformer. Inter-connections with-
in the power plug arrange the heater circuit such that it may be operated direct-
ly from a 12-volt battery source when operating mobile.
A resistor in the primary of the main power transformer protects the silicon
rectifiers from the high surge current produced when the unit is first turned
on. Relay contacts short this resistor when actually transmitting so that full
output voltage is produced.
De .GCONTROL GIRCUITS
Necessary
antenna
switching
is
accomplished
within
the
amplifier
by
relay
contacts.
When
on
receive
(or
transmitting
with
the
power
switch
in
LO
posi-
tion)
the
antenna
is
passed
directly
through
the
amplifier
to
the
transceiver,
When
transmitting,
output
from
the
transceiver
is
connected
to
the
grid
input
circuit
and
the
antenna
is
connected
to
the
amplifier
output
terminal.
In standby (receive) condition, cut-off bias to the amplifier tubes is applied
to prevent tube noise. When transmitting, the bias is reduced to the correct
value by grounding a voltage-divider network consisting of resistors R38 and
R40, and bias potentiometer R39 through KZ. Grounding this lead also actuates
relay Kl. Energizing voltage for this relay is supplied by diode D12 and the
heater supply voltage. This diode also prevents the high negative voltage exist-
ing on pin 9 (when in receive) from shorting to the heater line. In mobile oper-
ation, actuating voltage for K2 is obtained directly from the battery source.
Sections of the fil and power switches are inserted into the control line to re-
lay K2 so that the relay does not close except when the fil switch is ON and the
power switch is HI.
V. TROUBLE SHOOTING AND MAINTENANCE
No periodic maintenance is required on the SB2-LA amplifier. If the unit is
located in an unusually dusty position, the top cover should be removed oc-
casionally and accumulated dust removed. Remove six screws on the sides