Subject: TNC2 Duplex Mod

  I thought I would pass along a fix for a problem that has been cropping up

more and more across the country with packet on full duplex machines.



  It would appear that a lot of synthesized radios wander in transmit

frequency when the first key up. If your located in a crowded repeater area

there is a very good chance your keying up a repeater that might be within

50Khz of your transmitting frequency, on the input of repeaters. Especially

bad is the ICOM 2AT. We have spoken with ICOM about this problem and they are

working on it.



  There is a simple fix for folks running MFJ TNC's or TNC2 hardware clones.

This does not include the Pac-Comm Tiny 2's or their latest TNC's on the

market. It must be a TNC2 hardware compatible clone, such as the MFJ TNC's.



  The fix involves soldering a 4.7uf tantalum cap across R44 with the positive

side towards Q8, or the junction of R44 and R45. This will give you around

500ms delay of audio on the XR2206 chip, while allowing your radio to key up

immediately, without tones for 500ms. After you make the mod, you will need to

increase your Txdelay anywhere between 10-20 over what your using as a default

now. Some experimenting with the Txdelay is require to get things adjusted

correctly.



  Hope this mod will help users out there, and I'd appreciate a note back to

anyone trying this and finds it fixing this problem. It has cured our problem

in this area. You may wish to post this as a bulletin for users to try out and

circulate it to other BBS's.



73, Steve N7HFZ
NAPRA Technical V.P


SOURCE: The QRZ Windows Ham Radio CDROM