Repairing 'No RF Output' on a Kenwood TKR750 VHF Repeater

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Back to Home Repairing 'No RF Output' on a
Kenwood TKR750 VHF Repeater
Submitted by Henry Clark II KC4KZT
Turned into an article by Robert W. Meister WA1MIK

I found seven Kenwood TKR-750 repeaters that were used in an old coal mine for underground communications. I was surprised to find them on frequencies between 146.000 and 146.100 MHz. I did some research and found that there were several being used as repeaters, but I also read where they were notorious for having PA issues.

Out of the seven, three of them tuned up perfectly, but four of them had no RF output. After some digging with a magnifying glass, I located the problem that was plaguing the units. There is a choke (coil) located on the PA board that must have had bad solder from the factory. The coil or circuit board could also have gotten very hot and the solder softened enough to disconnect the coil from the pad on the circuit board. Silver-bearing high-temperature solder is recommended for this repair. This coil feeds power to the Final Power Amplifier transistor, so nothing else was damaged when this connection opened. A piece of the schematic with L7 indicated is shown below. Click on the image for a larger view.

I have attached a picture showing the culprit. You can clearly see where the coil L7 separated from the solder pad. I actually lifted it a little to give a clearer picture. If you didn't know where to look when you lost output power, you'd never see it. ALL had the same problem. After re-soldering the coil, all four units came back to life and tuned up perfectly. Click on the photo for a larger view.

Here's a photo of most of the PA board with a red tool pointing to L7. You can see the area near the bottom where it has been resoldered. Click on the photo for a larger view.

I thought this might be a good thing to put in the Kenwood section of the site. I'm sure there are many others lying around with this same problem.

Thanks for taking the time to read this, and I hope someone may get some use out of my findings.

Acknowledgements and Credits:

The schematic snippet came from the TKR750 technical manual.

The photos were taken by Henry Clark II KC4KZT.

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This page originally posted on 25-Aug-2014.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.