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| HOMEPAGE | THE WORLD OF FREE PACKET RADIO SOFTWARE AMSTERDAM THE NETHERLANDS |
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From: Steve the N7HFZ. Subject: The TNC2 in duplex modification. 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 50 Khz 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 500 ms delay of audio on the XR2206 chip, while allowing your radio to key up immediately, without tones for 500 ms. After you make the modification, 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 the N7HFZ NAPRA Technical V.P |
