Kenwood TM-721A Modifications

There were two modifications which I wanted to make to my newly acquired Kenwood TM-721A dual band radio. The main goal to replace and relocate the battery, as it is soldered to the front control board. And while I was in there, I’d enable crossband repeating, as the resistor R121 which needed to be removed is near the battery.

I had to remove twelve screws to get the top and bottom panels off. Then six more screws to remove the front panel. Then I could swing the front panel down, unlike my Kenwood TM-241A where I had to remove all the knobs and locknuts in the front.

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I found the battery, but the R121 resistor (which was supposed to be to the lower left of the battery) that I needed to remove to enable the crossband repeating was gone! Why didn’t I think to check to see if crossband repeating would work BEFORE I opened up the radio? I knew this thing had been taken apart before, because I bought it with the tone decoder and display upgrade already installed!

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Well, since I’m in here anyway, might as well remove the old battery. It had 2.4V, still pretty decent, but the soldering iron was already warm, so….

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… I soldered the Radio Shack CR2032 battery holder to a piece of wire, and then soldered that wire to the board.

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Then I put the front panel back on, and I relocated the battery to the bottom compartment of the radio, as the top compartment is a lot more congested with the speaker. I used a Dremel tool to carve out a groove in the metal for the wire to fit as it left the front compartment and entered the bottom. And lots of electrical tape around the battery and its holder.

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I put it all back together, did a hard reset by holding the “F” button while turning the radio on, saved a couple of memory presets, turned it off, crossed my fingers, turned it back on, and … success, the presets stayed intact! I checked the crossband repeating function and that also worked!

Update 25 October 2012: I had moved the TM-721A to the car, so I just picked up a Kenwood TM-731A to replace it in the “shack”. I found a way to route the battery wire to the bottom of the radio, even with the thicker gauge wire I had lying around. The only difference was that I had to hold the “MR” button while turning the radio on in order to do a hard reset.

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