AL-800H Instruction Manual
5
Timer - Overload Circuits
CAUTION:
Never modify or disable protection circuits without factory approval. Doing so with void
all manufacturer's warranties.
Grid Overload Protection
This amplifier contains a fast acting electronic grid protection circuit to enhance tube life. The amplifier
will quit operating and the
"OL"
and
"XMT"
LEDs will both illuminate if the safe limit of grid
dissipation is approached or exceeded. The grid overload protection circuit is reset whenever the
"STBY-
OPR"
switch is placed in the
"STBY"
position.
NOTE: This circuit has a much faster response time than the grid meter. This circuit also measures
the peak grid current. The operator may not observe the grid current meter climbing before
the overload circuit responds and disables the amplifier.
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 8)
.
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.
