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homebrewPosted - 13 November 2012 9:29  Show Profile  Email Poster  Edit Message  
I cannot tell much from the pic. The new set looks like a lot finer wire close spaced compared to your oatbox. That will give you lower Q, more resistance and losses. And probably lower volume with a bit more selectivity.
Go back to the original oatbox set and work on the antenna. Laying on the roof and tied directly to a vent will lower the signal level of the higher frequencies more than the ones at the bottom of the band.
Changing the antenna length with a good ground will have a very small effect on tuning compared to the coil taps. The longer antenna means more signal strength.
Assuming the variable is typical of it's kind and lower capacitance than needed for the tank circuit, put it in series with the antenna. It will not give you complete tuning control but may give you a different range and should make the tap adjustments more effective.
Garry NicholsPosted - 13 November 2012 12:8  Show Profile  Email Poster  Edit Message
Thanks for the links, T. No photos will show for me on the cap link, but I can see the specs. Looks like it is a two section cap with a minimum of 20 pF for each section, and a maximum of 59.2pf and 141.6 pF per section.

If you can figure out which terminals to use, you can wire the two sections in parallel to get a range of 40-200.8 pF.

After you get some experience, you may want to use the smaller section by itself to tune the top end of the band. The small range will spread the stations out.

201 pF minus 40 pF gives a range of 161 pF which will not likely cover the band. But by changing the amount of coil the cap is put across you can cover the band.

The oatbox coil is likely better than the coil that came with the kit. Do you know what size wire the oatbox is wound with? Is it plastic insulated or does it have a thin film coating?


TeamongerPosted - 15 November 2012 19:59  Show Profile  Email Poster  Edit Message  
Okay, I got my antenna suspended above the roof. Well it still touches at the edge, but the end is tied with rope to a metal vent pipe about 4 feet high. Didn't help the Elenco kit radio much. I think KGO 810 is a bit clearer on the oatbox, but no additional selectivity. I'll disassemble the kit and bring the capacitor back. Garry, it has three prong connectors in a row, two of which were pre-wired. Not sure about sections. Maybe you can recommend a better cap I can buy online? I might try making the cardboard and foil thingee...
My oatbox is wired with 22 gauge hookup wire, plastic insulation.

I did try wacking the white earphone, and it came to life... at least, until it unexpectedly goes dead again. A re-wack brings it back, but the kit earphone is less fussy.

homebrewPosted - 16 November 2012 8:56  Show Profile  Email Poster  Edit Message  
Try a lower value bleeder resistor across the white earphone.
Garry NicholsPosted - 16 November 2012 10:9  Show Profile  Email Poster  Edit Message

As homebrew suggests, is there a resistor across the phones somewhere in the circuit? If so, give us the color bands on it, starting from the band that is closest to one end. We can see what the value is. (It may say in your documentation.)

Not sure what you mean by, " cardboard and foil thingee".

The midnight science site sells variables (the click at top right of this page, then Radio Parts off to the left). If you think that you will stay in this hobby, I recommend investing in the two section cap over the single section. It will be more versatile in the long run, and it has offset plates which will spread out the tuning a bit at the top of the band. It gets bunched together more with regular semi-circular plates.

The cap that you have likely has two sections as I have said in a previous email. One terminal is for one section, one for the other and the third is the common ground return for both.

If you study it carefully (and it has a clear case) you may be able to see which terminal goes to what.

If you have a digital volt meter with a capacitance range on it you could figure out what is what by measuring different terminal combinations.

With the prewiring, what terminal went to what in the radio circuit? Maybe we can figure it out.


Edited by - Garry Nichols on 11/16/2012 10:11:32 AM

TeamongerPosted - 16 November 2012 16:57  Show Profile  Email Poster  Edit Message  
Yes, I'm using a 47K resistor 1/4 watt from Radio Shack, as suggested at That's also where the cardboard and foil cap plans are, May try that at some point.

The variable cap has a clear case, but I can't make out much. The prewire is on the middle and right terminal, when looking from the bottom and reading the CBM-223. In the kit circuit, the cap went in parallel to the coil.

The white earphone is balky, but when it works it does give a clearer sound than the kit earphone, though they both appear of identical design. There is a Phillips screw on the back, thinking of opening it up...

Garry NicholsPosted - 17 November 2012 8:39  Show Profile  Email Poster  Edit Message

The little variable cap that you have is not necessarily a "bad" one for the setup that you are currently using. It has two problems that I can see.

One is how to wire it up with the sections in parallel.

The other is limited range compared to the most often used 365 pF variables.

But there is a work around for that -- using different amounts of coil to tune segments of the AM band. Probably only two changes would need to be made.

Here's something that you could try. IF the cap has a metal shaft and IF you have some sort of continuity device (a digital volt meter, older volt-ohm meter or even a battery, a light bulb to match it and some wire or test clips) you could try this. Make the metal shaft one contact for the continuity circuit. Then, try the other three terminals (disconnect all radio circuit wires running to them!) one at a time to see if you get continuity.

I'm thinking that the shaft is likely common with the ground terminal. If so, positive continuity will indicate a circuit to one and only one terminal. Label it as "ground" or "return". Then you have the option of wiring the other two together to get the greatest pF range for that little unit.

This test will not allow you to determine which of the remaining two terminals are the ones with the lower and higher capacity ranges, but if you tie them together it does not matter (at this point for the present circuit!).

Does that test make any sense? Is the shaft metal?

Make sure that you don't get the diode into the continuity circuit or you risk burning it out! If all wires are removed from the little variable, this should not be a problem.


Steve CassPosted - 17 November 2012 12:45  Show Profile  Email Poster  Edit Message  
Your variable capacitor is capable of approximately 28 to 228pF if you hook it up as follows:

1. Connect the two outside terminals together and use that as one terminal. The middle lug is the other terminal.

2. There are two trimmers located on the backside of the cap. They are mostly composed of brass. Adjust each one to maximum pF by turning the adjustment screws to expose the minimum amount of brass sheet metal.

Steve CassPosted - 17 November 2012 12:47  Show Profile  Email Poster  Edit Message  
Your variable capacitor is capable of approximately 28 to 228pF if you hook it up as follows:

1. Connect the two outside terminals together and use that as one terminal. The middle lug is the other terminal.

2. There are two trimmers located on the backside of the cap. They are mostly composed of brass. Adjust each one to maximum pF by turning the adjustment screws to expose the minimum amount of brass sheet metal.

Garry NicholsPosted - 18 November 2012 18:44  Show Profile  Email Poster  Edit Message
Thanks, Steve!


Are you OK with how to hook up the variable across the oatbox coil? I think that coil will be better than the one with the kit. Bigger wire size and spacing between the turns provided by the plastic insulation. The kit coil has a thin film coating which does not space the wires from each other very much.


TeamongerPosted - 21 November 2012 15:41  Show Profile  Email Poster  Edit Message  
If I'm to use the other terminal of the cap, might mean I'll have to break down and get a soldering iron (gasp!). Steve, I see two recessed brass screws on the bottom, are you saying the pF goes up by backing them out? yes Garry the shaft is metal, but I don't have any meters right now...

Yes the oatbox coil does perform better, although the kit coil wasn't bad when installed in its place. I sanded off one side and tried using my alligator clip as a slider, that didn't work as well as just clipping to the antenna lead.

I'm going to have less time to tinker for awhile, but I appreciated all the tips, thanks again guys.

Garry NicholsPosted - 21 November 2012 16:40  Show Profile  Email Poster  Edit Message
OK, Ray. If you buy an iron, don't get too big a one. I use a 25W for most things.

If you were holding onto the clip when experimenting with the slider arrangement you likely influenced the result with your body's conductivity.

You can use test clips instead of soldering. Just try to have the wires not too long between things.

Almost always, tightening trimmer screws increases capacity (more pF).

Check in again when able.


Steve CassPosted - 22 November 2012 10:57  Show Profile  Email Poster  Edit Message  
Team, To gain maximum capactiance, turn each trimmer so that the sheet metal brass forms a triangle with three straight sides. If one of the sides of the brass is rounded, you will loose capacitance of 40pF or so. Get yourself a miniscrewdriver and do some twisting to see what I'm talking about. It really is a very simple deal.The use of a good (expensive) LC meter takes the guesswork out of the equation and is a necessary tool if you are to be serious about this hobby.


_J_Posted - 28 November 2012 12:8  Show Profile  Email Poster  Edit Message
I often remove capacitor trimmer screws and the trimmer plate (sheet of brass) for several reasons:

1. Trimmers add the same capacitance to both max and min thereby substantially reducing the tuning range. The broadcast band is so wide (2 octaves) it is hard to get anything that will vary the reactance enough to cover it anyway, and reducing range is counterproductive.

2. If I need lower frequency, I often try to get there by increasing inductance, not by tightening trimmers.

3. Capacitance added by trimmers is very lossy.

John Davidson

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