Ham Radio Goes Digital

My Ham Radio friends and I have finally moved into the digital world.  Just as audio tapes gave way to Compact Discs and DVDs the analog radio world is gradually giving way to digital technology.  In Amateur (Ham) Radio we have had digital radios for some time, mostly in the form of the Icom D-Star system.  This is probably the more popular system with many listings in many states.  In the commercial radio world there are several digital systems such as the more popular APCO Project-25 (P25) and NXDN®.  Unlike P25, NXDN® is an open standard with strong support from Kenwood and ICOM.

If you have ever listened to a digital radio signal on a conventional analog radio all you would hear is something that sounds like multiple rapid-fire machine gun noises.  If you have ever listened to the bank of channels associated with  a trunked radio system on an analog scanner the NXDN® digital signal sounds almost like the control channel.

Around 2006 Kenwood introduced NEXEDGE™, a line of commercial two-way radios and repeaters supporting conventional and trunking technology.  They feature 6.25kHz digital narrowband operation. NEXEDGE™ includes features such as FM Analog using 25 kHz and 12.5 kHz channels, NXDN™ digital mode at 12.5 kHz and 6.25 kHz channels, compliance with current frequency assignments, and NEXEDGE™ will easily convert to very narrow bandwidths in the future. All NEXEDGE® equipment operates in 25 & 12.5 kHz analog and 12.5 & 6.25 kHz NXDN® digital modes.

NEXEDGE® supports both NXDN® digital and regular analog modes via common transceiver technology. This means the radios and repeaters can function with both the new digital radios and the older analog radios found in both Ham Radio service and the commercial two-way radio service.  NEXEDGE® uses existing RF power amplifiers, duplexers and antennas.  NEXEDGE® Conventional operates in 12.5 kHz ‘‘Mixed Mode” so that analog and digital radios can share the same channel. NEXEDGE® digital trunked traffic channels can be shared with analog conventional, analog trunked or both types of traffic. NEXEDGE® uses the AMBE+2™ VOCODER, a state-of-the-art voice digitization and compression technology offering enhanced Forward Error Correction and noise reduction that offers superior clarity at varying signal strengths.

In the Amateur Radio service we do not take advantage of trunking technology at this time.  It's bad enough when we get bitten by the digital bug.  It's even worse if we start talking multiple repeater sites with towers, internet connections and electric bills!

In the old days we used to have to purchase crystals for each channel we wanted to opereate on.  Later the manufacturers developed frequency programmable radios that could change channels using the front panel keypad.  Because these Kenwood radios are business radios they are not user programmable.  You need to purchase the programming software and interface cable and program the channels into the radios with a computer. 

The nice thing about these radios is that they are FCC type accepted for commercial service.  That means if you have a business radio license you can program the radio for your frequency.  If you also have a Ham license you can press the radio into dual service and use the same radio to chew the fat with your friends on the Ham frequencies.

Right now you can catch me, WA1LMV and my friends Cliff, K1IFF; Don, K1ZSG and Clem, K1EM most days on the following UHF frequencies in either analog or NXDN digital modes.  If you call us in analog mode the repeaters will swicth over and we'll hear you.  Remember though that even in analog mode these are narrow band repeaters so you will have to back off on your microphone and turn up your speaker audio.

Time Delay

My church has a Klark-Teknik DN716 digital time delay that is part of the sound system.  This is a microprocessor controlled three channel analog to digital (A/D) and digital to analog (D/A) converter with three independently adjustable delays.  It isused to help control echo in the church by adding progressively longer time delays to the loudspeakers that are further from the front of the church.  There are four pairs of speakers mounted on the ceiling support columns.  The front pair are fed with no delay.  The subsequent pairs are fed with a 22.1, 44.25 and 66.3 millisecond delays.

The "B" channel in this unit developed static as the unit warmed up.  The sound system was installed in 1986 and this time delay is 26 years old.

Some time ago the unit developed a problem where the settings were not persistent on the next power up.  I wrote to Klark-Teknik in the UK and they promptly responded with the information on the replacible internal battery along with a PDF file with the service manual and schematics!

Now with this new static problem I wrote to them again.  The service engineers at Klark-Technic were fabulous!  Based on their experience they suspected the problem was with a defective filter module.  This component is a thick-film hybrid 7-pole eliptical filter that is used to convert the digital signal into a smooth analog signal.  The bad news is this part is $90!  It's a custom part only made for them.  The good news is they have them in stock and they are willing to sell one to me.

Can you imagine getting this kind of support after 26 years?  I don't know of many companies who would sell a custom circuit board component to an end-user customer especially after so many years.  It would have cost as much as $2,000 to replace this time delay unit today.

Before I started the repair work I turned on the sound system and let it warm up until I started hearing the static in the "B" channel.  That way I could confirm a successful repair if I did not hear the static after a reasonable warm up after repair.

Here's the top view of the DN716.  At first I thought I would have to remove the circuit board in order to unsolder the hybrid module.  Then I realized all I had to do was remove the bottom cover!

This is the bottom view.  It's a service person's dream to have access to both sides of the circuit board by only removing the covers.

This is the part I need to replace.  It is the hybrid filter IC8 part number SEM81-KTR009,  Don't expect to find one of these in your local enectronics store.

Here's a close-up view of the 4 hybrid filters.  One is on the input.  The other three are on each of the three output channels.  Since I was having static on the "B" channel I replaced IC-8.  It was fairly difficult unsoldering the filter.  The holes were plated through.  A solder sucker and solder wick were not very effective.  I was on the verge of damaging the foil pads before I got all the leads to be free.

Instead of soldering the new module to the board I installed a socket.  I made a socket by cutting a 40 pin dual inline socket making it into a 16 pin single inline socket.  I soldered the socket to the board and plugged the new module into the socket.  This way if anything happens in the future the module can be replaced without damaging the board.  

I was so pleased to hear the unit come back to life.  After waiting a reasonable length of time I was even more happy to hear not a single hint of static building after warm-up.

My deepest appreciation is extended to Andrew Shewring, technician at Music Group Research UK Limited for his expert advice and Eddie Roberts of Midas Consoles and Tina Dugas of American Music and Sound here in the US who sold me the part.