FT146
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22
Amateur radio magazines and handbooks provide a variety of circuits for RF
wattmeters and relative field-strength indicators, including methods of using
your VOM as an indicating device. CQ magazine for March 1990 offers an
article by KB4ZGC on how to make a highly accurate yet inexpensive dummy
load and wattmeter capable of showing 1/10-watt differences in RF power. If
you use a wattmeter characterized for the HF frequency region, it will not give
accurate results at the much higher two meter frequencies, although it will be
quite adequate for go/no-go testing.
MAXIMIZING RF POWER OUTPUT
The simplest way to ensure maximum reasonable power output without
component damage is to run the DC voltage in the 13 to 14 volt range,
observing a maximum limit of +15VDC. Typically, an automobile power source
is 13.6 volts when the engine is running and most mobile rigs are specified at
this voltage level.
IMPORTANT NOTE: If you are experimenting with this transmitter and see a
sudden and massive increase in power output and DC current, you have not
reached the promised land or created a 25 watt transmitter! Sudden surges like
that are a sure sign of amplifier self-oscillation. Kill the DC power supply
immediately, because your Q6 RF power transistor is heading to self-
destruction while probably interfering with every TV set in the neighborhood! A
poorly matched antenna along with higher supply voltages is usually
responsible for this occuring. Any prolonged "parasitic" emissions may also
overheat and destroy other components in the amplifier stages.
TROUBLESHOOTING HINTS
The transmitter is very easy to troubleshoot, providing you use some simple
common sense. If you cannot get any readings on the test points or RF power
output, check and see if the crystal oscillator is running - how? Well, take a look
at the crystal and see the frequency marked on it, it should be 16.280 MHz (1/9
of 146.52 MHz), right in the middle of the HF shortwave broadcast band, easily
received on any shortwave radio. You should be able to "hear" the oscillator
running quite easily.
If crystal oscillator operation is confirmed, let's move on a step further. The
oscillator is followed by a tripler stage, and 3 times 16.280 MHz is 48.840 MHz.
Once again this signal can be tuned on a nearby receiver such as a scanner.
One more tripler follows and that moves us up to the final 146.520 MHz output
frequency, easily tuned on a two meter rig. This proceedure will lead us to the
final amplifier stages where we can pretty much do a thourough visual
inspection.
Common problems to look for are solder bridges or interchanged capacitors -
