Mini-Mod 2021!

[Ian - G4JQT]

A bit of expectation management - in case it has been lost in the thread...

My modifications for the MkII are explained in posts #94 and #95 above. (Note that D3 in #94 is the wrong way round!). The mods do not increase the power or range, but address the slight FMing this simple design has, which I suspect may also have something to do with layout.

Goodness knows how many MiniMods have been built and presumably this hasn't been generally noticed, although one made with Veroboard did suffer more than most - possibly due to some extraneous long tracks not being cut.

Thank you Peter (Electronpusher0) for adding the voltage regulator for extra flexibility, developing your new PCB layout and getting the quality boards made.

However, a note of caution; if you use a SMPS some are quite noisy and may leave residual buzz on the MiniMod signal. If you experience this please use a linear PSU or battery.

Note that the inductor and transistor/IC part-kit sold by Tony Nailer at Spectrum Communications (on his eBay store) is for the original two-transistor version. You'll need an extra BF256 for the MkII. Tony also supplies the BC548B equivalent to the old BC109 - be aware of the CBE pin-outs.

Thank you for all the positive things that have been said about the MiniMod since I published the original version in the Winter BVWS Bulletin way back in 2011!

Regards,

Ian

[Electronpusher0]

The boards have arrived!

I have assembled and tested one board in all three variations
1. For battery use with no regulator fitted and a link in the D101 position.
2. For use with a 78L09 regulator - all components starting at number 100 EXCEPT R101 and R102 fitted.
3. For use with an LM317 – all components starting at number 100 fitted.

All three options work.

However there are two minor errors in the layout but they do not affect its operation.
The errors are:
The silk screen shows the tab of the LM317 facing the top of the board, it must be mounted with the tab facing the bottom. See pictures
The second error is quite embarrassing. The 3.5mm stereo jack socket footprint is upside down, this means that the socket must be mounted on the copper side of the board! Fortunately the socket is low profile so should not be too much of a problem.

The error occurred because this is a footprint I created myself and KiCad requires footprints to be constructed as viewed from the top of the board. I found the dimensions of the socket on line but of course they were viewed from underneath and I forgot to transpose them.

I will now offer the remaining boards on the appropriate section of the forum. The boards themselves cost £1 each including the shipment from China. Ian, whose design it is, has requested a £1 per board donation to Charity. Postage will be £1.45 first class but I will increase that to £2 to allow for packing. I will limit the offer to 2 boards per person initially just in case there is a rush!. I have 28 available.

Peter

[Electronpusher0]

Photos of the LM317 and scope images during testing.

Peter

[line sync]

Now I've bought a circuit board I'm now thinking of gathering up the components to build up the pcb.
I know spectrum sell a pack containing the coils & transistors.
Is there a parts list available ?

Robin

[Ian - G4JQT]

Originally Posted by line sync

Now I've bought a circuit board I'm now thinking of gathering up the components to build up the pcb.
I know spectrum sell a pack containing the coils & transistors.
Is there a parts list available ?

Robin

The original MK I version from the BVWS Bulletin in 2011 is posted on various threads on the forum. I'll post it again here at the bottom of this post. Descriptions of significant components (just the two inductors really) and where to get them are included in the article.

Please note there are some modifications, all of which are included in Electonpusher's latest PCB. The MK II uses an equivalent to the BC109 (although a BC109 is fine if you have one) and there is an extra FET as the buffer, with a few extra parts around it.

If I remember tomorrow I'll post links from relevant threads in this forum from relevant posts, if they are not already here in this thread. But really all you need to know is the two inductors are 90uH coils sold by Spectrum Communications: http://www.spectrumcomms.co.uk/Components.htm

And the relevant Spectrum Communications ebay page is: https://www.ebay.co.uk/itm/284204729...Bk9SR5CHk7aRYQ

Original MK I article from 2011: The_Bulletin_2011_MiniMod2.pdf

[line sync]

Thank you Ian , the link to the bvws 2011 bulletin was an interesting read.

Robin

[Ian - G4JQT]

Originally Posted by line sync

Now I've bought a circuit board I'm now thinking of gathering up the components to build up the pcb.
I know spectrum sell a pack containing the coils & transistors.
Is there a parts list available ?

Robin

Post 122 has Electonpusher's board circuit diagram showing all the components and thier values. The only parts that aren't clearly narked with values are the two inductors which are available from Spectrum Communications in my links in post 125 above.

[PaulDarzi]

Originally Posted by Ian - G4JQT

Post 122 has Electonpusher's board circuit diagram showing all the components and thier values. The only parts that aren't clearly narked with values are the two inductors which are available from Spectrum Communications in my links in post 125 above.

Thank you Ian - and many thanks for the design and generous sharing of it, appreciated.

[robinshack]

Just prepared a parts list for the MK2 Minimod.

I have checked it several times for any mistakes.....

Compiled using libre office as i do not have WORD on my pc.

Rob

[Ian - G4JQT]

Originally Posted by robinshack

Just prepared a parts list for the MK2 Minimod.

I have checked it several times for any mistakes.....

Compiled using libre office as i do not have WORD on my pc.

Rob

Thanks Rob.

For anyone having trouble finding 680n for C12, 470n will be good enough. Maybe a very slight reduction in bass. I'm sure someone can do the modelling to come with the theoretical figures, and I can see it on a scope, but not really heard a difference in most of the radios I use.

Repeat note: The MKII addresses the slight FMing of the MKI which I describe in the article and also in this thread post 121. The MKII does not have more range which is still only a few feet.

[robinshack]

My post 129, the BOM document. Steve_Bell notified me of an error. Thanks. C6 should be 4n7 and not 47n as listed.
I have revised the document with the change.
Thanks, Rob

[mictester]

My take on the "Mini-Mod" on a PCB:

[Ian - G4JQT]

Originally Posted by mictester

My take on the "Mini-Mod" on a PCB:

Attachment 271663

Yes, very tidy.

And one for the MK2...?

[tritone]

Originally Posted by Ian - G4JQT

The MiniMods are designed to have a range of only a two to three metres. This keeps them legal for those who are nervous of being accused of operating an illegal transmitter.

@ Ian,

Can the mini-mod be modded with some extra components and power for a longer transmission range, like about 10 metres or so without piling on extra outside interference and staying within the legal rules and regulations?

Just a thought.

Cheers.

[HamishBoxer]

Might be pushing the rules a bit, perhaps a longer aerial? But do check the range.

[Ian - G4JQT]

Originally Posted by tritone

@ Ian,

Can the mini-mod be modded with some extra components and power for a longer transmission range, like about 10 metres or so without piling on extra outside interference and staying within the legal rules and regulations?

Just a thought.

Cheers.

Yes, adding a longer aerial and an earth will extend the range. BUT... you're likely to exceed the tuning range of the output tuned circuit without reducing 270pF capacitor. This circuit must be resonant at the same frequency as the VFO for best modulation quality, maximum output and reduced harmonics - although harmonics are not likely to be a problem with only a few milliwatts of signal.

The MiniMod (MkI and MkII) is designed to have limited range. Unfortunately a completely different design is required for more range, as described in the original article posted in one or two of these threads.

There aren't any regulations for an unlicensed MW transmitter of any power - they are all illegal! But to be honest, I doubt a more powerful illegal MW transmitter with a range of a tens of yards is going to attract the wrath of the authorities, unless you're causing interference to legitimate stations. Ofcom have better things to do, but of course that's not their official policy and I'm not suggesting anyone try their luck!

Ian

[tritone]

Ok, Thanks Ian andHamish for giving me the heads up with that info. I was also chatting to Radio Wrangler and he was saying much the same along those lines, to keep it sweet and not try and go overboard with the transmission distance.

8 - 10 meters range is about what I would desire so I could keep the mini-mod beside my upstairs hi-fi and my vintage radio down stairs in the kitchen about 8 or so meters away.

Thanks for that info fellas.

Greg.