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Old 7th Oct 2013, 9:14 pm   #21
Ed_Dinning
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Default Re: MiniMod pantry transmitter

Hi Phil, a friend of mine "rebuilt" his transistor radio (MW & LW) on veroboard and had tremendous problems until he removed all surplus copper tracks and used a minimum distance layout.

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Old 7th Oct 2013, 9:23 pm   #22
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

Hi Ed. Yes, I can believe that, in a transistor radio with lots of gain at RF, IF and audio frequencies, but the Mini-Mod consists of a JFET Hartley oscillator, an integrated audio modulation amplifier with line-level input, and a bipolar modulator/PA with tuned collector load. There's also plenty of decoupling.

However, as you say it would be good practice to remove all unused copper tracks, and I'll bear this in mind if my finished circuit misbehaves. I'll report back in due course!
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Old 7th Oct 2013, 10:19 pm   #23
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Default Re: MiniMod pantry transmitter

Cut through the copper tracks at the end of each run, leaving the unwanted sections not connected to anything. Fit your soldering iron with a 2 mm. chisel tip, and run it back and forth along the unwanted track. This will soften the adhesive, allowing the copper to lift cleanly away from the Veroboard.
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Old 7th Oct 2013, 10:31 pm   #24
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

I was thinking of using (say) a 4mm or 5mm drill bit to remove the copper track, hole by hole
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Old 14th Oct 2013, 5:20 pm   #25
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Default Re: MiniMod pantry transmitter

The method AJS suggests works well.
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Old 14th Oct 2013, 6:26 pm   #26
David G4EBT
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Default Re: MiniMod pantry transmitter

Back in 1976 when commercial digital frequency counters were beyond the pocket of hobbyists, I built a 200 MHz counter using Veroboard for the logic boards and for the five nixie tube driver boards. I stripped all of the unwanted tracks off the boards by the method suggested - I cut through the tracks using a spot cutter, heated the strips with an iron, lifted them up at the end with a craft knife then pulled the tracks off with pliers.

The counter worked straight off, was (is) very stable, and is still working today. (It was from March 1976 Radcom). Though the counter is off-topic, the pics give an indication of the versatility and stability of strip-board at higher frequencies, albeit the pics show little of the tracksides of the boards, so I hope the pics have sufficient relevance to the Mini-Mod thread insofar as the suitabilty of strip-board is concerned.
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Old 14th Oct 2013, 7:38 pm   #27
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Default Re: MiniMod pantry transmitter

I built my MiniMod on unmodified strip board and it works fine.

For a spot-cutter I've got an ⅛" drill bit glued into a file handle.

- Joe
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Old 15th Oct 2013, 9:15 am   #28
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Default Re: MiniMod pantry transmitter

I would never be too happy at heating up srbp stripboard - probably quite safe on the
glass fibre version. For rf work, a version of stripboard with a ground plane (all copper)
on one side was made, but I don't see it today. If a copper track causes unwanted
coupling by capacitance it can be removed, but also consider connecting unused
tracks (or part tracks) to ground.
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Old 27th Nov 2013, 12:47 am   #29
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Default Re: MiniMod pantry transmitter

I finished building my second Mini-Mod - on Veroboard - last night, and it worked perfectly first time! As you can see, I wasn't tight for space this time so I used a piece of board and components that happened to be to hand. I kept the oscillator/PA physically separate from the audio input and modulation amplifier, and only cut copper tracks where necessary for isolation purposes.

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Old 27th Nov 2013, 1:07 pm   #30
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Default Re: MiniMod pantry transmitter

Thanks for the info and picture Phil.

I'll be taking three ready-built ones for demonstration and sale to the BVWS meeting at Wootton Bassett on Sunday 1 December. I'll have a table with other stuff.

Ian
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Old 27th Nov 2013, 9:18 pm   #31
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

They should sell like hot cakes, Ian - a brilliant design and the performance is excellent.

Did you ever try getting the circuit working on Long Wave, or Short Wave with component changes? The medium waveband is getting a bit crowded here, with up to four pantry transmitters including two Mini-Mods running at once...
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Old 28th Nov 2013, 5:50 pm   #32
Ian - G4JQT
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Default Re: MiniMod pantry transmitter

Hi Phil,

I have considered LW and SW and it would be possible, but there are a few things to consider:

1) The screened coils I use are definitely available in values suitable for short wave, but it's possible temperature and/or battery voltage changes would be more liable to cause frequency drift in such a simple circuit. This is where crystal control would be better, and as discussed on this thread, crystals for higher frequencies are easier and cheaper to get.

2) Long wave is also a possibility, but the screened coils I use are 90 uH, and that range goes only goes up to 125 uH. With appropriate capacitors, either could be made to resonate on LW, but I don't think the inductance is high enough for a good Q on LW and to give a decent output. I could make other coils, but the ones from Spectrum Communications are screened and very temperature stable - ideal for MW in my simple MiniMod.

3) Crystals for frequencies below about 2 MHz are very expensive, so I don't think that's an option for LW!

More and more stations are leaving MW across the UK, and particularly in Europe; there should soon be plenty of clear channels...

Ian
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Old 28th Nov 2013, 5:54 pm   #33
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Default Re: MiniMod pantry transmitter

The problem will be in the current MW band is used for purposes other than broadcasting.

- Joe
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Old 28th Nov 2013, 11:12 pm   #34
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

Thanks for the thoughts, Ian. I might just try shunting the tuning capacitors with a few thousand pF and see what happens...! Regarding stability on SW, I've had acceptable results from home-wound toroids at 3.5MHz, and they're often used on higher frequencies for low-power amateur transmitters. With suitable choice of capacitor temperature coefficient, I bet your circuit would work well on the 49 metre band.

You're probably right though, there may be no point, as MW is becoming less crowded by broadcasters. Not sure what alternative use is likely for MW, but I don't think pantry transmitters are going to be a problem.
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Old 29th Nov 2013, 2:32 pm   #35
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Default Re: MiniMod pantry transmitter

Quote:
Originally Posted by Ian - G4JQT View Post
2) Long wave is also a possibility, but the screened coils I use are 90 uH, and that range goes only goes up to 125 uH. With appropriate capacitors, either could be made to resonate on LW, but I don't think the inductance is high enough for a good Q on LW
AM transmitters on LW are a bit harder to design than MW. If you think about it, you don't really want high Q on LW else the amplitude of oscillation won't be able to follow the audio modulation, particularly at higher audio frequencies.

On MW this is not such a problem, as the RF is higher and if it takes 50 cycles of RF to increase or reduce amplitude,then no problem. But 50 cycles of 150kHz RF (the end of the LW band), takes 0.33msec which is significant when 5kHz AF has a period of 0.2msec.
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Old 30th Nov 2013, 10:19 am   #36
Phil G4SPZ
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Default Re: MiniMod pantry transmitter

That's interesting, Kalee. Where does the "50 cycles of RF" figure come from? I wonder if it applies to such low power PA stages where the inductance and capacitance are so small?
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Old 30th Nov 2013, 1:06 pm   #37
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Default Re: MiniMod pantry transmitter

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Originally Posted by kalee20 View Post
If you think about it, you don't really want high Q on LW else the amplitude of oscillation won't be able to follow the audio modulation, particularly at higher audio frequencies.
Yes, you have a good point there. High Q on lower frequencies may suppress the sidebands somewhat. In that case the output transistor has to have more gain or be driven harder to get the same P-P output at the antenna.

The simple design is more or less flat out already, so a LW MiniMod would just have less less range I expect - inches rather than feet!

Regards,

Ian
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Old 1st Dec 2013, 11:15 pm   #38
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Default Re: MiniMod pantry transmitter

A thumbs up from me too! I bought a ready made one from Ian today at W-B and I'm very pleased with it. That'll give some of my radios a new lease of life.

I'm going to try to experiment with different lengths of wire too.
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Old 2nd Dec 2013, 11:07 am   #39
Ian - G4JQT
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Default Re: MiniMod pantry transmitter

Hi Tazman, very glad you're pleased with it.

Just a note on lengthening the antenna wire; if too long it may not be possible to resonate L2/C13 output because the extra capacitance it will introduce may require reducing the value of C13, particularly at the high-frequency end of MW.

It's such a simple circuit that its parameters are necessarily quite restricted, but you won't do it any harm!

Ian
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Old 4th Dec 2013, 2:43 pm   #40
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Originally Posted by Phil G4SPZ View Post
That's interesting, Kalee. Where does the "50 cycles of RF" figure come from? I wonder if it applies to such low power PA stages where the inductance and capacitance are so small?
I took 50 cycles as an arbitrary figure, but for a tuned circuit with a Q of 100, the amplitude will decay to 20% of its starting value after 50 cycles.

So that won't matter on SW, is of minor relevance on MW, but merits serious consideration on LW, where 50 cycles of RF could take 330µsec and you may want the amplitude to drop much more, in just 200µsec. Similarly for the rise in amplitude.

Will it matter in low power PA stages? Yes, as much as high-power ones. However if L and C are both small, as you mention, then the operating frequency must be high. So no matter unless you have stupidly high Q's of several thousand, which are difficult to achieve anyway.
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