APRS / packet radio / TNC fun

Hi all,

I’ve had a crazy summer so far, and as a result I’ve had little time for tinkering and ham radio. Despite this fact, I’ve managed to acquire a new (to me) piece of kit: a Timewave DSP-232+ DSP radio modem.

DSP-232+ TNC / Radio Modem

I purchased this unit from Rob (VE3???) for a decent price – a little more than a standard stand-alone TNC, but this puppy can do Pactor and Amtor as well as a number of other functions.

The unit came with a USB-mini-b module already built-in which is pretty slick. I then proceeded to let the magic smoke out of it. The original “upgrade” is a U232 board from Timewave and carries a price tag of $70. Ouch! I know of a $20 solution – see below.

I wanted to ‘scope the output of the TNC so I hooked my trusty DS2202 up to the TX channel of “Radio 1”. Following best practices, I also connected the ground lead of the scope probe to the ground plane of the DSP-232+ which is what killed the USB to serial converter. A post-mortem of the situation revealed  that my Kenwood PS-30 power supply is happily producing 60Vac on both the (+) and (-) leads [reference to ground]. Not good.

RS232 to USB adapter module.

To remedy the situation, I finally ordered a DB9-USB-F from Digikey. I spoke of this before in a prior blog posting, and I can say that this little unit is fantastic! You must be sure to order the correct module as it is offered in three flavours: RS232 levels (± 15v), TTL (5v) and 3v3 levels and two “pinouts”: male / female. In my case, I ordered the female RS232 level version as the pinout matched the schematic diagram of my DSP-232+. Installation was simple and the module performs very well.

To interface the DSP-232+ to my Yaesu FT-857D I had to fabricate a cable which is a male 5-pin DIN connector to a 6-pin mini-DIN connector. Referring to the manual, I made the appropriate connections.

Inline soldering using hook method of joining wires.

To make the solder joint as small as possible, I use a method where I first tin the tips of the exposed wires, allowing me to form a solid “j” hook on each end. I then slide a piece of hear-shrink tubing on all of the wires, except for the one which will be the ground. Notice that in the photo, I’ve split the original insulation on the beige cord – this will allow me to slide the heat-shrink tubing away from the solder joint, but to re-envelop the wires after soldering.

Heat shrink detail

I like to use clear heat-shrink tubing on my solder joints to ensure good visual inspections – this is however just a personal preference. I can’t tell you how many times I’ve finished a solder job like this only to discover that I forgot the last layer of heat-shrink tubing. I make a point of placing a length of tubing over one of the wires as soon as I’ve cut it, ensuring that I will be able to neatly finish the job every time.

Completed cable union

Once all of the joints are complete, and the cable has been tested, I finish the splice by slipping the outer tube over the entire splice and shrink it down. In this case, it would have been nice to use a glue-lined (some times referred to as double-wall) heat shrink to add some mechanical rigidity. This is not strictly necessary but is advised for situations where vibration or other repetitive mechanical stresses may be present.

Notice how the diameter of the completed splice is only slightly larger than the smallest cable. This method leaves a really nice professional appearance.

In the end, I’ve got my DSP-232+ working nicely, pulling in APRS packets with ease. There are a ton of features in this unit – many of which will require some good-old manual time. Once I’ve got some good operating experience with this unit, I will be sure to post my thoughts. First I need to repair / replace my darn power supply – there should never be any AC on the DC outputs – this is most definitely a concern. Until then, I am operating my radio on battery power for testing purposes.

More to follow …