Subject: TS450S Mars/CAP Antenna Tuning



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.  



If it is a ribbon cable, it will be very difficult to tap into it.

You can lift one end of choke L106 instead, and run wires from the

lead and the empty hole to the switch.  You can snake the wires out

through a hole in the cabinet somewhere, or you could mount the

switch on the rear panel immediately behind the antenna tuner.  (I

was told that it contains a small, replaceable piece of metal.)



Remember, close the switch for MARS operation immediately above 4.000

MHz, and leave it open for all other operation.



Disclaimer: Perform this modification at your own risk, and be aware

that it may void the warranty on the radio.  (To the best of my

knowledge, Kenwood does not currently authorize this modification.)  

I assume no responsibility for any consequences of performing the

mod, although I do know of one case in which it was done and

operation was satisfactory.  --WA8MCQ, 29 May 1993





PS:

An error crept into my recent bulletin...excuse me...message on this

subject.  Near the end there was a recap, which said to close the switch

for MARS operation immediately above 4 MHz and open it for normal antenna

tuner operation.  This is backwards, and was done correctly earlier in

the message.  The proper positions are OPEN for MARS operation immediately

above 4 MHz and CLOSED for normal antenna tuner operation.  Sorry 'bout

that.  73...