Modifications for the Kenwood TM-255

Solution 9k6 TM255/455

Dear OM's

This is what I found a service technical report from Kenwood acc. TM255 and TM455 9600bps Date November 8, 1994

SUBJECT: Countermessure against modulation during packet 9600bps operation.

A trouble modulation malfunction during packet 9600bps operation was found in the manufacturing process even trough there in input in the DATA terminal.
Pay heed to this trouble in replacing the analog switch IC13.

This trouble is cuased by variation in the analog switch IC13.
Even if the 9600bps SW Tr.(Q20:DTA124EK) has been turned ON, the analog switch IC remains unstable.

Counter messure:
We decided to raise the analog switch IC13 voltage while the 9600bps SW TR. (Q20:DTA124EK) is in action by changing the chip resistor R117 in the TX-RX-unit of TM455A/E and of TM255A/E.....etc.

R117=10K to be replaced with 3K3

So if you have problems with those two say to your dealer to ask for SERVICE TECHNICAL REPORT No. B510-94-081E page 1/2 and page 2/2

Mods for the Kenwood TM-255E
From: Guy ON1DHT -

Adjust LOW power setting
The way to adjust low power is to regulate VR401. This trimmer can be found under the speaker (regulates power between 2 and 30 watts).

Adjust HI power setting
To regulate the power of the HI position is to adjust VR601. You can find this trimmer on the back left side (remove top and locate the alu cover).
Please be carefully when you dismount and locate the trimmer.

Adjust TX-Power/SWR ratio
At the same place you can find VR603 and that is the SWR protection control. Be carefull when adjusting this trimmer, because it protects your transceiver against bad SWR, but can be usefull when power drops to much on relative good SWR.

Adjust AGC
To adjust the AGC time constant of the transceiver locate VR402 that can be found on the left side at the center when removing the top.

Please be carefully with these adjustments! All modifications you make is totaly at your own risk.

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