Renovation, part 2..

Yesterday, Friday, was a day of thank’s to the fact that I was on call for my work last weekend. The “free Friday” award is always a welcome thing every five week. This Friday I decided to get the KAM+ modem fixed as I previously ordered new parts to fix the faltering negative voltage in the RS-232 interface. The cost for the parts turned out to be more modest than I first anticipated as the grand totalt with shipping included landed at under 5 euros. To be honest, I didn’t strive to get the most high quality components either.  The main thing is to get the modem running again.DSC_1291

As I told in my previous post my suspicions was the electrolytic capacitors in the power supply circuits. I also believed that there might be a -5V DC voltage regulator that was broken. To start with, I decided to change the 10uF capacitors around the LT1054 circut. To be more precise C50, C49 and C48. I choosed to change all of these capacitors as they might all in time be dried up. However changing just C50 might solve the problem as this capacitor is an important part of the LT1054 circuit providing voltage output to the 7905 voltage stabilizer.

The capacitors was changed and when measuring the voltage output of the 7905 stabilizer I now got the correct voltage! 🙂 I Hooked it up to the computer and It worked! 😀 Now I am a happy owner of two working KAM+ modems! And It feels good to be able to do the repairs yourself. After all electronics is a big part of this hobby! 🙂

73 de SM7VRZ


The Easter weekend is always a welcome pause in my hectic work life. A lot of energy is spent on my work nowadays and It feels like I haven’t got the time for my radio hobby. During this Easter, except for spending much welcome time with the family , I also had some time doing some fault finding on my Kantronics KAM+ modem that recently decided to be non communicative. It seems like there are only garbled data coming from the device and it also seems like the processor is not recognizing  any commands you send to it. As I have done some repair on thees devices in my early working career, I had my suspicions about some supply voltages and buffer circuits in the serial interface.


So I decided to do some fault finding on the KAM and after some searching on the internet for a schematic I found a VERY bad quality PDF copy. Since I thought that is probably the best I can get regarding schematics, I spent some time trying to interpret the strange blobs on the PDF. Rather frustrated I was close to giving up when I realized that I actually had a paper copy of the schematics in the original documentation I received when purchasing the modem a year ago. A quick search later, the paper copies was found and I started to look at the most qualified faulty components. I then got it confirmed, the serial interface is missing a negative DC voltage on the buffer circuits as well as a -5V DC-voltage. Some dried up Electrolytes and a voltage regulator having just given up seems to be the reason.


A quick check on the preferred components suppliers web page revealed that the cost of renovating the modem would be close to 20 euros just for the components. A small price to pay. 🙂  After asking one of my HAM colleagues about borrowing an oscilloscope  for the fault finding effort, I ended upp swapping an airband-receiver for another KAM+ modem. This gave me opportunity to use the other one as a reference for a working unit.


So, now I have TWO(!!) KAM+ modems! 🙂 Even though I really don´t need to fix the broken modem I will do it just for fun. It also might be fun to have two working modems for future applications. 🙂

73 de SM7VRZ 🙂

“New” toys

Today I’ve got a new toy for my small, yet sufficient HAM shack. As I’m in to digital modes and been in to it on and off since the early days of my amateur radio career, I felt I just couldn’t let the chance go by when it passed. In the ninety’s, when i received my first ham radio license, this toy was on the top of the wish list. At least until the PTC-2 controller hit the market.

The units is a Kantronics KAM Plus modem which can work Packet radio, RTTY, PACTOR 1, AMTOR, G-TOR and some other modes. I found wpid-wp-1425245879441.jpegit on DX-radio, a Swedish ” Buy and Sell”, site for about 50€. It was untested and not used for at least 15 years. The purchase was a gamble, does it work or not?

After I got home from the small trip to buy the TNC, I plugged it in to the computer, realizing that it´s not only the TNC that is old. For some reason the hyperterminal software is no longer delivered with Windows why I had to look for a replacement for it. I found PuTTY which did the trick! To my surprise the TNC was in perfect mint condition with the internal battery backup still working as it had log traces dating back from year 2000. A connection cable between my IC-7410 accessory connector and the TNC was quickly assembled, double checked, and plugged in. It was working perfectly!

Except for the TNC itself the unit came with the original box, manuals, schematics and also an original software on a floppy disc. Just as the TNC itself, the floppy might seem a bit out of date. 🙂

So why buy this old TNC When you can get the same functionality with a sound card and an appropriate software? Well, there are two answers to that.

  1. Even though it is a bit out of date, it is a dream come true to finally have one. In other words, for sentimental reasons. As it uses a terminal interface for issuing commands it´s not really a graphical wonder as the modern digital mode software, but for some reason i find it fun to work with the command line interface and the kind of robust feeling of a modem. Its also have those cool flashing LED:s on the front! 😉
  2. The second reason is that it is a useful unit for doing some research for the next article I’m planning to write here on the blog. What it is? Well stay tuned and promise you will not miss it! 🙂

73 de SM7VRZ