47.0-mc amplifier; diode CR7402, with the
trigger circuit to cause it to go from its
11.5-mc amplifier; and diode CR7403, with
RED (bad) condition to its GREEN (good)
the 5.65-mc amplifier. The diode associ-
condition.
ated with the rf amplifier in use conducts
b. The voltage divider consisting of re-
to allow +16 volts to be supplied to the test
sistor R7802 and rheostat R7801 and dc
switching transistor Q7801 sense the
probe rf preamplifier. The two diodes
associated with the two rf amplifiers not
maximum voltage supply limit. Transistor
in use prevent the supply voltage from
Q7801 is a unijunction transistor. With the
being fed to the amplifiers from their
Schmitt trigger circuit in its GREEN
common connection in the test probe rf
condition, the +16 volta dc applied to base
preamplifier supply line.
B2 of transistor Q7801 biases the tran-
sistor to cutoff. If the voltage applied to
the divier of resistors R7801 and R7802
9. Voltage Supply Limits Sensing Circuit
exceeds the maximum limit, the voltage
(fig. 6)
applied at the emitter of transistor Q7801
The voltage supply limits sensing circuit
overcomes the +16-volt bias and is con-
consists of two voltage dividers and a dc
nected through base Bl to _ground, and
switch. It senses the minimum and maxi-
transistor Q7801 is biased into conduction.
mum limits of regulated +16- and +12-
This action quickly reduces the voltage
volt dc supplies. Power, input, and output
applied to the Schmitt trigger circuit below
connections are made to the voltage supply
the minimum required to operate it, and the
limits sensing circuit when switch S7902 is
Schmitt trigger circuit returns to its RED
in position A or 8. The dc output of the
(bad) condition.
voltage supply limits sensing circuit is fed
c. When the test probe tip is first applied
to the Schmitt trigger circuit.
to a test point, a transient voltage can be
a. The voltage divider network consist-
applied to the voltage supply limits sensing
ing of resistor R7902 and rheostat R7801
circuit which could cause Q7801 to be
and the network of resistor R7804 and
biased into conduction even though the volt-
rheostat R7803 sense the minimum voltage
age being tested is quite lox. The network
of diode CR7801, load resistor R7805, and
supply limit. Rheostat R7803 is adjusted
so that at the minimum allowable level,
filter capacitor C7801 reduces such tran-
sufficient voltage is applied to the Schmitt
sient voltages by providing a relatively
Figure 5. Test probe rf preamplifier, schematic diagram.
8
