ATU Modification for the Kenwood TS440-S/AT

* Copyright (c), Amateur Radio 1995, Copyright (c), Ralph Holland 1995

Kenwood has placed the 440S ATU in the transmitter output chain only. This is a deficiency when operating the 440S mobile or into antennas that are mismatched to the nominal 50 ohms. In some cases the received signal strength can be several S-points down compared to what would be recovered when the antenna impedance is a conjugate match to the receiver's input impedance.

The ATU can be placed in both the receiver input and the transmitter output by a simple modification. Further, the ATU can still be taken in and out of the antenna circuit by the existing front-panel switch, when desired.

This change can be performed with only one additional part, a piece of thin 50 ohm coax approximately 7 cm long (preferably with a multi-stranded centre conductor).

See Figure 1 for the old block diagram and compare this with figure 2, the new block diagram. Figure 3 contains the approximate board and cable layout.


a. Remember the locations of all connections and wires that are mentioned in the following steps, or draw diagrams, as there are a lot of wires in this transceiver!

b. Disassemble the transceiver by removing the top and bottom covers.

c. Disconnect the speaker cable at the connector on the board behind the S-meter.

d. Turn the transceiver bottom-up and remove the cover plate for the filter unit.

e. Remove the five (5) screws holding the heatsink (final and filter units).

f. Turn the transceiver bottom-up again and disconnect the drive output coax from the main board near the antenna relay (black) - this wire will come out anyway if you don't remove it!

g. Lay the transceiver on its side with the mike connector closest to the bench.

h. Hinge the heatsink, carefully, towards the bottom of the transceiver so you can get access to the ATU control board.

i. Remove the ATU control cable (3 connectors J1, J2, J3 and 1 wire plug 14S).

j. Undo the ATU control board mounting screws and remove the board.

k. Disconnect the FILTER-OUT (FO) to ATU-IN coax from the plug on the filter board and unsolder the coax from the ATU board. Keep this coax for step n.

l. Unsolder the short wire going from the Antenna coax connector to the TX/RX relay centre contact. (The relay board is the small board behind the coaxial connector.)

m. Identify the short coax that runs from the ATU OUT to the Antenna relay coax socket marked S29. Remove this coax from the ATU OUT socket, re-route, and plug this into the FILTER OUT connector on the filter board. Ie the short coax now runs from the Relay board S29 to the Filter FO socket.

n. Take the long piece of coax, removed in step k; plug the small connector into the ATU-OUT socket on the ATU board. Solder the braid of the free-end onto the antenna socket ground lug nearest the side of the transceiver. Solder the centre conductor to the Antenna socket centre pin. Ie the ATU-OUT is now connected directly to the antenna socket.

o. Run a new short length of thin 50 ohm coax to the old ATU-IN connection wire on the relay board (centre contact of the TX/RX relay). Solder the braid to the same antenna socket lug as before, insulate the centre conductor with a sleeve of plastic and solder the centre conductor to the short contact wire, pull down the sleeve.

p. Solder the other end of this cable onto the ATU control board contact marked ATU-IN.

q. Re-route the coaxial cable and re-insert the ATU control board into the ATU.

r. Reconnect all four (4) connections on the ATU board. (Note the single wire is the ATU motor wire - don't forget it like I did!)

s. Thoroughly check the wiring against figures 2 and 3.

t. Replace the heatsink unit in the chassis and reassemble.

u. Reconnect the DRIVE-OUT to Power amplifier coax (near the relay on the bottom board).

v. Reconnect the speaker connector.

w. Replace top and bottom covers.


This modification has not disrupted any relay-timings or introduced stray reactance into the transmitter chain, so the transceiver should work at least as well as it did before the modification. Also note that the ATU has a by-pass relay actuated by the front panel switch - this was not indicated in the block diagrams for clarity.

You can try the unit out on various antennas and do S-points comparisons with the ATU tuned and inline against the ATU offline.


* Terms

This article was first published in Amateur Radio Volume 63 No 11, November199. Amateur Radio is the journal of the Wireless Institute of Australia. Both the author and the WIA hold copyright. No reproduction is permitted for commercial purposes without express permission of the copyright holders.