Wednesday, December 22, 2010

Changes to 2m gear

I have had a bit of a change round in the shack recently. Since getting the Kenwood TM-D710 which is used for 2m FM which is the vast majority of my VHF activity, the Icom IC-910H has really been under-utilized. 70cm is a dead band here and there isn't too much 2m SSB activity. Besides, I never really liked the Icom. So I decided to swap it for an old Spectrum Communications transverter that I have and sell it in the new year.

The Kenwood has been moved into the "second rig" operating position. And the desk mic I used with the Icom is now attached to the Kenwood. I had some CAT5 UTP network cables which are terminated in the same type of plug Kenwood uses for a mic connector, so I cut one in half and made up a cable for the desk mic. Despite being unshielded (UTP stands for Unshielded Twisted Pair) there doesn't seem to be any RFI even at 50W. And the reports I have had so far suggest that it sounds good.

The Spectrum transverter runs about 20W output. This is less than I would like for SSB. My plan was to set the transverter drive so the peak output was 5W and use it to drive a 50W linear amp that I have. But when I set the power to 5W, as soon as I put the cover on the case it dropped to 1.5W. So I thought that perhaps the whole thing needs realignment.

Unfortunately it did not go well. The transverter, which I bought umpteenth-hand in a private sale some time ago, had obviously suffered the depredations of the ham-fisted twiddler. Two of the ferrite tuning cores in the Toko coils were cracked and could neither be adjusted nor removed so they could be replaced. Despite this, the transverter receives pretty well.

However, while trying to realign the transmit side to solve the problem of changing power output when the case cover goes on, I noticed that there was often power out when the key was up. There was clearly some instability present. Although I could adjust the trimmers so there was no unwanted output, I could not eliminate the changing output as the case cover is put on. I do not have the test equipment (a spectrum analyzer) to be sure that the output is clean and I started to have doubts about the whole thing.

I did not want to risk wiping out the neighbour's TV reception whenever I use 2m SSB. So I decided to scrap the transverter and order an XV144 internal transverter module for my K3. It is on its way, and hopefully will arrive in the UK before the increase in VAT takes effect.
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