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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Jul 2017, 10:02 pm   #21
julie_m
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Default Re: First Radio Build - 12V HT system

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Originally Posted by bobskie View Post
EDIT - the picture seems to go strange when opened, try double clicking to open in a new tab which works or if you are on mobile hold down and click open in new tab.
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This is something you can alter in your favourite graphics editor. Quick fix is just to save an image as .jpg, which does not support transparency -- though it may cause visible artifacts on line art, as the compression was designed for photographs. Try it and see.
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Old 15th Jul 2017, 10:39 pm   #22
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Default Re: First Radio Build - 12V HT system

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""Ok I think I have a better understanding of this now, in the circuit is one wire connected 1/4 of the way up the coil in order to provide a tap? If that's correct then I could adjust the coil to fall within the MW range? I will probably not bother with the ferrite rod if that's the case. Sorry I'm very new to all this but I'm sure I'll pick it up eventually.""

Yes the cathode is connected 25% up from the ground end of the coil in the schematic.

The coil in the schematic will not cover the MW band, here's why, the coils inductance is given as 5uH (5 microhenries) the tuning capacitor has a maximum value of 365pF (365 picofarads) the minimum value isn't given but it will be around 30pF or so when all's done, plug those values into a resonant frequency calculator like what's in the link below, the two different resonant frequency results for a capacitance of 365pF and a capacitance of 30pF will be the approx tuning range limits of the tuned circuit using the values given above, eg: approx 3.7MHz to 12.9MHz:

http://www.pronine.ca/lcf.htm

For MW you will need a larger inductance, to get an idea of what value you can use the same calculator, you already know the approx minimum and maximum capacitance values.

For a single layer air cored coil you can use the calculator in the link below, for instance 131 turns of 26 SWG enamelled wire close wound on a 35mm diameter former will give an inductance of approx 273uH which would put you in the ball park for MW coverage using the 365pF variable capacitor:

http://www.66pacific.com/calculators...alculator.aspx

Lawrence.
Ok so say I got a 500pf variable capacitor instead, would that work for MW? I would like to understand the calculations but I have never been good at theory or mathematics in general! I suppose the best way is to experiment with coils until I understand. I imagine the higher the maximum the capacitance the better though, within reason of course. I will try to find a variable capacitor with a higher value.

EDIT- Upon rereading your post i realised I was clicking the wrong link for the calculator, I have done the calculation now and it doesn't seem to make much difference having a higher capacitance. I will need to modify the coil it seems.

Last edited by bobskie; 15th Jul 2017 at 10:44 pm.
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Old 15th Jul 2017, 10:41 pm   #23
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Default Re: First Radio Build - 12V HT system

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Originally Posted by julie_m View Post
Quote:
Originally Posted by bobskie View Post
EDIT - the picture seems to go strange when opened, try double clicking to open in a new tab which works or if you are on mobile hold down and click open in new tab.
Your picture is on a transparent background. It shows up cleanly in the thumbnail, against the white background of the forum post. But the overlay window has a black background, which shows through the transparent background of your image leaving it unreadable. It becomes visible again, against the white background of a new browser tab (unless you changed the default colours in about:config).

This is something you can alter in your favourite graphics editor. Quick fix is just to save an image as .jpg, which does not support transparency -- though it may cause visible artifacts on line art, as the compression was designed for photographs. Try it and see.
I thought it must be something to do with the transparent background, i will get that changed when I get the chance.
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Old 16th Jul 2017, 7:36 am   #24
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Default Re: First Radio Build - 12V HT system

Re: Post#22, a 500pF variable capacitor will tune the MW band with a suitable coil, but not with the 5uH coil that's shown in the schematic, to get a graphical understanding of what's going on have a look at the nomograph in the picture below, draw a line between an inductance value and a capacitance value then read off the frequency scale the resonant frequency in kHz of that LC combination. In that pic micromicrofarads is the same as picofarads (pF)

Lawrence.
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Old 16th Jul 2017, 8:04 am   #25
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Default Re: First Radio Build - 12V HT system

Thanks for all the help I have learned a lot. That scale certainly cleared things up a bit. I feel a bit more confident in building the coil now but im sure I will be back here asking more questions when the time comes

Cameron
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Old 17th Jul 2017, 2:32 pm   #26
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Default Re: First Radio Build - 12V HT system

Im back again with another question, which resistor in the diagram is the volume pot? The regeneration control is marked as a normal resistor too and I can't work out which is volume. Thanks.
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Old 17th Jul 2017, 4:47 pm   #27
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Default Re: First Radio Build - 12V HT system

The 100k pot is the volume control, it needs to have a logarithmic (log) track.

The 50k pot is the regen control, a linear (lin) track would be the type to go for I would say.

Lawrence.
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Old 17th Jul 2017, 4:59 pm   #28
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Default Re: First Radio Build - 12V HT system

I wasn't even aware of the different types, fortunately I bought a linear 47k for the regeneration control. How do you tell the difference between the types? I have a couple lying around that may well be the correct value which I will need to test. I suppose it's safe to assume one used for volume would be logarithmic?

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Old 17th Jul 2017, 5:40 pm   #29
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Default Re: First Radio Build - 12V HT system

Sometimes it's printed or stamped on the case, Log, Lin, sometimes it's a letter code such as A or B, with those codes though there's no definite rule, in most cases A will be Log and B will be Lin but for whatever reason it can sometimes be the other way round.

If a pot isn't marked Lin or Log you can check which type it is by measuring the resistance between one end of the pots track and the pots wiper then the other end of its track and the wiper, both measurements taken with the pot's shaft set to the mid rotation position, if those two resistance measurements are in or roughly in agreement then it will be a Lin pot, if the two resistance measurements differ by a factor 9 (approx.) then it will be a Log or Anti log pot, Log pots are the most common type compared to Anti log pots.

For more info do a google image search for...... log lin pot curve ......plenty to look at and read about.

Yes, volume pot would be a log pot.

Lawrence.
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Old 17th Jul 2017, 5:42 pm   #30
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Default Re: First Radio Build - 12V HT system

Assuming all pots are either log or linear law (there are other laws, some quite exotic, but you are unlikely to come across them) :

Set the pot spindle to the mechanical mid postion.

Measure the resistance between the wiper (middle terminal ) and each of the outside 2 terminals.

If the 2 resistances are about the same (and equal to half the nominal resistance of the pot) then it's a linear law one. If they are wildly different it's log law.

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Old 17th Jul 2017, 5:52 pm   #31
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Default Re: First Radio Build - 12V HT system

Well I will need to measure those pots. I've learned so much and I haven't even started building the radio yet!
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Old 17th Jul 2017, 6:55 pm   #32
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Default Re: First Radio Build - 12V HT system

You don't "need" a log volume pot, an linear will work just as well , it will feel a bit different using it though.
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Old 17th Jul 2017, 8:20 pm   #33
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Default Re: First Radio Build - 12V HT system

I have ordered up a logarithmic pot now the three I had were far lower than 100k.
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Old 17th Jul 2017, 11:41 pm   #34
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Default Re: First Radio Build - 12V HT system

ISTR about 50 turns on a typical ferrite rod will be about right for medium wave. Tap at about 8 turns from the "bottom".

You may not need an external aerial for local signals at all with the rod type coil.
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Old 18th Jul 2017, 12:21 am   #35
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Default Re: First Radio Build - 12V HT system

Hi,
Nice to see someone having a go at building a set from scratch. Takes me back to my youth and building projects out of Practical Wireless.
Paul.
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Old 18th Jul 2017, 11:00 am   #36
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Default Re: First Radio Build - 12V HT system

I will need to experiment with different coils, it would be handy to have reception without an external antenna but ferrite rods can vary so much for the calculation. It should be interesting trying to get that wire wrapped correctly. Does anyone have a good guide on doing it and how to create a tap etc? I can't seem to find a clear explanation.

Cameron
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Old 18th Jul 2017, 12:30 pm   #37
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Default Re: First Radio Build - 12V HT system

Simplest- get hold of some wirewrap insulated wire or even the thinnest PVC insulated wire you can get hold of. Wind on 8 turns, form a loop of whatever length you may need to reach its connection point in your build by twisting about half an inch together, then wind on the rest. Secure the coil on the rod using hot melt glue, ty-raps or even just sticky tape. leave the top tail long enough to add turns if needed for the right frequency coverage, removing a few is easy anyway.
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Old 18th Jul 2017, 2:28 pm   #38
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Default Re: First Radio Build - 12V HT system

Sounds simple enough though I expect it will be a bit harder in practice. I have bought some enamelled wire I'm just awaiting it's arrival. Thanks for the help
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Old 18th Jul 2017, 4:17 pm   #39
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Default Re: First Radio Build - 12V HT system

Enamelled wire? That's just gilding the lily


Just the job, really. You'll have to use a varnished bog roll core as a former though
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Old 20th Jul 2017, 1:11 pm   #40
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Default Re: First Radio Build - 12V HT system

Today I started assembling the circuit board(a piece of wood with nails in it). I notice that in the diagram the regeneration pot appears to have both connections on the outside legs. Is this correct? I can't see how it would function like that. Am I also right in saying that all three pins of the 100k pot are being used to create a voltage divider?

Cameron
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