The protection circuit for excessive input power should not be regarded as a 100% protection for
all levels of input power. Up to about 50W the circuit will work very effectively and will save the
amplifier from damage, above 50W for example accidental 100W or more applied to the input the
protection circuit can not be expected to provide complete protection. In most cases the protection
will act sufficiently quickly to protect the transistors from damage but this condition is not guaran-
teed!!! For this reason when using the amplifier with a 100W capable radio, caution should be used
as not to accidentally switch on the amplifier before the power has been reduced to 15W or less.
Switch on the power supply to the amplifier followed by the amplifier on off switch on the front
panel. After switch on there is a short delay of approximately 10 seconds whilst the amplifier car-
ries out a quick self test procedure. During this time the LED display of the output power and out-
put filter will flash on and off one at a time from top to bottom. When the test is finished the current
filter selected will be indicated by the relevant LED and also for the 3dB attenuator if set to ON.
SSB delay has no LED associated with it so the switch position must be checked if required on or
off.
Filter selection maybe be made manually but for most operation, if changing bands frequently, the
filter selection can be left in the AUTO position. Every time the amplifier switches to TX when
there is RF present the frequency is calculated and the correct filter latched in circuit. The filter will
remain selected until either another frequency (band) is used outside of the filters range or the am-
plifier is switched off. The default filter in Auto mode after switch on is the 10-12m filter.
Warning: Filter Selection
If using the manual filter selection care must be taken to make sure that the correct filter is selected
for the band in use. This is only of concern if the frequency of use is higher than that of the cut off
frequency of the selected filter. (For example transmission on 14.275 MHz (20m) with the 80m
filter selected. As the cut off frequency of the 80m filter is 4Mhz it will not pass RF at 14.175 MHz
and the transistors will see a very high SWR and could be damaged. The amplifier is protected for
these conditions but this should obviously be avoided especially at full power.
Selecting the wrong filter but having a transmission frequency lower than the filter cut-off fre-
quency does not cause an error or harm to the amplifier but any harmonics of the transmission fre-
quency will be transmitted if they fall under the cut-off frequency of the filter causing unnecessary
QRM on other bands. (e.g. transmission on 1.845 MHz with the 15-17m filter selected would allow
also all of the harmonics to be transmitted without attenuation, on 3.69, 5.535, 7,38........ All the
way up to the 11th harmonic with the 12th lying outside of the 12-17m filter this is almost the worst
example but as can be seen it’s quite important.
Warning: ATU / Non resonant Antennas
Non resonant Antennas and ATU’s (Antenna Tuning Unit). Before transmission you must make
sure that there is a suitable load connected to the antenna output of the amplifier. Transistor ampli-
fiers should not be operated into mismatched loads, (high VSWR) An acceptable level
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