The Kenwood TS-450S (and TS-690S, which is the same radio with 6
meters added) is capable of being modified for operation on MARS
frequencies. Unfortunately, the optional internal antenna tuner
cannot be used on those MARS frequencies which are immediately above
4.000 MHz. Although this modification is not currently authorized by
Kenwood, to the best of my knowledge, it will allow the internal
tuner to be used on those frequencies.
The tuner contains an inductance consisting of 3 coils: an airwound
coil with 4 taps, in series with a pair of coils wound on powdered
iron toroid cores. For operation on 80/75 meters, the entire
inductance is used. On higher bands, various points along this
inductance are shorted to ground to reduce the total inductance.
This is done by a set of 6 relays, controlled by signal lines
designated as 28A, 25A, 21A, 14A, 10A and 7A. For example, if you
wish to operate on 7 MHz, the 7A line becomes active, energizes relay
K106 (through choke L106) and shorts out one of the toroid coils,
leaving the other toroid in the circuit, along with the entire
airwound coil. When you get to 28 MHz, only a small portion of the
airwound coil is used.
The problem with operating on 4+ MHz MARS frequencies is that the
control logic of the radio activates the 7A line as soon as the
frequency hits 4.000 MHz. This reduces the inductance considerably,
and it is not sufficient to allow the tuner to tune at that
frequency. It will tune at frequencies lower than 7.0 MHz, but not
this low.
The fix is simple enough--keep the 7A signal from reaching the relay
when operating there. The simplest way to do it is to insert a switch
in series with the 7A line. To operate normally, leave the switch
closed so the relay will operate normally. To operate on MARS
frequencies just above 4.000 MHz, open the switch. The radio will
activate the 7A line but the signal will not get to the relay, the
coil will not be shorted out, and there will be enough inductance to
tune your antenna.
How to open the line? Unfortunately, I don't have one of these rigs
available to examine at the moment so I can only go by the drawing in
the service manual. I can't tell if cable W103, which plugs onto the
tuner board which contains the relays and coils, is a ribbon cable or
made of discrete wires. If wires, you can simply cut the proper one,
solder some long wires on both ends, and run them outside the rig to
the switch. The connector is along one side of the circuit board,
with one end near a screw hole. The pin nearest to the hole is
ground. The next one is 7A.
CONTINUED IN TS450S.4
