2 Meters for 3 Fish Suppers
Kenwood TM-241
A couple of months back I purchased a Kenwood TM-241E from a fellow member of the Stevenage Club. It was billed as having "The Kenwood problem" i.e. the display was intermittent. It seemed worth a "punt", as if the display could not be fixed, the PA might be useful.
Tested it out and all seemed to work well, but the display did occasionally fail. Interestingly I read that display problems are generally caused either by poor connections between the control and display boards, or by a low / expired backup battery causing the processor to get confused. I also found the service and operating manuals on the web. As the supplied operating manual was in German this would be handy for setting up memories etc.
Had fun struggling with the small Philips screws that hold on the front panel, so invested in another Philips screwdriver - £1.99 - Phew! It fitted correctly though, and the screws then came undone easily. Previous attempts using a "jewellers" screwdriver only succeeded in breaking the tip of the screwdriver!
Connectors and battery
Fiddly, but I got the control board and display apart, cleaned the connections, removed the soldered in CR2032 battery from the control board behind the display, and ran wires from the old battery connections back into the main body of the radio, as suggested on the web.
The original plan was to install a battery holder, but there was not much room, as the easy place to mount the battery was taken up by the optional tone board. Instead I just sandpapered each side of the battery, soldered the wires to it, and then wrapped the battery in some insulating tape.
After it was reassembled the on off switch did not work very well. Took the control board and display apart again and turned around the plastic "stick" that clips onto the switch, then it worked OK. Checked everything again and screwed it all back together. Now it held the last used frequency, memories etc, and the display worked. All it needed was a PSU.
PSU
A few months back I purchased a Tait T807-10 PSU from the Shefford Junk sale, knowing nothing of its "provenance" it was put to one side until required for a project.
The Tait turned out to be a bit of a find as it was originally used in a commercial repeater system and appeared to be working perfectly. Capable of 15amps it was perfect to power the Kenwood.
For ease of use it really needed binding posts fitted to replace the recessed + - screw terminals. Having removed the back plate there was space to mount some Maplins binding posts. The back panel was drilled with a hand drill; Oh for a pillar drill! Then the binding posts - which cost more than the PSU, were fitted; Never mind it was a good engineering exercise.
Carefully soldered up wires to the + and - on the PCB cut a small section of the PCB away to allow the cables to reach the binding posts. Soldered and screwed everything back together and switched on, to be greeted by a loud clicking noise. A quick investigation followed; of course the binding posts are not insulated, so they where both grounded. Oh dear! Two pieces of heat shrink tubing where then used to insulate the positive binding post. Checked, reassembled and tested OK.
Testing the Kenwood
More testing at Toms; it still held the memories and had not yet lost its display. It drew 9 to 10amps on transmit and seemed to be working well.
It has been hooked up at home for further testing and monitoring. There
is a surprising amount of activity on 2m.
Kenwood TM-241 Operator Manual
2M SWR Meter
The station was now missing an antenna. There are many options for simple home brew 2m antennas. But at a minimum a SWR meter is needed to set them up.
The value of SWR meters can be discussed at length, but this particular SWR meter was valued at just 50p by the car booter who sold it to me.
Although originally "designed" for CB (11M) it worked on 2m using 5 watts and a quarter turn of the calibration control.
As the voltage measured increases with frequency a larger calibration resistance would ideally be required for 2m, and almost certainly for 70cms. The "accuracy" of this meter will at best be limited, but it is a guide to help preserve the radio's PA.
With some careful rebending of the case it actually fitted together properly and was then throughly cleaned.
Many improvements are doubtless possible for Mk 2.
2M Antenna
Constructed a half wave wire vertical for 2m using 19 inches of copper wire soldered into the centre of a PL259 socket, insulation was then added to stop the wire shorting against the body of connector.
I checked it with the clubs MFJ SWR analyser; trimmed it slightly, then gave it an on air test. Signal reports and reception were slightly down on the “Full Wave” car antenna, but usable. Two half wave radial wires were then added, which reduced the SWR slightly further. It also seemed to work on 70cm accessing the local repeater with no problem and no smoke. SWR "measured" with the 2M SWR meter was 1:1.1 on 2m and 1:1.75 on 70cm.
Ideally a separate antenna is needed for 70cms as using a simple 2m antenna tends to mangle the radiation pattern; although hardly critical for this application.
Completion
The 2 Meters for 3 Fish Suppers Station is now being successfully used by M3NQG.