Amputating a TM-241's heatsink   

Many years ago I purchased a Kenwood TM-241 VHF mobile transceiver. The reason why I chose this specific model is a simple one: My car came with an ashtray built into the dashboard! I mean, what good is an ashtray to a non-smoking driver who is also a radio amateur? Certainly that space would be better used for a radio than for a silly ashtray! And the TM-241 has just the right width and height to fill the gaping hole left after removing the ashtray.
The only problem was that the radio was a bit too deep. It didn't fit entirely in the ashtray bay. That wasn't a big problem, though: I simply let its front stick out of the dashboard. It protruded about one centimeter, plus the length of the knobs. I lived happily with that arrangement for many years. It made a good combo with the TS-50 installed under the dash.

But then came a change in my life, I moved, and the car's role changed from being mainly a vacationing machine to that of basic transportation. I had always avoided parking the car in a street or any other unsecure place, because unfortunately here in Chile theft is rampant, and a car with radios in it exerts an irresistible attraction on any thief who happens to walk by. So, sadly, I had to remove the radios from the car, re-installing them only temporarily for major trips.

But I  dearly missed having at least a VHF radio in the car. So I went shopping for a smaller (shorter) radio, that would fit entirely in the ashtray bay, so that a cover could be placed over it to keep it out of thieves's lusty eyes. The Yaesu FT-1900 comes very close to that goal, but misses it by a few millimeters!  And I couldn't find any smaller radio. Another good option would have been a VHF radio having a detachable front panel, but alas, there doesn't seem to be any. All those that have detachable front panels are dual banders, and far too expensive, considering that I have no use for UHF, because I would be lonely on that band here...

So I decided to do an evil deed, unthinkable to most hams, cruel, sanguinary, and awfully gory: I would prune my existing TM-241 down to size, so that it would fit the available space!
After lots of measuring, I  determined the exact amount of heat sink that had to be cut off, so that the knobs of the radio would end up recessed just enough in the ashtray bay to allow placing a cover on it. Here you can see my poor radio, knobs against a board, with a line scratched into its heatsink. Note that the fins of the right third need to be cut a little shorter than the others, because there is an obstruction in the ashtray bay on that side.

The poor radio was shivering from fear!

All surgery requires some draping to keep the dirt out, right? I wrapped my radio in old paper (an IC pinout, and a schematic diagram), taped on with masking tape. This is to keep the aluminium sawdust from flying into the radio through every little opening it can find!

The tape also holds the cables out of harm's way.

By this time, my radio was seriously considering the possibility of running away, to escape surgery! So I had to restrain it on the operating table.
I don't have any anesthesia for radios, and this increases the need for very good restraining, or the radio WILL run away in the midst of surgery! It's a little like pulling a tooth in the middle ages.

This is the most delicate surgical instrument I found, that would do the task You surely know, an amputation without anesthesia hurts less if done quickly, so a powerful saw with a sharp blade is best.

I will spare you the gory scene of aluminium dust flying all around the workshop, and the poor little radio shrieking in desperation!

The rear 40% of the heatsink came off in one piece. Then I cut off small slices of the rightmost several fins. I then rescued the poor little radio, even smaller now than ever, from its surgical draping. It was still shocked, but soon recovered.

This is a 50 watt radio, and the output module is only about 50% efficient. That means at least 50 watts of heat is put into the little heatsink stub that remains, during full power transmission. The original specs state a maximum TX duty cycle of only 25%. It's clear that after cutting off half of the heatsink, this has to be degraded further.

On the other hand, at most times it's fine to use low power. At 5 watts output, the module is awfully inefficient, but still the dissipation decreases to about 15 watts. So I think that the radio is still safe to use for pretty long transmissions and high duty cycle at 5 watts output, but when using 50 watts the duty cycle will have to be kept VERY low. Other than that, there should be no effect from the heatsink amputation.

Many if not all TM-241 transceivers were delivered with the output module mounted dry, without heat conductive compound! I added a thin layer of this stuff, which is very easy to do, and that should slightly attenuate the effects of the loss of heatsink area. What counts, after all, is the total thermal resistance from the power transistor junctions to the air.
The radio has been reinstalled in its rightful place, well recessed in the ashtray bay of my car. It's easy to reach, easy to see, and well protected. The area where the radio sits also houses the air conditioner, so it is very cool there whenever the air conditioner is running. That should help in keeping the radio safe even with that small remaining heatsink stub.

When I need to park the car in dangerous places, which include basically any place where people freely stroll past, the cover is placed over the TM-241, and the front panel of the music radio is removed and taken away. That should help quite a bit in making thieves move on to the next car.

I can imagine that many hams would never ever do something as cruel as this to their radios. These are the same kind of people who will never ever drill a hole in the car's roof to mount an antenna! But I believe that technical goods are here to serve us, and we do not have any duty to preserve them in pristine, like-new condition. We have the right to adapt them to out needs. Let's face it: My TM-241 was almost worthless anyway. It's an old radio. You can buy a brand new FT-1900 for 150 dollars in many places of the world, or for 240 dollars here in Chile, where everything is more expensive than elsewhere. That radio has slightly better overall performance, 10 times as many memories, and many more features and functions than the old TM-241. So, a perfect, like-new TM-241 might be worth 60 dollars, perhaps. But mine had suffered several failures, which I fixed one after another. It's not a like-new radio. Also I replaced the power connector by a different type, to avoid needing different connectors for every radio. And I made several internal modifications to it, to suit my needs. So, it might have been worth 10 dollars... and I don't think it lost much of that fabulous value by amputating half of its heatsink! But to me that gory amputation means getting back on VHF mobile, without the constant fear that a car window will be destroyed to steal the radio.

And if a thief happened to steal my pruned TM-241, I would be eager to see how he manages to sell it!

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