FUNCTIONS OF CIRCUITS IN THE ECG MONITOR
Knowing the signal paths will help you trace the cause of malfunctions to a general
area. Knowing the functions of circuits will help you pin point the malfunction in terms of
precise voltages/signals in the ECG monitor.
a. Functions of the Circuits in the Power Supply, Deflection, and Battery
(a) The MRL Porta Pak 90 Monitor can be powered by its self-contained
12v AH battery or from line (120/240v, 50-400Hz). When connected to the line (unit ON or
OFF), the battery is recharged at a constant rate. A low voltage cut-off circuit avoids deep
battery discharge by turning the unit off at a battery voltage below 9.5v. A high frequency
switching power supply converts the battery voltage to the particular requirements of each
section of the monitor.
(b) Under normal operating conditions, the monitor requires
approximately 6.0w from the battery. This enables it to run continuously for at least four
hours from a fully charged battery.
Main dc-dc invertor.
(a) U20 (TL493) is an integrated circuit which not only has a free running
clock, but also has error amplifiers and circuits necessary to deliver a push-pull pulse width
modulated signal to the power field, transistors Q20 and 021. The constants of the clock
are selected to deliver a 25kHz signal to the outputs and hence to the primary windings of
T2 which has several secondary windings.
(b) Pin 1 provides approximately 65v P/P for the input of the high voltage
multiplier (MRL 490247) and also feeds the second grid and focus circuits for the CRT.
(c) Pin 2 provides approximately 650v peak for the negative, polarization
and blanking circuit related to grid number one of the CRT. The remaining windings feed a full wave
rectifier bridge CR27 through a dual inductor (L, L2) to provide a regulated plus and minus
voltage of approximately 12v each. It also provides the energy to operate the patient cable
(d) The positive voltage is fed back to U20 through a zener diode (CR28)
to present a nominal voltage of 2.5v at pin 1 of U20. If this voltage increases above that
value, the duty cycle of the drive pulses at pin 9 and 10 of IC U20 is reduced. As a
consequence, less energy is transferred to the secondary of T2 which in turn lowers the dc
output voltages until the original values are restored. By this arrangement, a 5 percent
regulation is obtained for all dc voltages against load and/or battery voltage from 13.6v to