Golborne Vintage Radio

This battery-operated design by Ian Liston-Smith, G4JQT, uses the headphone output of any portable audio player to generate an AM signal for reception on a nearby medium wave radio. It has a range of just a few feet so there is little or no risk of radiating a signal beyond the room in which it is in use. Ian's design criteria was that it should have high quality sound; be simple to construct and put into use; would accept headphone audio feed from MP3 players, CD players etc; would be battery operated, and would be reproducible. It certainly fulfils those criteria.

The design featured in the Winter 2011 BVWS Bulletin since which time it has generated quite a lot of interest, and quite a lot seem to have been built. It uses two 90uH 'Toko' style cored coils, cheaply available from Spectrum Communications. Some constructors have built the Mini-Mod on strip-board - others on printed circuit boards, for which Ian has kindly provided a layout as shown in the second pic below. I used the same basic component layout as Ian's, but adapted the tracks to give a little more clearance between the tracks and the ground plane so as to make it easier for me to etch on a DIY basis, and to minimise the risk of 'bridges'.

Initially I had problems in getting it working, which stemmed from the BF256 FET I'd used having different pin-outs from the data. Among my collection of BF256s I found some which had the pin-outs marked, and this confirmed that pin-outs don't always correspond to data sheets, as shown from the pic of the transistor below, where the pinouts marked on the BF256, they don't correspond with the datasheet in that the drain and gate are the opposite way round, with the source still in the centre, but luckily, they're marked on that particular transistor. (I'd tested the FETs on my cheapo Chinese 'multi-tester', but it gave ambiguous results). When I changed the drain and gate leads over, the Mini-Mod worked fine.

I've attached a pic of Ian's circuit, his PCB layout, the one that I designed based along the lines of Ian's, the completed PCB, and in the last pic,  the Mini-Mod boxed up in it's 'wooden overcoat', which measures 11.5cms x 8.5 cms x 6 cms (approx. 4.5" x 3.5" x 2.25").  I've since built a second one, which worked straight off with no problems. I have it close to my Ekco A22 with about a Metre of flex draped on the floor as an antenna.

Full marks to Ian for conceiving this neat little circuit!

Hope that's of interest.

I have built 2 of these, they work very well. But I can endorse Davids remarks regarding the BF256.I have several of these devises and upon checking the leadouts when building the Minimod I found that the leadouts vary on all of them.
David I like the wooden overcoat that yours are in. Mine are housed in plastic boxes from Rapid.

Dave
M0HBV

The BF244 FET is electrically identical to the BF256 but the gate is the middle lead. Both of these types come in A, B and C groups which have different cutoff voltages. I have no idea which group is best suited to this design. The 2N3819 is electrically fairly similar to the BF244/BF256 but with a very wide range of cutoff voltages.

As Horris has already said, Yorkie has done a lovely job on the box.

(19-08-2015, 07:52 PM)ppppenguin Wrote: [ -> ]The BF244 FET is electrically identical to the BF256 but the gate is the middle lead.

Thanks for sharing that little gem, I wasn't aware of that.

Rob.

The source and drain are interchangeable on these and quite a few other small signal JFETs. Somewhere I hope I still have an old (Siliconix?) FET data book with very comprehensive data on these and many other FETs.

On the subject of reversing transistors, the OC140 is a unique bipolar silicon transistor which is symmetrical. Emitter and collector can be freely interchanged. If you do this with most bipolar deivices they still work but you get very low gain. As a bonus you also get very low saturation voltage. This has (rarely) been used in some designs.

Sorry, I'm drivelling all over Yorkie's thread.

I must add that I received the first one David made, clad as said in the beautifully dovetailed overcoat. I had had components for a long time and wasn't going to get around to building it for sometime, maybe years. Particularly as I have never got set up to make PCBs and strip board and I don't get along. So I got David interested and now we both have one.

A very good and useful piece of kit, thanks again.

Gary

(19-08-2015, 08:34 PM)ppppenguin Wrote: [ -> ]Sorry, I'm drivelling all over Yorkie's thread.

Nah - you're not doing that Jeffrey - just drifting a bit, which is a nice feature on here and reflects the realities of life!

(20-08-2015, 07:54 AM)Radio Fixer Wrote: [ -> ]I must add that I received the first one David made, clad as said in the beautifully dovetailed overcoat.  I had had components for a long time and wasn't going to get around to building it for sometime,  maybe years.   Particularly as I have never got set up to make PCBs and strip board and I don't get along.   So I got David interested and now we both have one.

A very good and useful piece of kit, thanks again.

Gary

Thanks for the donations of the bits Gary!

They only slight downside - which some may argue is an advantage - is the short range. It needs to be quite close to the radio, so you can't have the mini-mod in one room and the radio in another, but then neither are you going to be spraying RF all over your neighbours curtains or whatever! It does what it says on the tin, and the audio quality is good. Best to keep the input gain level turned down and the volume on the radio turned up. Simple, neat, effective. What's not to like?

Doesn't need to be on a PCB in a fancy box - lots seem to have been built on strip-board and work just as well, albeit there's more scope for wiring errors.

(20-08-2015, 09:09 AM)Yorkie Wrote: [ -> ]They only slight downside - which some may argue is an advantage - is the short range. 

With respect, David,  "I have ... about a Metre of flex draped on the floor as an antenna." is hardly a recipe for an effective MW antenna!


A vertical whip might help with the height, alternatively use a longer wire round the curtain rail.


Note that antenna is attached to the tuned (high impedance) side of L2 and will tend to detune it so I would retune L2 to suit whatever aerial you use.

Thanks for reading Terry.

I may have given the wrong impression is referring to the short range as a 'downside'.  The short range isn't a problem for me - I only want to use it alongside the radio, hence the short antenna, but I know that there are other designs with greater RF output and a greater range, which some users might prefer. I haven't tried it on a longer antenna - I've already got the 'Worked All Cottingham Tellies' Award!