Specifications

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