Voltage And Continuity Checks - Heathkit SB-104 Operation Manual

Page 272
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VOLTAGE AND CONTINUITY CHECKS
To measure voltages, use a voltmeter with a high input
impedance (11 megohms or higher). A meter with a low
input impedance may load down a circuit and cause the
readings to be abnormally low.
An ohmmeter is very useful for measuring resistors,
determining the continuity of conductors and inductances,
and making
a
rough check of the serviceability of diodes and
transistors (as in "Checking Transistors and Diodes," Page
274).
First, before you check a component on any circuit board,
check for the presence of the proper DC input voltage and
check the ground connections from the circuit board
connectors to the chassis. These are shown on the individual
schematics for the circuit boards in the "Ma~ntenance"
section Page 275 and on Figure 5-1 (fold-out from Page
3231.
Check all nongrwnded circuit board connectors t o make
sure there is not a h o r t circuit (zero resistance) to the
chassis or to adjacent pins. Do this because one fine strand
from a conductor can sometimes protrude and touch an
unintended location, thus causing a short circuit. These are
usually difficult to see.
When you are told t o check a component, refer t o the part
of the "Maintenance" section which deals with the circuit
board upon which the part is located; then measure the
voltages around the component and check them against
those shown i n the ''Voltage Chart." It will usually be
necessary to use the extender board to gain access t o the
points to be measured. Any voltage deviation of more than
20% should be considered as possibly indicating some sort of
malfunction.
There can be various causes of a voltage variation. For
instance: the supply voltage may have changed, there may
be a malfunction in the Transceiver's power supply, some
resistor between the test point and the voltage source in the
Transceiver may have changed value, or a bypass capacitor
may have short circuited. Use your voltmeter and trace the
voltage path back until you get a normal voltage reading.
When this point is reached, the probable causes of trouble
can be limited to a very few components, which can then be
thoroughly investigated.
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CAUTION: When you check transistors, be very careful that
you do not touchtwo leads at the same time with the meter
probe. This can destioy a transistor instantly.
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The complete absence of a designated voltage indicates a
poorly soldered connection, a break in the foil, a power
supply failure, or a similar problem.
In such cases, turn the Transceiver OFF and use your
ohmmeter Ion i t s RX1 scale) to check thecontinuity of the
path to the voltage source. Use the "X-Ray View" (see
"Maintenance," Page 275 and the schematic diagram t o
determine where the path is.