Mini-Mod 2021!
 

As so many members have now obtained PCBs due to the kind generosity of Peter, 'Electronpusher0', hopefully quite a few are now nearing completion or even already completed. Since In Liston Smith first published the Mini-Mod in the BVWS Bulletin back in 2013, it's proved hugely popular. There have been several threads and bursts of activity on the forum - notably in 2015 and 2018, and in 2021, it still has legs!

I thought it might prove useful to create a 'Mini-Mod 2021' thread where constructors of Peter's boards can share their experiences, results, performance, and any difficulties in constructing and adjusting the project (hopefully few!). I've completed mine and it worked straight off and performs equally as well as did others that I've produced in the past, one of which I still have for comparison.

I used what components I had to hand, so in some instances I bent the legs of 5mm capacitors to find 10mm spaced holes, and I used a mixture of ceramic, Mylar and polystyrene capacitors. Ian Liston-Smith had said that 680 uF capacitors are a bit uncommon, but 470 uF should do fine instead. The only 680 uFs I had to hand had 5mm spaced pins so I used 470 uF, which seems fine.

The only small challenge I guess was soldering the Audio input but there are two options - one is three holes spaced for a 3 pin socket, or separate holes if preferred. I used the three holes to solder the wires to anyway and had no problems. Likewise, if it's desired to fit an LED, the LED holes are close together (2.5mm), but I had no problems with that either. (The LED is an option anyway - if it's not fitted, neither is R9 required). I fitted an ultrabright one which only consumes 1mA and might prevent a PP3 from being run down if inadvertently left on.

The transistor pads are close together, but again, with a fine pencil-tipped iron it's no hassle and the plated-through holes wick the solder in really well.

To box it up calls for a box with internal dimensions at least 75-80mm wide x 100mm long and maybe 50mm deep to comfortably accommodate the PCB and the PP3 Battery. I guess those on a higher pay grade than me might created a 3D printed bespoke box with a battery compartment, but I just made yet another little comb-jointed box from scrap mahogany and created a waterslide decal for the front panel.

Daft in a way - that took me longer than building the PCB, but if it was a chore, I wouldn't do it, and all it costs is my time. The most pleasurable aspect of retirement (of which I've had 26 years to date), is that you dance to no-one tune but your own. (Except loved ones of course). I screwed the PCB to the bottom of the box with 6BA C/S crews from beneath, and used wires from the PCB which were long enough to fit the sockets, switch and LED to the front panel before screwing it down.

The PCBs are superb.

How wonderful that JLPCB will produce small batches of boards of such high quality at such a low price and cater for our modest needs, and will give an instant quote online. JLCPCB is a massive outfit - 800,000 customers in 170 countries, 3,000 employees, 20,000 orders a day, Five factories totalling 450,000 sq M working 24/7. They turn orders around in 24 hours, with 0.23% fault rate. 600,000 sq metres per month capacity. They also do auto SMD assembly (35 million parts/month), and yet they'll sell us hobbyists half a dozen boards for little more than a tenner post free.

Worth watching their video here:

https://jlcpcb.com/aboutUs

I've attached a few pics of my Mini-Mod:

Pic 1: Completed PCB.
Pic 2: Unmodulated 1 MHz (300 Metres) output at the aerial.
Pic 3: Output modulated by 1kHz sine wave. (albeit at low amplitude).
Pic 4: An angled view if the little box I made.
Pic 4: The waterslide decal I made for the front panel. (If it's any help to anyone, feel free to copy it).

Hope that's of interest and that others will add their experiences to the thread.

Re: Mini-Mod 2021!
 

Hi David, I am nearly there, fell at the last jump as to speak, the 680nf or 470nf I will have to buy in. I originally thought I only needed to buy the inductors and ordered them, then found I had nearly every value of electrolytic caps in axial and radial except 330uf. Then the FETs I have is the original one specified but chose to buy the second option for which Peter designed the PCB for. I have a pack of the LM386 but need to find them. I hope to get back to this at the weekend. Any one else got started on theirs?

John.

Re: Mini-Mod 2021!
 

Mine worked first time. I built one previously on stripboard but couldn't resist the prospect of building another with a more professional board layout - the quality is superb. Luckily I had most of the parts including a salvaged 2N3819 and BC109 that had been lurking in the spares box for years. Both worked. like others I found the 680nF cap was too big when it arrived and so I fitted two 330nF in parallel which was a snug fit but OK. For a box I found the following 100mm x 100mm junction box ideal: https://www.ebay.co.uk/itm/Plastic-J...-/352876701871 Even the holes in the minimod board line up with the box mounting holes and plenty of room left for the battery. I didn't bother to fit the LED so I will have to mark the on-off position of the mini-toggle switch. Cheers, Jerry

Re: Mini-Mod 2021!
 

These look much nicer than my veroboard one!

For an LED I ended up using a white one cut off a skip-find string of Christmas lights. So if anyone else wants one I have about 79 left, but post will be more than the thing is worth :)

Re: Mini-Mod 2021!
 

Thanks for starting this thread David.
Everyone who ordered from the first batch will have received them by now so we should see more completed projects.
The second batch have been manufactured (it took 5 days rather than the usual 2 because of the quantity) and are winging their way to me.

Regards
Peter

Re: Mini-Mod 2021!
 

Here are pics of my two, the Veroboard version on the left. The latest Minimod box still needs finishing off. On the first one I bent up an aluminium clamp for the battery on the underside of the box lid to keep it clear from damaging capacitors, and I'll probably do the same on the new one, unless I'm feeling lazy and decide that a cable tie will do! Jerry

Re: Mini-Mod 2021!
 

I have the PCB and most of the parts, the remaining ones on order. If everything is here by the weekend (and work doesn't interfere) I intend to build it then. Hopefully mine will work as well as the others.

What frequencies are people setting them to? Obviously it will vary around the UK. What is the range and does it vary with frequency? What length aerial wire?

Re: Mini-Mod 2021!
 

I’ve tweaked mine to 295 Metres, (just above 1 MHz) as that’s clear for my location.

The one I made a couple of years ago is tuned to 325 Metres (just below 1 MHz) which for me, is also clear. Of course, for use with radios that ordinarily need an external aerial, interference from stations isn’t an issue anyway, and for radio with a ferrite rod, the radio can be turned to null out an interfering station or the Mini-Mod tuned away to a clear spot.

I mostly use mine with an Ekco A22 with no aerial plugged into it, so it soon finds the Mini-Mod as there’s nothing else on.

As to the Mini-Mod aerial, as it sits next to the radio, I find half a metre of flex is more than adequate. The signal fades out under a Metre away from the radio, which is just fine for me as I know it’s not going to cause anyone any bother.

Good luck with the build Paul.

Re: Mini-Mod 2021!
 

I'm looking forward to receiving my boards when peter gets delivery.

Whenever I look at David G4EBT's beautifully crafted cases and graphics I feel more than a twinge of envy!
I was that kid with 10 thumbs in woodwork and metalwork at school and never had lessons beyond the age of 14, so I have to doff my cap to anyone with the innate skills, its a gift. One that my father had and only failing eyesight and arthritis stopped the old boy, he despaired of my inability at any sort of manual craft.

I've got enough coils to make 3 so I was thinking to knock one up on a bit of perf board in the meantime.

Has anyone made one that works off a mains PSU? I was thinking to use a unstabilised wall wart and put a voltage regulator in the box.

Andy.

Re: Mini-Mod 2021!
 

Hi Andy, there's a thread here concerning an issue while using a mains PSU, and the solution: https://www.vintage-radio.net/forum/...d.php?t=167094 . Also, in Post #1 of the following thread, Crackle has attached a stripboard layout: https://www.vintage-radio.net/forum/...d.php?t=133001 Cheers, Jerry

Re: Mini-Mod 2021!
 

The one I have was made by Ian, I find that an external box with 6 AA batteries lasts a long time, not worth having a mains PSU.

Re: Mini-Mod 2021!
 

Great to see another MiniMod thread!

To answer Paul, the antenna needs to be only about a yard long. Too long and the final tuned circuit will never tune. But remember, this only transmits a signal for few feet. It's meant to be used quite close to the radio.

The capacitor value (C12 in the original article) was 680 nF not uF! 470 nF should be almost as good, but with a theoretical slight drop in bass response.

As for frequency to use, it won't tune the whole MW band and but it tunes about +/- 400 kHz centred on about 900 kHz. I think the Q of the tuned circuits means it has slightly more output at the higher frequency end of the range.

Re: Mini-Mod 2021!
 

Ooops, thanks for spotting the typo Ian! :-)

For the avoidance of doubt, it was a 470 nF cap I fitted - not 470uF.

Re: Mini-Mod 2021!
 

After receiving several requests I am running third batch, if you are interested please leave a request on the original offer thread.

Peter

Re: Mini-Mod 2021!
 

Just to say a big thank you to Ian and Peter for the circuit and circuit board.
I have at last got mine working well, listening to it on a Decca TP22 as I take the photos. I managed to zap the FET I don't know how but I had to change it and then the Minimod worked.
I used Servisol Flux remover on the PCB, This softened the finish on the PCB, left my fingerprints all over but it dried out ok.
Also David's addition the LED indicator did not work first time, not my mistake, or David' or Peter's. Any guesses? I should say it lit with my LED tester before I used it.

John.

Re: Mini-Mod 2021!
 

Does anybody know the unit of C6? It is marked as 4n7.

I can't wait to get my MM up and running!

Re: Mini-Mod 2021!
 

4n7 means 4.7nF (nanofarads).

Re: Mini-Mod 2021!
 

Writing 4.7n as 4n7 or 2.2u as 2u2 etc arose because the decimal point can so easily be lost as documents age and fade.
We were told to use this method when I was working and it has stuck with me.

Peter

Re: Mini-Mod 2021!
 

Likewise. Also used for resistors, 4k7 for 4.7k for example. It is also possible that a speck of dirt when a drawing is duplicated could be misinterpreted a decimal point, whereas with this format we know to ignore any such stray dots. Another note is that because keyboards don't have the ohm symbol we tend to use "R" instead.

My Mini-mod is about 90% finished. Waiting for some replacement capacitors (the ones I received were wrong spacing and leads too short to spread) and 390R resistors. Hopefully I'll get it finished and working at the weekend.

What have people fabricated for adjusting the inductor cores? I know it needs to be non-metallic or at least non-ferrous.

Re: Mini-Mod 2021!
 

Not needed to adjust mine but I think they require a flat blade, in the TV trade 50 ( or was that 60) years ago a plastic knitting needle with the end filed into a chisel shape did the job.