Sencore TF151 - Other Semiconductor Devices; Solid State Servicing Hints

CHECKING OTHER SEMICONDUCTOR DEVICES
There are many other semiconductor devices other than the transistor, FET, or
the diode. Such things as the unijunction transistor, SCRs DIACs, TR ICAS, and
others are being used in industrial applications. These devices require a very el-
abrate test which can not be performed on the Sencore TF1S1 accurately or with the
voltages and currents available.
SOLID STATE SERVICING HINTS
1.
Be sure the power to the equipment is turned off or the line cord is removed from
the AC receptical when making in-circuit tests or repair work. Transistors can
be damaged from the transients developed when changingcomponents or inserting
new transistors.
2. When servicing solid state TV receivers, use a meter and high voltage probe to
measure the 2nd anode potential. If you arc the 2nd anode lead to the chassis
for a spark test as done in tube circuits, you can destroy the high voltage rec-
tifier and possibly the horizontal output transistor . The transient spikes de-
veloped may also destroy some of the small signal transistors in the receiver.
3. When working on Solid state TV or radios, do not operate them with any parts
disconnected, such as the yoke or speaker. If you do, you are removing the
load
from the transistor and it can draw excessive current and destroy itself in
a
short period of time.
4. When replaCing power transistors, be sure to use silicon grease to insure max-
imum heat transfer from the transistor. Both sides of the mica insulation should
be covered for best results. Be sure also that no foreign matter such as metal
shavings adhear to the mica insulator. This can cause a breakdown at a later
time.
5. If an FM stero receiver still has poor separation after the mUltiplex section has
been realigned, check the switching transistors or demodulator diodes. The tran-
sistors should have approximately the same beta figure and the diodes should
have the same front to back ratio for best separation and circuit operation . If
the devices are not rna tched, it will cause an unbalance in the circuit and max-
imum separation will not be obtained.
6. Transistors and FET's a,re "solid" devices, but there are some cases where an
intermittent can occur. If you run across a "tough dog" and suspect that you
may have an intermittent, you can check the transistor or FET in-circuit and
' tap it lightly while watching the gain on the TFlSl meter. Any changes will in-
dicate a defective device. A can of freeze spray will also prove to be quite help~
ful. If the transistor changes its gain drastically or no gain indication is obtain-
ed after spraying the suspected transistor, it is an indication of a defective or
intermittent transistor.
7. If you work on a particular make or model of set, log the beta or Gm readings
from a good working unit on the schematic for future reference. You can take
the minimum figures from the Sencore Transistor manual or get them from the
manufacturer as a speed and time saving servicing convenience. TIlen, you merely
look at the schematic to get'all your necessary information. In-circuit leakage
readings may also be logged in the same manner. ' Although the readings taken
will include the circuit impedances, they can show up a defective stage quite
readily if the readings are drastically different than those from a good working
set.
8. When servicing transistor radios or amplifiers, be sure that the volume control
is setto mid-range. If the control is set to minimum, the first audio transistor
will appear to be shorted and cause a misleading reading on the in-circuit tran-
sistor tester.
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