TM 9-4910-700-14 & P
THEORY OF OPERATION
through R24, established by the fixed
zener voltage on the base of Q10 and
the theory of operation of the test set.
Refer to figure 3-1, schematic diagram.
resistor R25. When the points are
open, Q9 is on and Q10 is off. The
voltage drop across R24 is thus a
circuit is powered by the vehicle batt-
constant value when the points are
ery power (12 volts). The power is
closed, and zero when the points are
applied through CR1, the polarity pro-
open. The average drop relative to
tection diode, to R1 and CR2 which pro-
the peak voltage is the percentage
vide the base drive for Q1, the voltage
dwell angle. When the meter is calib-
regulator. The signal input is applied
rated in degrees for an engine of a
to the base of Q2, a switch, which pro-
given numbers of cylinders, the meter
duces a positive pulse at its collector.
will read directly in degrees of dwell.
This positive pulse is then applied to
Switch S2 selects either R28 and R29
the base of Q3 through C3 making the
or R26 and R27, in series with the
collector of Q3 go low, therefore start-
meter. R28 is calibrated so that full
ing the fixed timing cycle which is made
scale on the meter corresponds to either
up of R6, C4, Q4, and Q5. The fixed
100% of percentage dwell, 60 degrees
width pulse at the collector of Q5 is
of dwell for 6 cylinder engines, or 90
inverted by Q6 and applied to the base
degrees of dwell for 4 cylinder engines.
of Q7 which is in series with M1 and S3.
R26 is calibrated to that 100% of dwell
Switch S3 allows current to flow through
reads 45 on the 60 scale, for 8
Ml, the RPM meter, for the duration of
cylinder engines.
the fixed length pulse. Since the num-
ber of pulses is proportional to the
b. Positive Grounded Systems, For
engine RPM, the meter displays engine
positive grounded system. the DWELL
RPM . R16 provides an internal calibra-
lead is connected to the base of 08
tion adjustment for the 1000 RPM range
through R18. When the points are
and R14 for the 5000 RPM range.
closed the voltage at the collector
Q8 is approximately zero, with respect
to the battery + lead. Q8 is there-
cuitry measures the length of time that
fore on and Q9 is off. When the points
the points are closed compared to the
are open (negative with respect to
total time for firing one cylinder of
battery +), Q8 is off, and Q9 is on.
the engine. This is accomplished by
The rest of the dwell circuitry func-
attaching the DWELL lead to the primary
tions identically to the negative
ignition wire connecting the spark coil
grounded system.
primary to the distributor breaker
points. When the points are closed,
there is approximately zero volts be-
set measures the voltage drop across
tween ground and the DWELL lead. When
closed points for both positive and
the points are open, the voltage across
negative grounded systems. In both
the points is near the battery voltage.
cases, the DWELL meter is connected in
The circuit will measure dwell for both
series with a fixed resistor (R1) to
negative and positive grounded systems.
form a voltmeter with a, full scale
sensitivity of one volt. The GO-NO-GO
mark is calibrated at 0.2 volts. For
a. Negative Grounded Systems. When
negative grounded systems, the meter
used with negative grounded systems,
is connected from the DWELL lead to
the ground switch is in the "-" position BATTERY by S1, the ground switch.
The distributor points are thus connect- positive grounded systems, the meter or
F
ed between the base of Q9 and ground
is connected from the DWELL lead to
through R20. When the points are open,
BATTERY + by S1.
Q9 is on. When the points are closed,
Q9 is off. Q9 controls Q10. When Q9
is off, a constant current flows
3-1
