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Author Topic: Kenwood mobile TM-281a restarts when keyed  (Read 204 times)
KJ4RWH
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« on: July 10, 2019, 06:05:32 AM »

I have an on & off problem that has returned and would like your input. I installed a TM-281a in my Ford Van. I ran a 8awg power drop from the battery post to a barrier strip behind the driver's seat and then ran 12awg up and over to the radio. Grounds are made through the sheet metal with self tapping screw and star washers. The radio is attached to a Tram base loaded antenna which shows a beautiful match thru-out the 2 meter band (MFJ-259b). It worked like this for over 2 years. I have High power selected for most all the memorized channels.
Recently when I key the radio it resets the power as if the 12v was lost for an instant. Low power does not cause this problem. I've been told that it could be RF coming in the power leads so I added 2 snap on chokes with the power leads looped through twice. This worked for a couple of minutes and the problem returned 100% of the time
I once had the same model of radio inside as a base unit and had to add B+ chokes for the same thing. What do you think?
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KA2ODP
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« Reply #1 on: July 10, 2019, 09:39:55 AM »

Check your ground connection.  Did you use a self-tapping screw through the floor?  I have seen poor radio technicians do this as a speedy way to install a 2-way radio. The screw is literally sticking through the floorboard and is exposed to the outside world.  Works fine for a while, but eventually the exposure to rain and road slush causes the screw to rust.  The rust wicks its way up the screw threads to where the threads contact the sheet metal. Once that area gets rusty the connection is no longer 100%.  The positive side connection is still good, but a bad ground connection will cause power problems as well.  The radio draws less amperage when transmitting at low power, so the flaky ground connection will work OK at 5 watts but not at 50 watts when you are drawing around 13 amps during transmit.  Check your ground connection and make sure it is still clean and secure.  If the sheet metal is painted, use sandpaper to remove the paint around the hole so the star washer can bite into bare metal.  Otherwise, counting on the small contact area between the self-tapping screw threads and the thin vehicle sheet metal can fail to provide a good enough connection for higher power current draw.
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WA3SKN
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« Reply #2 on: July 10, 2019, 02:33:54 PM »

Does it do it with a dummy load?
Yes,,, its a power problem.
No... it is RF feedback getting into the rig, check antenna and feedline, grounding, etc.
73s.

-Mike.
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M0GVZ
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Posts: 388




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« Reply #3 on: July 11, 2019, 08:12:56 AM »

Fuse holders are usually the main culprit due to corrosion building up on the contacts. Try taking out the fuses then replacing them again.
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