Fixing FM transmitter cavity

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Fixing FM transmitter cavity

Postby Bob Reite » Mon Dec 04, 2017 5:56 pm

I made a reference to projects that "pay the bills around here". Here are photos of one of them. Part of the plate cavity of an Armstrong 3500 Watt FM transmitter got "tired". The finger stock for the sliding part of the plate tuning control started burning up, and went into a runaway condition, as the remaining fingers had to try to handle the RF current. Factory had a replacement sliding ring, but "good luck" on the center tube. I dare not try to run it with all the pitting, that would just burn up the new finger stock ring (the last one the factory had in stock) so time to fire up the lathe.


View of burned up part before machining.

OK, stepping back a bit, I did not have a bull nose center large enough to handle the 4" nominal tubing. No way I was going to try to turn this without both ends supported, so I had to make one out of a chunk of 6061 aluminum.


4 inch diameter 6061 blank


I needed to be able to turn the majority of the outside diameter, so the first order of business was to cut a hole, to allow clamping on an inside diameter.


Finally turn a slight taper by setting the compound side at 2.5 degrees.


All done!

I had to take about 0.010 off the part to get rid of the pitting, yeah, it burned not only through the silver plating, but the copper base coat as well. No pits and ready to go to the plating shop. For what it's being used for it does not need to have a mirror finish like a flute or silver serving dish.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Bob Reite
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Re: Fixing FM transmitter cavity

Postby Doug Coulter » Mon Dec 04, 2017 6:14 pm

Nice work, Bob. :D So few people understand that a big part of the game is the prep, often making the tools that then help you do the job.
Welding is easy...prep, not so much without specialized tools.
Painting ditto.
Machining, as above...
And so many other things.
Any software dev worth their salt has a truly decent setup. While I can and have write code on a piece of total crap, as long as it doesn't crash too often or take days to boot - of course I don't do that often or by choice.
It's a poor workman who complains about his tools. I always interpreted that as meaning at least in part, that a good workman takes care to have good tools, making them if there's no other way.

See above...and a lot of the rest of the board. Even though my own priority activity is fusion work - all this other stuff I post on is in support of that, and one could easily be excused for missing the fusion part entirely!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Doug Coulter
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Location: Floyd county, VA, USA

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