Instruction manual

AL-800 Instruction Manual
8
This circuit is located on the Timer / Overload board P/N 50-0800-5. IC501 (pin 5) senses the
voltage across the grid shunt (power supply board, R106) through terminal 1. If the voltage at pin 5
exceeds the regulated voltage at the junction of R505 and R511, the output of IC501 (pin 7) will go
high and turn on Q504. Q504 will energize the coil of RL501. RL501 will latch "on" and remove
the voltage from terminal 3 and relay RLY101 (power supply board).
Thermal Overload
The transformer in this amplifier contains an internal temperature sensor. This sensor disables the
transmit function if the transformer's temperature ever happens to approach the limit of safe
operation. Thermal overload is indicated if the
"OL"
LED illuminates without the
"XMT"
LED
being illuminated.
The amplifier will not operate during a thermal overload indication. The thermal protection system
automatically resets after several minutes of a non-transmitting cool down period.
Cathode Warm-up Timer
Indirectly heated transmitting tubes normally require a few minutes of "warm-up" time before plate
current can be drawn. This protection is provided by IC501 (see
Tube Life
on page 10). Pin 2 of
IC501 is connected to a regulated reference voltage. C509 begins to charge when the main power
switch is placed in the ON position. The voltage at pin 3 of IC501 will exceed the voltage on pin 2
after approximately three minutes, causing the output of IC501 (pin 1) to go high.
Q503's collector pulls low whenever the output of IC501 (pin 1) is high. When the collector of
Q503 is low, Q502 and Q501 are forward-biased. This allows 12 volts to appear at terminal 5.
Terminal 5 supplies 12 Vdc to the keying circuits and the plate and grid current meter lamp.
Power Supply
Heater Supply
The heater circuit of this amplifier satisfies all requirements of the tube manufacturer related to tube
performance and life. The heater voltage and inrush current are controlled by the power
transformer's internal resistance and impedance, heater choke resistance, heater wiring resistance,
and the step-start circuit. For maximum tube life, NEVER replace any circuit components or wiring
with substitute parts. Always consult the factory before making modifications that may affect tube
life.
Plate (HV) Supply
The plate supply uses a full wave doubler circuit with 200 amp surge rated diodes and a heavy duty
32 pound grain oriented transformer with internal thermal protection. Filtering is accomplished by a
bank of high quality computer grade capacitors totaling 63 mFd. Large 50k-ohm 7-watt bleeders are
used for safety and superior voltage equalization. The transformer allows user selection of fourteen
different line voltages (See the
Transformer Connections
section on page 15 for details).