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TM-V71A to TNC Troubles

Discussion in 'General Technical Questions and Answers' started by W2LIE, Oct 20, 2008.

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  1. W2LIE

    W2LIE Ham Member QRZ Page

    Good Evening..
    I've decided to try using my TM-V71A with my TNC to get more power for my APRS station until I can get the correct connectors for my old TR-7800 at next week's hamfest.

    Since I'm having troubles getting the TM-V71A to work correctly - now I'm determined to get this radio on Packet just to prove that I am better than my radio ;)

    I've followed the pin-layout here:
    I am connecting a Kantronics KPC-3 (not +3, but the 1992 version of Firmware) to the rear data port of the TM-V71A.

    I found an old PS/2 mouse laying around that was going to the landfill, so I cut the cable off, and used the mouse's tail as my data cable.

    So - here is my problem. 2 parts

    1. When the Data cable is plugged into the radio and the TNC, the radio keys up. This is with the TNC on or off. If I send out a beacon, I can hear my data, so I know that pin is correct.... Once I remove Pin-3 from the circuit, the radio no longer keys up.
    2. When the PTT line is removed, the TNC always thinks its receiving. When I check my terminal, I can see data coming across, but the TNC is a solid RCV light...

    Neither situation is good for me.. I've checked my wiring a few times, I even unsoldered the connection and tried it all over again by starting lower down the cable.

    Anyone have this problem? Am I missing some info in the circuit?? Should this be a strait wire connection with no capacitors or resistors in the circuit?

    Thanks for the help and 73
  2. KA1MDA

    KA1MDA Ham Member QRZ Page

    These types of problems often stem from confusion about the connector pin-outs. Is the pin-out shown in the diagram looking at the pins in front of the plug, or is it looking at the solder connections at the rear of the plug? I noticed in your diagram the PS/2 connector pins are reversed (mirrored) from the pin-out shown in the Kenwood TM-V71 manual. So one of these is drawn viewing the connector from the front, while the other is drawn viewing the connector from the back. If the PS/2 connector were wired from the wrong "side", the PTT line would be connected to the squelch line instead of the 1200 and 9600 baud receive lines being tied together. This could cause the radio to go into TX.

    I think the first thing you need to do is isolate possible cable problems from TNC problems as follows:

    Plug the cable into the radio, but do not connect the DB-9 end to the TNC. Open the squelch so you hear noise out of the speaker, then close the squelch so audio is muted. If the radio goes into TX during any part of this step, PS/2 connector wiring is reversed.

    Next, try momentarily shorting the PTT pin to the ground pin on the DB-9. If the radio does not TX when these 2 pins are shorted (CAUTION- Do not short any other pins to ground or each other!), check for wiring error on DB-9.

    If the radio goes into TX when the pins are shorted and back to RX when the short is removed, chances are good that you can rule out the cable as the problem and focus on the TNC.

    73, de Tom, KA1MDA
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