Storz 201340 20, Service Manual

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3.2.3 Lamp timer circuit
The lamp timer circuit keeps track of lamp usage in hours, and warns the operator that a scheduled
lamp change is approaching or overdue. The red LED (D902) on the front panel will light at approximately
450 hours (90% of the lamp’s expected life) and start blinking at around 500 hours to indicate a lamp
change is imperative. Upon changing the lamp, the user must manually reset the timer by pressing the
reset switch located inside the light source on the front panel. The circuit power is maintained by a
lithium battery when the light source is not in use. The battery must be replaced every 10 years to insure
continued operation.
The first stage of the lamp timer circuit consists of a CD4060 binary counter (U1) driven by a 20 kHz
crystal (Y1). The CD4060 counter divides the 20 kHz crystal frequency down to a 1.22 Hz square wave at
U1-3, which provides clocking for the next stage of the counter. When the light source is in use, the
yellow LED (D1) will blink to indicate the circuit is running. A test jumper is also provided to reduce the
overall 500 hour count to about 30 minutes for testing purposes. This section of the circuitry is powered
by the +12 V bias supply output and is only active when the light source is in use.
The second stage of the lamp timer circuit consists of two CD4020 binary counters, a CD4017 decade
counter, a lithium battery (BAT) and a reset switch (SW). These counters are driven by the 1.22 Hz square
wave from the first stage of the counter circuit and keep track of lamp usage hours. Pins 9 and 11 of the
CD4017 decade counter (U4) provide the logic signals that activate the FET’s that control the replace
lamp LED (D3). The lithium battery powers this section of the circuitry at all times, whether the light
source is on or not. When pressed, the reset switch sets all three counters back to zero.
The final stage of the lamp timer circuit consists of 3 FET’s (Q1–Q3), the replace lamp LED (D3), and the
LED current-limiting resistor (R7). At approximately 450 hours U4-9 goes high and activates the Q2 FET
which turns on the replace lamp LED (D902). At approximately 500 hours U903-9 goes low and U903-11
goes high, activating Q3. Since Q3 is in series with Q1, and Q1 is continuously clocked by U1-3 at a
1.22 Hz rate, the replace lamp LED will start to blink. In addition, when U4-11 goes high, it disables pin
13 of the CD4017 which is the count-enable pin, preventing any additional counting of the CD4017
counter (U4). This holds the count at 500 hours until the SW RESET switch is pressed. Since this stage
of the circuit is powered by the +12 V bias supply output, the replace lamp LED and associated circuitry
do not draw power from the lithium battery.
XENON NOVA
®
300
MODEL 20
1340 20
3-6
06.11 / V1.6