share their loads r19 and r20 are protection resistors for jacks j3 and j4 through jacks j3 and j4 measurements can be made however if an accidental short is placed on these tip jacks no harm will be caused diodes cr3 and cr4 are backlash diodes preventing any reverse voltages from being put on the transistors as well as providing input polarity protection so that if someone inadvertently puts a reverse battery on the input the transistors will not be destroyed these diodes will then forward bias and consequently trip the input circuit breaker cb1 as can be seen either q1 and q2 are on or q3 and q4 are on alternately driving the two halves of the primary winding of the power transformer t2 the control board j2 controls the time that these transistors are on and consequently the output from the power transformer t2 which is rectified by br1 will have a varying pulse width as the pulse gets wider naturally the voltage will go up or more power can be provided to the load resistor r9 and capacitor c5 provide an rc circuit which suppresses inductive switching spikes that could cause the rectifier bridge br1 to be destroyed l3 provides input filtering the current limit monitoring circuit that exists in this device will now be explained the control circuit monitors the voltage across r10 and r11 and the closed contacts of switch s1 in normal operation however the switch s1 is provided for calibration or testing.