TB7100 Service Manual Transmitter Fault Finding (50W/40W) 275
© Tait Electronics Limited October 2005
Transmit Tests
The following points need to be borne in mind when carrying out transmit
tests:
■ secure main-board assembly
■ ensure proper antenna load
■ limit duration of transmit tests
■ protect against accidental transmissions
■ avoid thermal and RF burns
These points are discussed in more detail below.
Secure Main-Board
Assembly
Before conducting any transmit tests, ensure that the main-board assembly
is adequately secured in the chassis. This is essential if overheating of the
radio is to be avoided. (As mentioned earlier, the heat-transfer block must
already be secured to the main board of the assembly.) It is good practice to
secure the assembly by at least the two external screws and one of the
internal screws. The screws are labelled
I and E in XREF. There is no
need, however, to secure the lid of the radio body.
Ensure Proper
Antenna Load
The radio has been designed to operate with a 50Ω termination impedance,
but will tolerate a wide range of antenna loading conditions. Nevertheless,
care should be exercised. Normally the RF connector on the main-board
assembly will be connected to the RF communications test set as shown in
Figure 6.2 on page 83. But for those tests where this connection is not
necessary, a 50Ω load may be used instead. Do not operate the transmitter
without such a load or without a connection to the test set. Failure to do so
might result in damage to the power output stage of the transmitter.
Limit Duration of
Transmit Tests
After setting the frequency and power level (if necessary), enter the CCTM
command 33 to perform a transmit test. This command places the radio in
transmit mode. After completing the measurement or check required,
immediately enter the CCTM command 32. This command returns the
radio to the receive mode. Restricting the duration of transmit tests in this
way will further limit the danger of overheating. The reason for this
precaution is that the transmit timers do not function in the CCTM mode.
Protect Against
Accidental
Transmissions
Under certain circumstances the microprocessor can key on the transmitter.
Ensure that all instruments are protected at all times from such accidental
transmissions.
Avoid Thermal
and RF Burns
Avoid thermal burns. Do not touch the cooling fins or underside of the
radio body when the transmitter is or has been operating. Avoid RF burns.
Do not
touch the antenna or the RF signal path on the circuit board while
the transmitter is operating.
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