New radio in the shack: Kenwood TM-742E

The Kenwood TM-742E

Kenwood TM-742E
A while back I bought a second hand radio: the Kenwood TM-742E. A great radio with 2m, 70cm and 23cm module!
The man who sold it is a heavy smoker, so I had to clean the rig intensively. But after doing a good job the radio is like brand new. No scratch on it.

Back-up battery

The back-up battery was dead so I had to replace that.  This repair gave me a good impression of the inner being of the rig. The Kenwood TM-742E is really a brilliant piece of RF-engineering. The radio is made of three modules (one for each band) and each module is about exactly the same as Kenwood mono banders TM-531E, TM-431E and TM-231E. I think the Kenwood engineers found out it is possible to combine these RF-modules and make one control unit (with a little more functionality then the mono banders) . That’s exactly what you find when you open up a TM-742E. The back-up battery is placed directly behind the display. You have to remove the front unit to get access but that’s fairly easy.

A lot of modules!

Some research find out the Kenwood TM-742E was available (back in the day) with a maximum output of 50 watts (2 m) and 35 watts (70 cm). There are also four optional FM band units — 10 m (50 watts), 6 m (50 watts), 1.25 m (25 watts), 70 cm (35 watts), and 23 cm (10 watts) — to endow the TM-742A/E with tri-band status. For low-power operations, there are 5- and 10-watt settings (1 W with the 23-cm band unit). I even heard rumors about the existence of a 13-cm module.
My guess is this is one of the first Kenwood radio’s with detachable front panel.

It makes a perfect combination with my Diamond X-6000 tri-bander antenna. First reports are great. I’m really impressed from the performance of the 23-cm module. I hear a few repeaters on 23-cm and I am able te work them too.

Service manual

Really handy is the availability of online material for this radio, even the service manual is out there to find!

Latest updates on the WSPR-RX: WSPR-RX 2

Latest situation of the WSPR-RX

A while ago I did build my first 30m WSPR-RX to participate in the WSPR-challenge. I must admit it is very addictive! It did not took long before I was building my second WSPR-RX. This time a little bigger since the first one was too much miniature. Also I wanted to add some tweaks.

Temperature compensation

WSPR is a very small band signal. Actually it only covers 200 Hz on (30m) 10,138.70 MHz. When you build an x-tal oscillator for this receiver, it has to be quite stable to be able to hold the WSPR-RX between its lines. And the original design drifts. Too much for me. At some point I could read the rooms temperature from the drift of the decoded WSPR-signals.
In the original PA2OHH-design of this receiver there is an Negative temperature coefficient thermistor (NTC) with a varicap to compensate for frequency drift. I think Remco PA3FYM has left these parts out for simplicity but I wanted to build it back in.

Voltage regulated fine tuning

Furthermore to put the oscillator on frequency is a little challenging by itself. Since the original design almost never hit the 1500Hz band middle of the WSPR-band right away, you have to tune a little to get it right. Great, but how do you tune an oscillator that has no tuning options?
So I did build (with tips and hints from Remco PA3FYM) a voltage regulated add-on. As it turns out, you can re-engineer an 7805-voltage regulator to a variable regulator pretty easy. I did use this concept to design a voltage regulator for the whole design. I’m able to set the voltage between 11 to 14 volts which is enough to set the oscillator somewhere between 1400 and 1600Hz.


In the meantime I am able to reach in the top 20 of the WSPR-challenge. Consequently to get into the top 10 seems very hard! I need all the (WSPR)-ears I can get. As a result I took everything I could find (including Roeland PA3MET‘s Sony ICF-SW7600G short wave radio) to create as many WSPR-receivers as I could in addition to the home made WSPR-RX’s. But it is still not enough to reach the top. I need to come up with another plan.

New item in the shack, a birthday present: RaspberryPi-3



My latest acquisition to the shack is a RaspberryPi-3. A great birthday gift to get! I already owned two RaspberryPi-1’s and one RaspberryPi-2. One of the RaspberryPi-1’s is doing a great job to put out the telemetry of my weather station with Xastir. I wanted the RaspberryPi-3 put to work as a WSPR-decoder as extension on my latest project: the 30m WSPR-RX. Still working on tweaks for the RX though.


My first hunch putting an OS on the RaspberryPi-3 was to use a special light version of Ubuntu (Ubuntu Mate). Unfortunately WSJT-X (the software to decode WSPR-signals) as a Ubuntu package is pretty old (version 1.1 or something while the latest is 1.6). So I decided to compile from source. That turned out to be a little laborious since you need a lot of extra source code packages that all named different from the documentation. After an hour or so searching the source was compiling. When WSJT-X was ready it did run. But after a few starts it gave errors which I couldn’t fix. It drove the RaspberryPi-3 to a load of 100% and couldn’t recover. Back to square one. I changed Ubuntu Mate for Raspbian. And I was already compiling again from source when I stumbled upon this link. A simple PPA with the latest and the greatest of WSJT-X: version 1.7!


I did a few changes to the standard setup of Raspbian; since I run the RaspberryPi’s kind of headless, I installed tightvncserver. I just switched off the default lightdm and enabled the vncserver from boot.
Something else I did was adding WSJT-X as startup application. This way after a power failure or a spontaneous reboot, WSJT-X would start automatically again to pick up it’s task.
I still need to add a cronjob to check every 5 minutes or so if the program stil runs and if not: start it again.


After a few hours playing around with the new RaspberryPi-3 I very much like the better speed! The RaspberryPi-1 is just plain slow, the RaspberryPi-2 is a little better, but this latest edition is very good workable as all rounder in my shack. I put in a cheap Chinese USB-sound stick to decode the WSPR-signals from the WSPR-RX. It works problem-less. Booting is actually pretty fast and Raspbian is very usable. The RaspberryPi-3 isn’t getting warm or anything (you read about these errors lately) I run it on a good but simple Apple 5v/1A adapter which runs fine. Tonight I’ll do another adjusting session at the club’s lab of the WSPR-RX and then I’ll let this combination run for a few weeks.