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Author Topic: gang capacitor markings  (Read 2765 times)
K5CQB
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Posts: 223




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« on: August 09, 2011, 09:54:33 PM »

The variable gang capacitor in my GDO is marked;

r7c273 6136 140
p16-302

Does this mean it is a 16pf to 302pf ?. 

Thanks,
Jim
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G3RZP
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Posts: 4477




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« Reply #1 on: August 10, 2011, 12:09:01 AM »

Maybe. Maybe not. What make of GDO? Does it look like it could be that capacity range (which seems a bit big for a wide range GDO)?
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K5CQB
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Posts: 223




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« Reply #2 on: August 10, 2011, 01:17:49 PM »

I am not familiar enough with them to guess at it's range, although it also sounded a bit wide to me.  The GDO is a PACO g-15.  I bought it without coils and have read that if I find the capacitors range I can determine the inductance needed and homebrew my coils.
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VK2TIL
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Posts: 318




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« Reply #3 on: August 10, 2011, 02:51:19 PM »

There is a manual on BAMA;

http://bama.edebris.com/manuals/paco/g15/

It shows that P16-302 is the Paco part number but gives no value.

I would guess that the "140" in the marking may be the capacitance; 100-150 pF is usual in a GDO.

If you have no way of measuring L or C you could wind an inductor of a certain L (there are several on-line inductance calculators which will give reasonable accuracy) and try it in your GDO while listening for a whistle in a receiver; it might take some fiddling but you should find a GDO dial setting that "whistles" at a frequency shown on the receiver dial.

For example; 50 pF (probably about half the GDO C) requires 1.2665 uH to resonate at 20 MHz (an on-line calculator tells me this).

Thirty turns of 0.4mm wire spread over one inch on a 0.25 inch diameter former is going to give us close to that (another on-line calculator).

Try something like that in the GDO and see how close you get to the calculated frequency.

If it "whistles" at, say, 23.165 MHz you can estimate the inductance you require to achieve correct dial readings; it's just proportion.

Once you have one range working you can calculate the max & min C of the capacitor (which will include some "stray" C) and design coils for all ranges.

I built a GDO many years ago;

http://imageshack.us/photo/my-images/850/photohqe.jpg/

The exercise of calculating & making the coils was very instructive.

Fiddly; yes, but you will know a great deal about resonance at the end!



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K5CQB
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Posts: 223




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« Reply #4 on: August 13, 2011, 09:15:02 PM »

Many thanks!  By the way that's a very nice looking GDO.

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KB9BVN
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Posts: 116




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« Reply #5 on: October 05, 2011, 05:04:12 PM »

Your Garage Door Opener has a varicap in it?

<grin>

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WB6BYU
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Posts: 13152




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« Reply #6 on: October 05, 2011, 07:59:49 PM »

Yes, just wind a coil of known inductance (the calculator here is a good one)
and then check the frequency of the GDO at each end of the dial.  From that you can figure out how
much inductance you need to cover each range.

A frequency counter is handy, but you can couple the GDO to a general coverage receiver and tune
around to find the signals.
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