(b) As long as the signal amplitude on the junction point of R44, R45,
CR6, and CR7 is not approaching the zener voltage of CR6, CR7, the system is in the full
diagnostic mode. If any of the zener diodes enter into conduction, Q5 (in case of a
negative overload signal) conducts, which pulls its collector to +6v and drives the gate of
Q3 from -6v to +6v. This action makes Q3 conduct, and shorts out R34 which reduces the
time constant to 1/10 of its original value. This is, of course, a temporary monitoring
frequency response which returns to diagnostic as soon as Q3 ceases to conduct because
its gate is returned to -6v, which is below its "pinch off" value. Similarly, a positive
overload signal makes CR6 conduct which drives Q4 into conduction and consequently
(c) The combination of CR5, C4, and R48 provides a fast switch over
from diagnostic to monitor quality and a gradual recovery (approximately 1.5 seconds)
from monitor to diagnostic mode. By this means, huge slow drifts of the baseline are
practically avoided, and total loss of signal is minimized.
Isolated power supply.
(a) The 25kHz pulse width module signal drives the primary of isolation
transformer T1. The secondary of T1 feeds a full wave rectifier bridge CR1 and provides
plus and minus dc voltages in reference to its center tap (pin 5). A combination of
monolithic capacitors C16, C17, and tantalum capacitors C14 and C15 filter the high
frequency components out. Since the plus and minus sides of the regulated power supply
are exactly mirror images from each other, we will describe only the positive regulator.
(b) A pass transistor Q1 is driven by one section of U5 whose input is
comparing the nominal voltage drop of zener diode CR2 against the true output voltage. If
the output voltage is low, pin 12 of U5 will be negative against floating ground (pin 13). As
a consequence, pin 14 goes negative, which makes Q1 conduct harder and therefore
overcomes the lack of voltage at the output.
(c) The opposite condition (output voltage too high) makes pin 12
positive, and as a consequence, Q1 conducts less. The departure from the nominal
voltage or reference CR2 is very small since the full open loop gain (minimum 20,000
times) of the particular section of U5 is present to correct the detected error.
(d) The resistors connected across Q1 and Q2 aid in starting up the
regulators. The power supply works perfectly with a differential voltage of only 0.5vdc
between input and output voltage. The transient response is improved by output
capacitors C12 and C13.
(e) For paddle signal pick-up, SW2 enables the infrared diode of K6
opto-coupler which in turn enables, on the grounded analog section, the paddle
preamplifier signal to reach the output (CRT and Chart Recorder).