Mini-Mod 2021!

[Jolly 7]

Originally Posted by technotel

Hi Jolly
I have someone already requested it although he was going to send payment for postage, he hasn't done so yet. So give it a little longer and I will get back to you. He may have decided he doesn't want it after all? So you will be next in line.
Terry

Thanks Terry.

[fastpete]

Here are a couple of pics of my Mini-Mod 3d printed enclosure. Not as period correct as some of the other beautiful boxes here but it's nice and low profile.

HERE is a link to anyone wanting to print their own.

I can't get over how well these work. They really bring a new lease of life to these radios. Particularly in Ireland where there are few AM stations left.

I have 3 more to build!

[Ian - G4JQT]

It's really good to see that this project is still living. I wrote the MiniMod article that got published in the BVWS Winter 2011 edition!

It always was a compromise between complexity and performance. Great to see the variety of enclosures and the efforts made to make them - I just used plastic project boxes from ebay.

As stations continue to close, it's possible Ofcom will be even less likely to take an interest in what's going on in medium wave - as long as a station's content doesn't draw complaint.

(I was told unofficially in 2014 by an Ofcom inspector that MW pirates are no longer on their radar, as long as the content isn't political, racist, etc. and that would only come to their attention by a report from a member of the public. Our pantry transmitters put out less energy than many SMPS, and don't cause wide-band interference. Nevertheless, unofficial broadcasting is still illegal.)

In future I may develop a simple, more stable and effective AM signal source using CPP Field Programmable Blank Oscillators. They are MUCH cheaper then quarts crystals!

[Electronpusher0]

I agree that its good to see the minimod still going strong.

Depending on demand I may be tempted to obtain and offer more boards at some time.

Originally Posted by Ian - G4JQT

In future I may develop a simple, more stable and effective AM signal source using CPP Field Programmable Blank Oscillators. They are MUCH cheaper then quarts crystals!

If you need a pcb laid out on Kicad just let me know. - FOC of course, least I can do.

Peter

[Pamphonica]

As a small minimod mod, I have experimented with a tiny twin gang cap, about 20pf per side, in parallel across the two main (270pf) caps, but reduced those to 220pf. This allows a small swing, about 30kHz, just enough to get rid of birdies and interference on some sets. I’ll report back once I’ve used it for a bit.
The cap was NOS but was probably designed for a small vhf only set. Nice metal framed air spaced job, with a built-in 3:1 drive. Only about 20mm across.
-Jeremy

[Telegraph-Hill]

Hello lovely people, I am a Radio Presenter since 1992 and do my German shows via Internet for the last 10 years. I am also into electronics and all the studio gear I am using.

I just want to make a show sounding like the real 60s. All affords to use EQ's are not really successful. The best sound I got out of an Leak30 Amp. They are using very good high filters to get rid of the hiss from the old 78's.

My plan was to get 2 Broadcasting systems running using either one of the one discusses in this thread and 2 receivers (I have some brill hacker units here) to push the original stereo signal coming out of the mixing desk into 2 Mini-Mod's (one for each channe) and receiving by my hackers.

In Theory I do not need a long areal. I was wondering about the interference from this setup as I broadcast 2 different frequency's.

What do you think?

[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. But of course the range depends upon local noise, channel occupancy (you'd choose a clear one anyway) and receiver sensitivity. Extending the MiniMod radiating aerials works to some extent, but if too long it will detune the output tuned circuit unless you change the fixed capacitor.

I don't think they will be suitable for your application. Nevertheless, having two low-power around-the-house MW transmitters on different frequencies shouldn't present any problems. I've built various transmitters to serve more than just my house and had them all operating without any problems. Just ensure that they are more than three or four channels (at least 27kHz) apart.

[rambo1152]

Originally Posted by Telegraph-Hill

My plan was to get 2 Broadcasting systems running using either one of the one discusses in this thread and 2 receivers (I have some brill hacker units here) to push the original stereo signal coming out of the mixing desk into 2 Mini-Mod's (one for each channel) and receiving by my hackers.


What do you think?

Are you going to mic the Hackers like guitar amps, or take a direct audio output?

Either way I don't think this is the way to create an authentic '60 sound unless you want everything to sound like "The days of Pearly Spencer".


There must have been more to studio audio processing in the '60s than just EQ, what about dynamic compression?

Were the 60s pirates using Orban Optimods? what was being used before those?

Luxembourg sounded very "bright", and I'm sure there was some reverb added too, perhaps that is the definitive sound of '60s AM radio?

[Telegraph-Hill]

Hi Graham,


Are you going to mic the Hackers like guitar amps, or take a direct audio output?

Direct audio output of course

Either way I don't think this is the way to create an authentic '60 sound unless you want everything to sound like "The days of Pearly Spencer".

I just want to give the listeners the feeling to sit in front of an old AM Radio and enjoying the show.

There must have been more to studio audio processing in the '60s than just EQ, what about dynamic compression?

I do use dynamic compression in my setup with additional vintage Tube compressors

Luxembourg sounded very "bright", and I'm sure there was some reverb added too, perhaps that is the definitive sound of '60s AM radio?

Yes I can remember Luxenburg using reverbs. This is also available in my broadcast studio.

It looks like I will do what noone else has been done before I will keep you posted, and thanks for the feedback.

[murphyv310]

Hi.
I made up a minimod a few months ago. The problem I have here is severe QRM, not only on Long and Medium wave but well above, I cannot even transmit on the HF amateur bands as the receive signal is obliterated, so the HF side of the hobby is a waste of time.
OFCOM won't investigate interference on LW & MW their response is to switch to another platform! Fortunately I am just able to listen to Caroline in one room upstairs that is at the opposite side to my gadget ridden next door neighbour!
The minimod although it works hasn't enough power to flood the interference unfortunately, three feet from the radiating antenna and the QRM is taking over and 6 feet the signal from the minimod is too weak and obliterated.
The annoying thing is I believe this QRM is being radiated through the mains and I can receive it well outside both front and back, I've switched off my mains supply to make sure it's not from my home or shack and of course its not generated from me.
I've built many pantry transmitters in my time some of which can be seen on my channel, now there is little point here in bothering due to all this.
Funny though how OFCOM won't investigate when you genuinely use MW which I do.

[Ian - G4JQT]

Yes, if you have very strong local interference the range of the MiniMod may not be sufficient.

All I can suggest is that the frequency chosen is as clear as possible and that the MiniMod output coil is carefully tuned to match spot-on.

If you have an receiver with an antenna socket, you can stick the MiniMod antenna wire into that. A direct connection shouldn't be necessary.

Ian

[murphyv310]

Hi Ian.
There are very few clear spaces TBH. I made a pantry TX with Russian Rod Pentode, using the 1ZH42A that has an HT of 6 to 9v it was microwatts but required a tank circuit for any range, which eventually I got around 10 to 15 meters and even that struggles now. The interference is far more powerful than any pantry transmitter I've made or used.

[paulsherwin]

Reopened by request.

[Ian - G4JQT]

I've done a little bit of design work on the MiniMod. It's getting a bit long in the tooth now (and never was cutting edge!), but I've done a couple of modifications; one following a suggestion from Tony Nailer of Spectrum communications who sells the part MiniMod kit on eBay.

I have never been particularly happy with the level of FMing of the circuit. On most examples this isn't noticeable, particularly when received on a set with a wide IF bandwidth. It's caused by the modulation applied to the output transistor varying the load on the VFO. With a free-running, un-buffered LC oscillator running at the output frequency, some pulling is inevitable. It's the price paid for its simplicity. I think the variations between MiniMod examples are caused by varying characteristics of the individual output transistor.

A buffer stage could be added, but the problem isn't really that bad and it adds an extra layer of complexity for what is a simple circuit intended to have only a limited range.

Nevertheless, I may investigate adding a buffer stage and to make that worthwhile, beef up the design's output somewhat.

If anyone has noticed a problem, the attached circuit shows the changes, i.e. the addition of D3, C14 and reducing C3 to 100pF and increasing R2 to 10k.

On some of my MiniMods these changes made a difference, on others hardly any difference at all.

Regards,

Ian

[Ian - G4JQT]

I've added a buffer stage to the MiniMod circuit to increase the frequency stability.

I did consider this in the original design way back in 2011. At the time I didn't think it worth the effort, and generally if you're listening on a vintage set with a fairly wide IF bandwidth it's not really necessary. But on transistor sets which often have narrower IF bandwidths, the MiniMod's slight FMing is sometimes noticeable. (The FMing is caused by the modulated signal in the output transistor varying the VFO load.) The addition of Q2 buffer isolates the VFO from the output transistor.

I'm still using the BC109 because I have plenty, but the BC548 is one of many suitable equivalents.

The suggestion of adding C14 was made by Tony Nailer (spectrumcomms) who has been selling the coils and semiconductors for the project on eBay for about a year. Some constructors added an LED to the circuit, which I have also included now that low current types are so common.

There are many ways of getting more power out of such a simple circuit, but apart from the increased battery drain, matching the output to a tiny and inefficient transmitting antenna starts to get a bit complicated. Then there's the harmonic filtering to be addressed. The output tuned circuit of the MiniMod takes care of that. The Q of the tuned circuits give maximum output in the approximate frequency range 1000 kHz (300 metres) plus or minus 50 kHz or so. But this depends upon the exact values of C1 and C13.

There are also much better ways of generating the signal than an L/C VFO. The ProgRock programmable crystal kit (from QRP Labs) and a PLL synthesizer (PCB and crystal available from medtechnica on eBay) are both viable alternatives, but again, probably a sledgehammer to crack a nut for a simple battery-operated device. However, they'd be ideal for something more substantial...

The PCB layout is included which anyone is free for anyone to use/amend. I designed the original layout in Express.pcb which doesn't generate gerber files. I only use Express.pcb to generate a drawing which I use to hand wire the 'tracks' and use single-sided PCB using the copper as the ground plane. A ground plane is not really necessary, it's just easier in my construction method. Square crosses are drilled and soldered to ground plane; round crosses just soldered to ground plane.

Since it is not possible to solder grounded connections of the 90uH inductors directly to the ground plane, C5's earthed side is soldered to one side of L1 to earth and one side of the unused winding of that inductor to earth. C10 does the same for the earthed side of L2.

The linearity of the modulation envelope is visible in the photos.

I made at least two dozen of the initial design a few years ago, and I think many more have been made reading the various MiniMod threads on this forum. I don't intend to build any more.

There are a number of ready-made low-power ''pantry' transmitters already available on eBay and elsewhere, most of which give good performance.

Note: I have no link with any of the vendors mentioned above.

[Ian - G4JQT]

A few more photos:

[Electronpusher0]

As many of you will be aware I laid out a pcb for the original MiniMod and sold just over 200 on the forum at just over cost (with Ian's permission).

I have been in contact with Ian by PM and we have agreed that I can also supply the Mk2 pcb if I wish.
Ian has however requested that I add an extra pound per pcb which will go to a charity Ian will designate.

Time to get working on KiCad, watch this space.

Peter

[fetteler]

Peter,

If you are doing a new pcb for the Mk2 may I suggest that you keep to the original overall dimensions of the board? Hopefully the extra bits can be squeezed in (I don't recall the original to be particularly densely laid out) and this will enable anyone who has a Mk1 to keep the same enclosure and mounting should they wish to upgrade to a Mk2.

Best wishes to you and many thanks to both Ian and yourself for a great little piece of kit.

Steve.

PS A bit more spacing on some of the transistor pads and (I seem to recall...) the audio input pads would be nice too

[Electronpusher0]

Hello Steve,
thanks, all good points.
I certainly intend keeping the same board dimensions and basic layout.
I will also change the transistor and audio pads as you suggest.
The Mk1 MiniMod was my first KiCad layout, I have discovered larger pad footprints since then.....

Ian has suggested holding off on ordering the pcbs until he has made sure there are no other changes, he says this will be after the end of October.

Peter

[Electronpusher0]

Despite Ian advising to hold off in case of any other changes I simply could not resist trying to fit the new buffer stage to the pcb.
See the images attached.

I have taken on board Steves suggestions.
The board is the same size as before and the connectors are in approximately the same place which allows direct substitution for the Mk1.
The transistor pad sizes are larger and better spaced. The test point pads are also larger.
I have kept the 3 pin audio input socket the same in case anyone used the matching plug and socket.

I will not order the boards until Ian gives the go ahead in case of late changes.

Peter