[aprssig] Problems with APRS Harness

Stephen H. Smith wa8lmf2 at aol.com
Tue Mar 21 18:45:11 CST 2006

fred at WB4AEJ.COM wrote:
>      We put the APRS harness together today.  I can see the DCD light flashing when I adjust the audio.  It's behaving like it should.  
>      But when I put the microphone connector from the harness in the microphone jack (Kenwood TR-7850) and tighten down the ring, the transmitter keys up continuously.  
>      As soon as I loosen the ring, the transmitter unkeys, even if I don't remove the connector.  
This radio actually has a 6-pin mic jack rather than the now-universal 
8-pin one?

Sounds to me as though the PTT line inside the mic plug is shorting to 
the metal shell of the plug.  When you screw down the coupling ring, it 
makes metallic contact with the radio's chassis which in turn is common 
to the pin 2 (Mic Gnd) somewhere inside the radio. 

Do you have the clear PVC insulating sleeve present that is normally 
inside the shell of the plug that prevents leads soldered to the pins 
from contacting the inside of the metal shell? 
>      The drawing for the harness can be seen at http://paws.wcu.edu/fatkinson/APRS/APRS_Harness.pdf.  
I assume the "1/8-inch Phone Jack" (which is actually a 1/8th In Phone 
PLUG ) is a two conductor device - the pins should be tip and sleeve.  
Ring is the third contact on a stereo 3-conductor plug.  

If the radio really has receive audio on the mic jack,  as labeled in 
your diagram,  why are you bothering with the speaker jack fork of the 
cable at all?  The audio on the mic jack is probably fixed level 
unaffected by the volume control which makes if far more convenient -- 
you can set the speaker volume at any level for monitoring channel 
activity by ear without affecting the TNC receive.

>     Any suggestions as to what might be causing this?  I am using an MFJ-1270B Turbo TNC-2 with the Kenwood TR-7850.  
>      I ohmed the harness.  Everything is wired as it should be.  
>      I pulled the plug and put my ohmmeter across the PTT (3) and the Microphone Ground (2).  When I issue a connect command to the TNC-2, I think I'm seeing a very brief ground (it's too fast for the meter to track accurately) over and over until the TNC-2 times out.  

Sounds like the TNC is trying to key PTT properly.
>      Also (you can tell I've been out of packet for a long time), what is the command to make a TNC-2 display all the packets it hears on the Hyperterminal screen?  
It should do that by default, assuming the usual comport stuff (COM 
number, baud rate, etc are set correctly). 

Bear in mind that even today, TNCs power up in the archaic 
7bit-EvenParity-1Stop bit mode (7-E-1) until you issue the commands 
"AWLEN 8"  and "8BIT ON"  to command them into the 8Bits-No Parity 
(8-N-1) format universally used today. 

This means you may initially have to set HyperTerm to 7-E-1 to issue the 
commands, then switch it to 8-N-1.    [ APRS applications all expect 
8-N-1 format. ]

After you get the cabling issues resolved, I suspect you will have to 
play around a lot with transmit delay (TXD) settings on the TNC.  These 
are old radios with slow-settling synthesizers and (I think) mechanical 
relay PTT switching rather than modern all-electronic T/R switching.  
The result is the radio will be rather slow on the draw when the TNC 
tries to key it.  You will probably need hundreds of milliseconds TX 
delay to avoid chopping off the packet header on transmit. 

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