18 



(3) Check the gap on the LWI pen arm relay. It may be 

 necessary to reduce the gap by turning the gap adjust- 

 ment screw clockwise. 

 3. Trouble-shooting the unijunction assembly. 



a. If (1) and (2) have been completed and the pen arm is still 

 not functioning or is responding only slightly, the fault 

 may lie in the unijunction assembly. The malfunction is 

 most likely due to an improperly functioning unijunction 

 transistor. This transistor is sensitive to very high tem- 

 peratures, such as temperatures encountered when solder- 

 ing. 



b. Replace the unijunction transistor, being careful not to 

 overheat during soldering. If trouble persists, proceed as 

 follows using a simple voltmeter. 



(1) Make sure the voltmeter is adjusted to read DC volts. 



(2) Push and hold the unijunction accelerator button. 

 This activates the 4.7k ohm resistor. 



(a) Place the positive probe of the voltmeter on the 

 positive end of the capacitor (a. Fig. 17). 



(b) Place the negative probe of the voltmeter on the 

 negative end of the capacitor (b. Fig. 17). 



(c) The voltage should increase to about 8.5 V, dis- 

 charge, increase again to 8.5 V, and discharge 

 again in a cyclic fashion, with each cycle lasting 

 several seconds. 



(3) The circuitry may be checked without activating the 

 unijunction accelerator switch. 



(a) Place the positive probe of the voltmeter on the 

 junction of the 200 ohm and 470k ohm resistors (c, 

 Fig. 17). 



(b) Place the negative probe on the positive end (a. Fig. 

 17) of the capacitor and observe the ohm scale on 

 the voltmeter. 



(c) Cycling should also be observed on the ohm scale if 

 the negative probe is kept on the capcitor while the 

 positive probe is moved to the 220 ohm resistor (d, 

 Fig. 17). 



(d) If cycling is not observed, check for loose wiring 

 and cold solder joints or replace resistors. 



Testing the Wetness-sensing Element for 

 Its Response to Wetting and Drying 



The sensing element should be 'conditioned' before it is 

 attached to the sensor housing, but this does not insure that it is 

 going to function properly. Thus, it should be tested for its response 



