1 One of the fault phenomena and its maintenance
Trouble phenomenon: The stylus turns to one side after the power is turned on. The baseline adjustment button does not work, there is neither millivolt standard voltage signal nor ECG signal. Other keying functions are normal.
Maintenance analysis: To solve this type of failure, we must first determine the approximate location of the failure, and then check it level by level. For this reason, the various function keyswitches and regulators on the machine panel can be fully utilized to compress the faulty parts.
2 Failure phenomenon two and its maintenance
Trouble phenomenon: The lead selection control and display are normal. However, when recording an ECG, the ECG signal of some leads is seriously distorted, and the signal waveform of some leads is inverted.
Maintenance analysis: This failure is caused by the superimposition of signals from certain leads when recording the ECG, and sometimes even the lead signal whose R wave should be a positive amplitude becomes the negative amplitude of the R wave inverted. The cause of the failure in this example generally occurs in the lead selection control circuit in the preamplifier circuit. The above-mentioned failure occurs when there are more than two sets of electronic switches simultaneously strobing the lead signal. The lead selection electronic switch is composed of four analog electronic switch integrated blocks such as IC103. The strobe and blocking of the switch are controlled by the IN terminal, which is the input level of the prohibition control terminal (pin 6). When the IN terminal is low, the lead gating is allowed, otherwise the lead gating is blocked.
3 Failure phenomenon three and its maintenance
Trouble phenomenon: After power on, when the speed change switch is selected, the speed display cannot be changed. The actual paper speed is not changed.
Maintenance analysis: The RS trigger composed of IC208A and IC208B is the key circuit to control the conversion of paper speed. This bistable trigger state determines the selection state of the paper feed speed. If any of IC208's gate A and B is faulty, the trigger will not be able to reverse, so it will not work when switching the speed selection switch. If the IC211 or the speed changeover switch is in poor contact, it can also cause the failure in this case.
Inspection method: use an oscilloscope to check the output of IC208's 6th and 9th pins. When the 25mm/s paper speed selection switch is pressed, its pin 6 should be high level and pin 9 should be low level. When the 50mm/s paper speed switch is pressed, the 6th pin of IC208 should be low level and the 9th pin should be high level. If one of the two paper speed switches works and the other one does not work, there may be a problem with the inoperative switch. If the output level of IC208 does not change when the two switches are switched, it means that IC208 is malfunctioning.
