Radio for Grandson

On Monday, we have one of our grandsons here. Whilst he is here, he wants to build a radio as part of his Design and Technology (DT) work. I have plumped for a TRF circuit based around the TD7642 IC, which is like the ZN414 of old.

Very many years ago I built two around a ZN414 for my boys. The plan is to build it myself first to prove it all works, then disassemble it so my grandson can build it after some soldering practise. In the past this circuit worked fine with a ferrite rod antenna.

I am finding things far harder now: my eyes are finding close work more problematic and my fine motor skills are poorer.

He wants to do a design brief and we can draw the schematic together. I can explain how it works in simple terms. Hopefully he can take it home.

Simple radio project

My original post was banned by the censors as it violated blogger guidelines, probably as the algorithm saw certain words.  Talk about nanny state!! In the gaps, insert letters!

With my own ch***n, I totally failed to get them interested in amateur radio! In 2 weeks time my gran###n is here.  His d*d has asked me to work with him on a radio project.

I have ordered a TA-7642 (ZN414) TRF chip from GQRP club sales, but realise I shall also need a small slab ferrite rod, a high impedance crystal earpiece and a 1.5V battery holder. My building skills are really poor compared with before my stroke: what would have been really easy is now hard.

If I cannot find these parts, I shall have to build an LED signaller instead.

It would be good to see another novice operator. Oh look, there's another flying pig.  😉 

Simple TRF based AM radio ICs

Many years ago when my sons were little I made them a couple of Medium Wave AM radios using just a ZN414 3-pin TRF receiver IC. It worked very well considering how simple the circuit was using just a small ferrite rod antenna and a crystal earpiece. This IC is no longer available but there are other similar parts available such as the MK484. Although I have not tried one of these as an IF stage I have no doubt they could be useful in simple rigs like the Tenbox currently under development. The advantages over a super-regen would be selectivity. These devices have input impedances of around 4Mohm so if a ceramic filter was to be used ahead of the IC a step-up transformer would be needed to minimise loss. As they only work to around 3-4MHz they would only be suitable as an IF in the Tenbox 10m AM transceiver.

A useful page I've just found on how to get the best from these simple ICs is http://theradioboard.com/best-of-the-best/mcgillis-mk484.htm.  The datasheet for the MK484 is available here.

A page showing the use of the MK484 as an IF stage in a simple 160m RX is  http://www.vk6fh.com/vk6fh/mk484radios.htm . See also http://www.youtube.com/watch?v=qdlXVKOITe4. I am told that a similar design appears in 'Radio Projects for the Amateur' Volume 4 by Drew Diamond VK3XU. 

I'm still likely to stick with a very simple super-regen RX in the Tenbox - they ARE the best in the intended application - but I may make it in a modular form so that different TX strips and RX strips can be used and compared.  I can see a whole series of simple AM designs in the pipeline, HI.