Ron, I use the TM-271a and a kantronics KPC3 in my home APRS station...it works great.
I did the mod as described here:http://www.kb2ljj.com/data/kenwood/TM-271A-E.htm
AND followed this bit of advice from the Yahoo Group for the tm-271a: ( YOU REALLY NEED TO HIT THIS GROUP!)
Just make sure your TNC is set to Carrier Detect CD Software. Running open squelch and not CD software, you will NOT transmit.
I've modified several of these radios.
DB9 pins are : Pin 1 PKD, Pin 3 PKS, Pin 5 PR9 (I was using PR1), Pin 5 GRN.
I make a direct connection to the board with a BD9 pigtail. I don't use a pigtail with a mini plug and then make up a mating mini to DB9 to the radio. Cleaner connection if you know which TNC the radio will be connected.
73 Bill, W1GTT
This allows me to use the radio with the hand mike as needed - and the aprs connection is permanent with no worries about squelch or audio settings, and NO NOISE!
JC - KI4OIU