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Author Topic: Convert aerionautic amp for 70Mhz use.  (Read 4564 times)
LA9XNA
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Posts: 103




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« on: September 17, 2013, 01:02:32 PM »

I just got 4 transistor amps previous used on aeronautical frequencies.
It is a triple stage amp with 10 or 100 mW drive for full output.
The amps have 50w transistor on the final stage and do probably run in class C at 24V on the two final stages in the amp and probably 12V on the first stage.
Can anybody give me some advise about how to tune them for 70 Mhz.
I guess a start would be to bypass the output filter and add a variable cap in the output matching network of the transistor.

For testing I'll  probably use an attunator to keep the output on a low level.

I'm a bit uncertain on how to change the class C to a class AB and get the bias correct.

If anybody have done similar I'd be pleased to get any advice.
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W9GB
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Posts: 2597




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« Reply #1 on: September 17, 2013, 01:30:49 PM »

Start with the 4-meter (70 MHz) website.
http://www.70mhz.org/index.php?categoryid=1

70 MHz Low Pass Filter
http://www.70mhz.org/index.php?categoryid=6&p2_articleid=264
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G3RZP
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Posts: 4366




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« Reply #2 on: September 17, 2013, 02:06:39 PM »

Which aeronautical frequencies? There are aero frequencies at 75 MHz ( ILS outer beacons around 315 MHz, communications /VOR at 108 to 136 - Upper end depends where you are.
What frequencies are you referring to?
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LA9XNA
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Posts: 103




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« Reply #3 on: September 17, 2013, 08:49:15 PM »

It is in the 108-136 range
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G3RZP
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« Reply #4 on: September 18, 2013, 12:53:21 AM »

Quite a job then. You'll need to redesign the matching networks for the lower band: start with the transistor data - it is possible, but unlikely (depending on age) that the transistors are internally matched for the 108 - 136 MHz band, in which case it really is a job, and in any case, you may well  find it easier to start from scratch or follow a published design.
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KB1GTX
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Posts: 460




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« Reply #5 on: September 22, 2013, 11:59:44 AM »

If it's for the us aircraft band 108 - 136 , it's designed for AM so it's already biased unless
It's a high level modulated board.
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LA9XNA
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Posts: 103




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« Reply #6 on: September 22, 2013, 10:15:12 PM »

If it's for the us aircraft band 108 - 136 , it's designed for AM so it's already biased unless
It's a high level modulated board.
I was planing to redo the bias because it is probably class C. It will be biasd to Class AB or A depending on how linar it is.
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G3RZP
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Posts: 4366




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« Reply #7 on: September 23, 2013, 02:19:44 AM »

It might be linear or it might be Class C - depending on where the modulation was done. You need to start with the individual transistor data to redesign the coupling networks.
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