MALFUNCTIONS AND DEFECTIVE MODULES
Section I. ISOLATE MALFUNCTIONS TO MODULE LEVEL
Performing PMCS and calibrating equipment will reveal malfunctions. As a medical
equipment repairer, you are responsible for locating the causes of malfunctions to
determine their probable cause(s). In this lesson, you will learn how to isolate a variety of
malfunctions in the MRL Porta Pak 90. Maintaining the defibrillator/monitor in a proper
operating condition is essential for ensuring its safe use during surgery.
SIGNAL PATHS OF THE PORTA PAK 90
You must understand signal paths in order to trace the circuit and locate the cause
a. Signal Paths in the Monitor.
(a) Each patient cable signal, with the exception of the reference, goes
through a buffer stage. The very high input impedance is converted to a low impedance
before the signal reaches the lead selector switch or paddle enable circuit.
(b) The switch is followed by a common mode rejection circuit which, in
turn, feeds two high gain stages and a fast recovery circuit. This temporarily (in the
presence of a signal overload condition) switches from the diagnostic to monitor frequency
response and back. Long recovery times and eventual loss of signal during that period
(c) The signal gain of the two amplifier stages is inhibited during the
duration of a pacer pulse. This is detected by the pacemaker pulse detector whose output
signal "punches a hole" into the main signal, avoiding amplifier overloads and, as a
consequence, signal distortion.
(d) The amplified signal modulates a Lo voltage frequency converter and
is passed on to the grounded analog section through opto-coupler K6.
(e) K7 enables the paddle pick-up signal at the grounded side to
complete the isolated patient cable amplifier, if the selector switch is in that position.
A 6v regulates the power supply which is driven by a 25 kilohertz (Khz) pulse width
modulated signal. The signal is derived from the main switching power supply.