Friday, February 11, 2011

Replacing And Relocating The Battery In A Kenwood TM-721

Not a tutorial, just a pictorial. I might put step-by-step notes in at a later date.

I will note that the knobs don’t have to be removed. I highly recommend you leave the main faceplate on unless you absolutely need to dig around in there, i.e. lamp bulb replacement. Those little buttons are hard to get back in once they fall out.

Note: After completing the work, please look for anything that might possibly short-out and tape it up before applying power.

(Click on pictures to enlarge)

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The top and bottom covers must be removed in order to swing down the head unit. The battery is in the front of the radio behind the LCD display.

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There are 6 screws that must be loosened or removed in order to swing down the head. Don’t forget the ones in the middle on the top and bottom… Leave the lower two screws on each side in (you’ll know which ones when you look at it) so the head only swings down – providing strain relief for several cables and connectors.

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Battery was soldered where the electrical tape has been placed. The positive connection is on the right –closest to the white ribbon cable.

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The positive lead is closest to the white ribbon cable.

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New battery will be placed in the main compartment of the radio.

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Don’t forget to wrap all this up with tape so it doesn’t short out against other components in the radio.

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Wire routed to where the new battery will be placed. This is a tight fit so don’t use too heavy a gauge of wire… take your time and keep poking the wire with your finger to keep it in place when you swing the head unit back in place.

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3 Volt battery from Radio Shack, etc.

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Checking for voltage at the board before taping it all up.

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It’s a tight fit but you can fit it all back together. I wish I had found a smaller 3 Volt battery and holder. The top cover houses the speaker and it’s a really tight fit. It took me a while to figure out how to fit the new battery among the electrolytic Caps.

Battery relocated to main body of radio. Take your time as the speaker makes it a tight fit. Of course I checked for stray capacitance, etc., after doing the replacement… Smile

Note… it worked at first but then lost memories again after an hour. Checked the battery, it was down to 1.5 V. Re-taped all connections again and entered some freqs. Two hours after turning the radio off, the mems were still there. I must have shorted the positive side of the battery to ground somehow in the first go-round and partially drained it. At least it will be a lot easier to replace when/if needed.

Note: After being shut off and not powered up for 4 days, the TM-721 still had it’s memories intact when powered on. Nice.

Note: Just completed battery replacement on a TM-731. This one went about like the first one. The one hang-up was one of the battery wires breaking loose from the original battery leads when I swung the head unit back up. Solution: a new arrangement in routing the battery wires and resoldering to the leads. Afterwards, the memories are being retained after power is removed. Good deal.

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TM-731A – same fix… and it worked!

Now I’ve got another 731A to perform the battery replacement AND the lamp replacement on…

Note- Decided not to tackle the lamp replacement at this time as the radio is used in a well-lighted area… maybe next month.

However, the previous owner had already replaced the original battery in a similar manner – except they routed the new holder and battery to the bottom section of the radio. Much more room!

I did have to replace the holder as the one there wasn’t quite right for the 3 Volt batteries I had purchased for this project. However, it did make for an easier install and I thank whoever did the mod.

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ps. It seems that some Kenwoods of the TM-721/731 vintage on occasion “lose their minds” and won’t hold memory regardless of the condition of the battery. I think I have had one of these radios in the past, but I’m not sure. Regardless, if you have a radio that doesn’t seem to hold memory even with a good battery, you might search around on the Net for a mod concerning this.


JB said...


Thanks for the article. It helped me replace the battery in my 721A.

I first tried soldering leads to the replacement battery but it exploded (had I not been wearing eye protection I would not be typing this now).

Plan B (no holder available): tightly wrap leads to battery using "Rescue Tape" to keep everything in place. It's highly elastic, bonds on contact, and a great insulator.

Located the new battery in the bottom of the radio - as you say, plenty of room.

Well, so far so good! Thanks again & 73,


Paul-e-wogg said...

Thanks for the write-up. I've fixed a few Kenwoods like this to great success. Too bad Kenwood didn't think to make it easy!

On the notion of the radio "losing it's mind"... I think you are thinking about some Icom radios. There was a whole series of those that could basically lose their firmware if the battery died. It's why I will never own an Icom (stupid engineering and "they always break").

And on replacing watch batteries and the like and then having them go prematurely dead... many people don't realize that a little oils from your skin, bridging that tiny gap between battery poles can cause enough current bleed to make them drain in short order. It's why you should only handle the new ones while wearing latex or similar gloves. That said, I am guessing that you might have very briefly shorted your new battery while installing it in the holder. That might explain a quick drop in voltage that then sustained itself.

I'm always remember to wear the gloves and fixate on not letting the battery get shorted even for a brief instant. I've thrown away brand new batteries that I glitched like that. Just better off starting fresh and doing it right. They are not expensive if you search Amazon for the best deal and buy a lot of 10 or more. You'll need them for something else.

hugues said...

hello, probleme lcd no light
the numbers no visible....
thank's for the contact