DSP222H_Manual_A
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Possible components at fault are the crystal X1 or microcontroller U1.
f) Reset input to microcontroller
Voltage at pin 1 of U1 should be 5 volts ± .2 volts. Check to insure pin C on the detector
is high--approximately 24 volts.
Possible components at fault are transistors Q9, Q10, or diode D5.
g) Loop oscillator output
Connect an oscilloscope to pin 13 (pin 12) of U1. Observe the square wave to be the
same frequency as the loop oscillator. At this point, a simulated call should be
introduced at the test box. Observe a slight frequency increase at this point.
If there is no square wave at this point a defective component in the oscillator circuit
may exist. Possible components at fault are Q1 (Q4), Q2 (Q5), Q3 (Q6), D3 (D4) or the
potcore isolation transformer T1 (T2).
h) Outputs and indicators
Verify that the opto-isolated outputs conducts only when the front-panel detect LEDs are
on.
Possible fault areas are: Transistor Q7 (Q8) or the dual opto-isolator U2 or LED2
(LED1)
