Repairing a Kenwood TM-241

I purchased a defective Kenwood TM-241 mobile VHF transceiver a while ago. It transmitted and received as it should, but the display was garbled and rendered the radio practically useless. Some times it did not respond to button presses either. Finally, the backup battery was near its end of life. Repairing the unit was not too hard, and once again it works like a charm. Read on and see how I did it.

Garbled display on Kenwood TM-241

Background

Many pieces of equipment are known for their weaknesses. Kenwood TM-241 have a four of these, as far as I am aware of:

  • Defective display, showing random garbage.
  • Buttons not working.
  • Defective backup battery.
  • Defective output power module

The majority of reviews on Eham.net says that this is an alright radio in terms of usability, but it usually dies in one of the mentioned ways. Many say Kenwood has done a bad job with this one. It turns out these problems are easy to solve, making the radio as good as new.

Defective backup battery

Defective backup battery is something that eventually gets relevant on any unit containing one. Batteries do not last forever, but what happens when the battery dies varies. In some cases the radio starts acting weird, and in other cases only memory is affected. If you experience that your radio is doing strange things, replacing the backup battery may be a good place to start.

I were not experiencing any of these things, but since I had to disassemble my radio I chose to replace the battery at the same time. Measuring low battery voltage after removing it implied that it was about time anyway.

Defective display and buttons

Inside the TM-241 are three connectors that cause malfunction on several units. Bad contact causes the display to flicker or display garbage, and may lead to buttons not working. Getting the radio back on track is a matter of re-establishing contact between the boards inside the unit.

I have read that some choose to replace the contacts with wire, but I successfully repaired the contacts in a fairly easy manner. Replacing them with wire would be a last resort for me.

Defective output power module

The output power stage has died on several of these radios. Replacing it is somewhat expensive, and those experiencing this usually replaces the entire radio. For unknown reasons, Kenwood has not applied thermal paste between the power module and the heat sink. When transmitting, the module heats up, and as it is unable to dump the heat, it eventually dies.

It is easy and cheap to prevent this failure, but if it has already been overheated it may not be saved. If you have a working unit, you should definitely consider applying thermal paste before it is too late.

Repairing the TM-241

Here is how I repaired my transceiver. Tools and equipment I needed was:

  • Narrow, flat headed screwdriver (approx. 2 mm)
  • Philips #0 screwdriver
  • Philips #1 screwdriver
  • Pliers
  • Thermal paste (optional but highly recommended)
  • Solder iron, solder containing flux
  • Electronic cleaner (spray)
  • Sewing needle

Disassembling the unit

Note: Follow these steps at your own risk.

Step 1 – removing buttons

Remove the VFO knob plus volume and squelch knobs. Pull the knobs straight out with your fingers.

Disassembling the front of TM-241, removing buttons

Step 2 – opening the housing

Remove the two screws near the heatsink.

TM-241 removing screws to open case

Pull up the rear end of the cover. Then pull the cover towards the heat sink to free the tabs securing the front end.

Opening the TM-241 housing

Step 3 – removing the plastic front cover

Both over and under the transceiver there are two tabs holding the front cover in place. Lift the tabs to free the cover and pull the cover straight forward. I find it easiest to loosen one side at a time, by loosening both the top and bottom tab and pulling the cover gently away from the main body. Be careful not to break off the tabs (like I did on mine, as shown in the picture).

Removing the plastic front cover on TM-241

Step 4 – removing the display

The display is held in place by two nuts; around the VFO knob and around the microphone connector. Use a pair of pliers to unscrew the round nut around the connector, but be careful not to damage the threads. Remove the nut and washer around the VFO knob. A wrench is more suited for this, but a pair of pliers works as well.

Removing the TM-241s display

Pull the display away from the main body. Wiggle it carefully to let the connector between the boards release. Be careful not to lose the buttons when removing the display. They are loosely held in place by a foam pad. The foam was decayed and fell apart in my unit. I did not replace it, but function of the buttons does not rely on it.

Step 5 – removing the metal frame

Beneath the display board there is a metal frame. This is held in place by four screws: One centered on top, one on the bottom and one on each side. Pull the frame away from the main body after the four screws have been removed.

Removing the internal metal display frame on TM-241

Step 6 – removing the inner control board

Two screws secure the inner control board. One is located above the “Call” button and the other goes through the transistor (?) between the volume and squelch knobs.

Unscrewing the TM-241 inner control board

Wiggle the board while pulling to release the connectors. Be careful not to bend the board too much, or it may break.

Removing the control board

Repairing connectors

On my unit it seemed like both of the connector sets caused trouble. All of them are the same age, so it is wise to have a closer look on all of them anyway.

Re-soldering connectors

Since there may be some vibration in a mobile unit, solder joints may break after some time. I chose to reflow the solder joints on the connectors in case any of them were cracked. I simply heated them with my iron and applied a little new solder containing flux.

Resoldering connectors on control board

Before flipping the radio over, it is wise to remove (or secure) the speaker. It is placed loose on a bracket and will fall out when you turn the radio.

Remove the speaker

Underneath the radio, on the main board the mating connectors are located. I reflowed these as well.

Resoldering connectors on TM-241 main board

Re-establishing contact between connectors

My main suspicion with respect to the faulty display was bad contact between the mating contacts. I flushed them a few times with electronic cleaner, letting them dry in between. To help resolve any oxidation or residue, I sprayed the contacts and then connected and disconnected them several times before letting them dry completely.

Re-tensioning the contacts on TM-241s control board with a needle

Using a sharp sewing needle, I bent the connector pins a bit inwards (towards the center). This makes the connection tighter, and reduces the risk of insufficient pressure on the contacts. Be very careful with this. Do not bend the pins too much, or the connectors may not mate correctly.

Re-tensioning the connector on the TM-241 display board

Changing backup battery

The backup battery was holding low voltage on my unit. I replaced it with an ordinary CR2032 battery from my local grocery store. A special battery with solder tabs should be used, but they are much more expensive. I simply soldered wires directly to the battery (while wearing safety glasses!). To avoid short circuit, I added som shrink tube around the battery. With long enough wires, the battery can be hidden inside the main body.

Kenwood TM-241 backup battery replacement

Applying thermal paste

When the unit was already open, I added som thermal paste to the power module. I do not know why Kenwood left it out, but many before me have experienced failure due to excessive heat.

The power module is held in place with two screws. I unscrewed those and carefully bent the module slightly away from the heatsink. Then it was simply a matter of applying thermal paste and replacing the screws.

Applying thermal paste to save the TM-241 power module

Assembling the unit

  1. Attach the control board. Make sure the connectors seat properly.
  2. Replace the two screws securing the control board.
  3. Attach the metal frame.
  4. Screw in the four screws securing the frame.
  5. Attach the display board. Make sure the connector seats properly and that all buttons are in place and moving freely.
  6. Replace the washer and nut over the memory select/VFO knob. Be sure not to over tighten.
  7. Replace the nut over the microphone connector. Be sure not to over tighten.
  8. Slide the plastic front cover back on. It should fasten with a snap.
  9. Push on the volume, squelch and VFO knobs.
  10. Replace the speaker bracket and speaker. Make sure all wires are well away and will not be pinched when replacing the cover.
  11. Replace the top cover. Screw in the two screws.
  12. Replace the bottom cover and replace the two screws.

Conclusion

Kenwood TM-241 working once again

My transceiver works again, just like new!

The repair cost me next to noting. All I needed to buy was thermal paste and a new backup battery (CR2032). Apart from that I used a little shrink tube, solder and electronic cleaner.

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