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Old 1st Mar 2015, 7:38 pm   #61
julie_m
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Default Re: MiniMod pantry transmitter

Quote:
Originally Posted by mark_in_manc View Post
Well, the breadboard experiment worked, as does the transistor, and having found my wiring error...it now works!
Just out of curiosity, what was your problem with your wiring?
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Old 1st Mar 2015, 7:47 pm   #62
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Default Re: MiniMod pantry transmitter

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Originally Posted by mark_in_manc View Post
Well, the breadboard experiment worked, as does the transistor, and having found my wiring error...it now works! Thanks for your help, folks - great fun, and I know a lot more than I would have had it worked straight away.

A couple of remaining questions, if anyone has the stamina:

Why use a FET in this application, and not a normal transistor?
I think FETs have a reputation of more stable internal electrode capacitance than transistors. For whatever the reason they are usually preferred for VFOs. Perhaps a semiconductor expert could elaborate?

Quote:
Why on AM does the signal seem to 'clean up' (whilst dropping in level somewhat) when one tunes to one side of the strongest signal? I don't mean in terms of distortion - more in terms of AF spectral balance where, tuned for maximum volume, the sound appears rather 'middly'.
If you are referring to the MiniMod signal in particular, it's because it has no significant audio bandwidth restriction. This is a function of the bandwidth of the output tuning in the antenna cct. This means that when you detune slightly on a radio, you get more of the higher frequencies from one sideband. If you have a receiver with a wide IF bandwidth this effect is much less noticeable.

It probably isn't as noticeable on 'real' AM stations because the transmitted audio bandwidth is restricted. It varies from station to station, but not usually more than about 5.5 kHz. (Theoretically no more that 4.5 kHz for 9 kHz channel spacing, but in practice daylight stations are no closer than three channels.)

Curious to know what the original problem turned out to be?

Ian
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Old 1st Mar 2015, 8:07 pm   #63
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Default Re: MiniMod pantry transmitter

Quote:
Curious to know what the original problem turned out to be?
Just a silly mistake with one of the legs of C2 resulting from getting confused over which way up the board was; rather than tagging onto the FET gate along with the diode and R1, I managed to attach it to the drain end of R3

Thanks for your explanation regarding de-tuning. I once learnt some maths for all that, a rather long time ago - I must try to find it.
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Old 1st Mar 2015, 8:12 pm   #64
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Default Re: MiniMod pantry transmitter

The Mini-Mod is quite sensitive, and the two I have built only require quite small amounts of audio input to produce a fully-modulated signal. Do make sure you're not over-driving it!
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Old 1st Mar 2015, 10:53 pm   #65
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Default Re: MiniMod pantry transmitter

Distortion in the AF bit of the circuit I understand and can handle - I'm just an RF numptey!
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Old 5th Mar 2015, 10:18 pm   #66
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Default Re: MiniMod pantry transmitter

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The Mini-Mod is quite sensitive, and the two I have built only require quite small amounts of audio input to produce a fully-modulated signal. Do make sure you're not over-driving it!
I was hoping to use it to broadcast whatever was happening on the front room stereo (via the headphone out on the amplifier), so as to be able to also listen in my kitchen some distance away.

For appropriate modulation at normal listening levels in the front room, I've ended up with a switchable input attenuation, where the normal 27R resistors get swapped out for 10k. Even then I could go a bit higher - 20k might be more like it. I guess it would be better practice to build-in switchable gain on the op-amp...perhaps I'll get around to this.
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Old 6th Mar 2015, 2:18 pm   #67
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Default Re: MiniMod pantry transmitter

The AF input of the MiniMod was designed to match the headphone output of an iPod/Walkman type player. I think if the 27R and 100R resistors are removed you could go it at C12 with phono-level inputs.

But you'd still need a reasonable way of adding L+R (just joining them up isn't really how it should be done - even on something simple like this!), and a pot could be used instead of the 10k with wiper to pin 3.

I don't think I tried that so not sure how well it would work. But in principle it seems OK.

(I hope to have some more ready-built MiniMods available in a few weeks.)

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Old 6th Mar 2015, 3:58 pm   #68
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Default Re: MiniMod pantry transmitter

Here's how I build mine. It's happily works with anything I've tried.

- Joe
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Old 29th Mar 2015, 2:47 pm   #69
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Default Re: MiniMod pantry transmitter

Well, here are some photos - I'm having great fun with this! The board is mounted in an old B&K measurement mic box, which reminds me of a former life.

Also pictured is my 'aerial' - a previously somewhat superfluous ornament belonging to the wife, which has found a new use. In that previous life, I might have been able to write some code to calculate its directivity

I have a VHF61 in the attic with a long wire on it which picks this up (from ground floor) very well. A Cossor 501 in the next room really struggled - but then it struck me that some RF power might be picked up by the radiator in the front room. So I connected the 501 aerial to the radiator in its own room - and great reception appeared.

This makes me think there might be something iffy with the earth-bonding of my central heating...if that turns out to be the case, perhaps I can earth it through a big choke!

I hadn't realised how little time we spend in the front room (with the telly off) - and how much more music I'd listen to, with this available. It's really got me listening to music again.

Now to acquire a few more radios
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Old 29th Mar 2015, 3:27 pm   #70
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Default Re: MiniMod pantry transmitter

Nice job Mark. That is definitely an unusual aerial.

Colin
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Old 29th Mar 2015, 9:02 pm   #71
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

Yes, nice to see these little modulators in their completed housings.

The attached picture shows my second MiniMod, concealed inside the cabinet of a pre-war home-made piece of test gear that I picked up from last year's NVCF. It's beneath the Avo case. I was able to re-range the voltmeter to function as a battery voltage indicator, and used the 0-25mA meter to display the current drawn by the MiniMod. There's enough space left inside the case to house an auto-reverse cassette player, more recently replaced by a tiny cheap MP3 player, plus half a dozen D-cells in a holder to create one completely self-contained 1939 pantry transmitter.

I fitted two aerial outputs, using the terminals already present on the front panel. One is the MiniMod output direct, to which is attached the usual throw-out wire to permit general reception in the workshop, and the second is loosely coupled to the main output via a capacitor of a few pF formed by a couple of inches of insulated wire, twisted together. This output can be plugged straight into the aerial socket of a receiver under test without fear of overloading. Since I built this, we haven't had to use the Avo signal generator!

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Old 30th Mar 2015, 8:14 pm   #72
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Default Re: MiniMod pantry transmitter

hi Phil - very nice!

Driving mine with the hi-fi headphone out means I can mess about with bass and treble to optimise whatever is coming out of the radio - so long as I don't run out of modulation depth by going for too much bass, for example. I've found good results with a fair bit of bass and treble boost which compensates for a normally middly performance.

Reading up a little more on the LM386 (TI application notes) it is suggested that omitting the cap between pins 1 and 8 will cut gain from 46dB to 20dB (which might be OK for me, with a large signal source) and then using a series RC combination in feedback from pin 5 to pin 1 will give a (presumably 1st order) bass boost. By extension I guess I could parallel this with appropriate LR feedback to also get a top-boost - without adding more op-amps and draining the battery quicker. Any reason this might not work?
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Old 30th Mar 2015, 10:35 pm   #73
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Default Re: MiniMod pantry transmitter

The low-ish power output from an MP3-type player headphone jack does allow the frequency response to be changed if the player has that facility.

Most radios need a bit more treble in my experience to compensate for the IF bandwidth restriction, and more bass too if it's a portable with a small speaker.

The AF response of the MiniMod is flat from about 75 Hz to about 8 or 9 kHz from memory. It rolls off at the high end due to the the Q of the antenna tuned circuit. (I suppose you could put a 100k resistor across that and see if it does anything useful.)

I also chose the LM386 because it will happily work into loads higher than an 8 ohm speaker.

The gain can indeed be changed by removing C9 or putting a resistor in series with it.

If you have a phono output, putting it in at C12 via a series 1k (say) resistor in left and right and changing R10 to a pot (wiper to pin 3) should work. Trouble is phono outputs seem to vary so much, it may work on some bits of kit, but be too weak on others. That's one reason why I kept it simple, designing it as I did - but experimenting won't do any harm.

I'll be making some more in a few weeks.

Ian

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Old 3rd Jun 2015, 10:52 am   #74
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Default Re: MiniMod pantry transmitter

Sorry to dredge up this old thread, but I skimmed over it a bit at the time, but now have a situation where having a small AM modulator might be handy.

I'm also about to send another order in to element14 so it's an ideal time to get the appropriate parts!

Can anyone tell me where I get buy the circuit? I see it's in a BVWS bulletin, but I can't see any way to buy it (in electronic form or otherwise) on their website.

Also, is there a standard equivalent to the coils available from the likes of element14, or am I going to have to buy them in from the UK?
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Old 3rd Jun 2015, 11:35 am   #75
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Default Re: MiniMod pantry transmitter

Andrew, given the more relaxed regulatory attitude to microtransmitters in NZ it might be worth going for something with a bit more grunt. The Minimod is deliberately designed to be a very low output device.
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Old 3rd Jun 2015, 1:10 pm   #76
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Default Re: MiniMod pantry transmitter

Andrew,

If a more powerful transmitter is an option for you, I can recommend this:

http://www.antiqueradios.com/forums/...?f=12&t=193042

I built one, and it works very well.

Regards,

Colin
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Old 3rd Jun 2015, 9:47 pm   #77
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Default Re: MiniMod pantry transmitter

That looks interesting, Colin, but as the clock oscillator is basically generating a 1MHz square wave and with only a single tuned circuit following it, does the circuit put out a lot of harmonics?
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Old 3rd Jun 2015, 10:33 pm   #78
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Default Re: MiniMod pantry transmitter

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Originally Posted by paulsherwin View Post
Andrew, given the more relaxed regulatory attitude to microtransmitters in NZ it might be worth going for something with a bit more grunt. The Minimod is deliberately designed to be a very low output device.
The more relaxed attitude is really towards FM microtransmitters. Usage of unlicensed devices is covered by General User Radio Licenses (GURLs), with a specific one for low power FM broadcasting (up to 0 dBW EIRP on specific frequencies officially in the guard band) and one for short range devices which includes allowances for -50 dBW EIRP across the FM band for audio senders and -16 dBW from 107.0 to 108.0 MHz with no specified use.

Power isn't too important in this application - I'm mainly thinking of something I can loan to the guy who operates Cave FM to run a few of the vintage radios he's collecting with his own output. I have a 70's era BE broadcast limiter with an AM/Asymmetric setting which I can run from a clean feed to provide input to a modulator like this, and it only has to cover a room. Worst case I build a couple.

I liked the look of this unit because it's tuneable, has good reviews here, is solid state, and it looks like it'll fit in a cheap Jaycar "jiffy" box with no problem. The only downside seems to be getting the circuit, and the coils potentially only being available from the UK - but they are cheap enough, so I'd just order enough for a few more of these so that the postage per unit is hopefully a little more reasonable!

If there's an alternative, proven design that's similar then I'd be happy to try it instead . I've got an order going to element14 today, and from the initial photo and what I've read here I'll throw a couple of LM386s and 2N3819s into the order, both parts are cheap enough!

Quote:
Originally Posted by Colin Ames View Post
If a more powerful transmitter is an option for you, I can recommend this:
http://www.antiqueradios.com/forums/...?f=12&t=193042
I built one, and it works very well.
I've seen either that one, or very similar, before. I think I'd prefer to go for something where the frequency can be tweaked - certainly 1 MHz will be a problem here, with Southern Star on 1026. It may work in Gore, but I think something that's a little more agile would be useful.

Frequencies I need to avoid are:
* 558 - Radio Sport (Dacre)
* 720 - RNZ National (Dacre)
* 864 - Newstalk ZB (Dacre)
* 1026 - Southern Star (Tussock Creek)
* 1224 - BSport (Kennington)
* 1314 - Parliament/Southern Star (Dacre)
* 1404 - NZ's Rhema (Tussock Creek)

Kennington and Tussock Creek aren't that strong into Gore, so I thought something around 1100 would work for there.
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Old 4th Jun 2015, 12:46 am   #79
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Default Re: MiniMod pantry transmitter

I didn't mean to imply that there was anything wrong with the Minimod, I was simply pointing out that it has limited range.

The problem with all simple tunable microtransmitters is that they tend to drift. A crystal controlled one won't drift, but you obviously need to choose an appropriate frequency.

1MHz would seem to be OK for the list of frequencies you give, though presumably it isn't a standard broadcast frequency (I think NZ uses 9kHz spacing like most of the world outside the Americas).
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Old 4th Jun 2015, 12:53 am   #80
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Default Re: MiniMod pantry transmitter

Phil,

I wish that I was knowledgeable enough to answer your question about harmonics. Sadly, I am not. Sorry.

Regards,

Colin
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