FET TRF.

[Joe]

Today I amused myself by making a TRF radio out of a book I was given for my ninth birthday.

           

It worked but I can see why this sort of thing put me off as child: if I hadn't got a Bits Box it would have cost a good few bob and been inferior to a radio from PoundLand.

It seemed distinctly temperamental and I could barely breath without upsetting it, though the construction method is asking for dodgy connections, and I soldered the wires to he variable capacitor and pot. The controls have knobs as touching them effected things as well.

When it was tuned in the sound quality was surprisingly good, though for "Crystal Earpiece" read "LM386 and a filleted R600." There was nothing as crude as a battery on the bench either!

- Joe

[pwdrive]

Mmm...does it squeal?

Lawrence.

[ed731pdh]

Ahh, the dreaded S-Dec boards. Misery in a small plastic package.

[Joe]

A bit of squealing but more farting. - Joe

[Tedzed1]

A bit of squealing but more farting.

Sounds like you have been at the "homebrew" again Joe, in more ways than one.

[pwdrive]

Yeah, that sounds about right, the S Dec stuff is a blast from the past, had some of that once, it superceded my plank and drawing pins, even progressed to vero board once, I miss my experimenting, will have a go this winter coming. When I used to build stuff like that I used to replace all the fixed resistors with presets then tweek them all for maximum result and minimum heat, you can probably guess, I'm not a design engineer. (quick edit before any confusion arises!) I was refering to post No:4 not post No:5, No:5 popped up as I was submitting this post (No:6)

Lawrence

[Joe]

I've more progressed TO bits of wood. I certainly agree on the pre-sets as "Suck it and See" is my main design philosophy.

Actually, with the S-DeC the holes are far enough apart for leads not to be too prone to shorting, unlike modern 'Breadboards.'

- Joe

[Skywave]

The circuit for the drain of Tr1 simply isn't right: amongst other things, the wiper of VR1 is 'hot' as far as R.F. is concerned. If you are finding that the reaction is too harsh and kicks in suddenly, increase the distance between L1 and L2. That will reduce the mutual inductance and thus reduce the degree of feedback. It's also worth bearing in mind that the gm of the '3819 can vary widely from one example to the next - as I found out many years ago when I first started using them for audio and V.H.F. pre-amplifiers.

For circuit development work, especially R.F. circuitry, S-DeC is the last thing you want to use.    Here's what I do: take two pieces of double-sided copper laminate board, typically one 6" x 5", the other 6" x 3". Solder the two 6" sides together at right angles. The narrower piece becomes the 'front panel'; the wider piece becomes the 'chassis'. Drill a number of holes in the front panel first - you can use these holes to mount things like pots., variable caps, switches, etc. For the chassis, simply use point-to-point connections with the chassis as the 0v./ earth point. You can usually arrange the component layout such that those earthing points will give sufficient rigidity to the associated components. Sometimes the use of conventional tag-strip helps: again, the earthing tags of those strips can be soldered direct to the chassis. For critical circuits where unwanted feedback may be a problem, simply solder in additional pieces of copper board to act as screens.

When you're happy with the final cct. and go to 'production, you can strip off all those components and then the 'test chassis' is ready for the next development project.

Al.

[Paula A]

Agreed.. for TR1 it would be better to take a lead from a 30's trf set, use a cap and variable resistor on the RF groundy end of the reaction winding and set the stage bias sensibly with a fixed audio takeoff point from the RF "hot" end before the bias resistor. I would be tempted to add an RF wideband common base simple transistor "buffer" rather than pre-amp stage, just to cool down the "grid" loading. Maybe 6 more components.. 1000% improvement.

Nice to see a proper blast from the past. The white plastic box and trailing bits takes me right back.

In my teens I used to build all kinds of weird things using the ubiquitous EF80.. from "duplex" trf sets to audio amplifiers. I generally found Gilbert Davey's designs worked pretty much out of the box and were not too critical on using the exact bits and bobs. My record DX on a thing with home wound coils and a random ex-tv EF184 was hearing the Olympics on the Pacific regional broadcast of the Australian World Service, and some rock music broadcast from Tokyo (frequency??.. your guess is as good as mine, only info in English was the station id and "from Tokyo, live" ) was a regular.

[Joe]

Here's the Mark II version:-

   

I made no changes to the circuit, not because I think I know better than Al or Paula but I thought it only fair to give the original the best chance of working and, frankly, some of the suggestions passeth all understanding.

It now works MUCH better; in fact it was pulling in interference until I earthed it. It's as rough as the proverbial bear's posterior as I only regard it as temporary.

I don't know how interested people on here are but, it being Tuesday, I've got visitors, one of whom was fascinated and could barely contain her excitement at proceedings.

   

- Joe