MW Modulator Nov 2004 EPE Mag

[David G4EBT]

I've been meaning to build the MW modulator which featured in 2004 EPE Mag and have just got round to it. This enables input from audio sources such as an FM or DAB radio, or ipod etc, to be fed into the modulator and played through a valve radio at a point on the medium waveband, so that you can play your favourite programmes through your favourite old time radio.

It's a simple project, consisting of a VFO with a centre frequency of 650kHz (c 460 Metres) which tunes from about 610 - 690 kHZ, (490 - 435 Metres). This is then modulated by the audio input, and the resultant signal fed into the aerial input of the radio on whch you want to play your programme.

It should not be confused with a micro-transmitter.

The only snag is that the original design uses a Toko coil, now unobtainable. (Only one tapped winding of the coil is used, so it isn't used as a transformer). I wound my own
coil on a scrap transistor radio IFT core.

Initially I wound on 270 turns, then tapped it and wound on another 30 turns to see how it panned out. The original design specified a 50 pF tuning cap, but I only had a 75 pF to hand, so I used that. To get the VFO on frequency, I had to add 47pF in parallel.

The VFO is very stable and didn't drift when left runnig for several hours.

Once I'd got the VFO working, I built the rest of the modulator - the modulator board and PSU board, and built in into the case. It has three controls - the tuning cap to vary the tuning to a suitable spot on the dial of the radio, the modulation control, to adjust so that it isn't driven into distortion, and an attenuator control, so that the radio isn't overloaded.

I wouldn't say it's "Hi-Fi" but I've tried it with an FM Radio, a DAB, and an i-pod.

The editor of EPE had said in a recent mag that they might re-visit the project to see if it could be brought up to date, and overcome the Toko coil problem. The difficulty which magazines have is that they cater for people who may not have a scope or frequency counter etc.

I've attached some pics, which might be of interest, showing the inside and outside of the project, a scope trace of an unmodulated 650 kHz sine wave from the VFO, then another with it modulated by a 1kHz tone.

Hope it's of interest.

David,
G4EBT

[Alf]

I built this some time ago, whilst not perfect, it does the job, it's got a range of about three yards, depending on aerial and earth connections.

[Antlong]

Hello all,

Spectrum Communications, to be found in PW magazine at least, is having a (limited?) range of coils manufactured to replace Toko.

Regards Ant

[Ian - G4JQT]

I've had some experience making these little devices. They are fairly easy to get working, but there are often three problems with a simple design:

1) Keeping the VFO stable long term (15 minutes or more)
2) Stopping the VFO pulling with the modulation and creating an AM as well as an FM signal.
3) Getting really good modulation – surely one of the main points of doing it in the first place.

Using a crystal cut to a clear MW channel is the easiest way of getting around the first two problems, but can be quite expensive and not worth the cost in a very low-power, simple device. A drifting VFO at best can mean retuning the radio and at worst drifting towards another station and causing a het, although David doesn't have this problem.

Slight FMing isn't usually noticeable unless there is a weak signal underneath when it makes a sort of "squeging" noise as the two carriers mix.

Careful selection of mod transformer, or better still series mod using a transistor, (or valve) overcomes the third problem.

What's the mod envelope like on yours David, - scope triggered on the 1 kHz audio tone, not the RF. Can you let us see it at say 90 per cent mod?

A trapezoidal display really shows what's going on, i.e. the audio signal in the Y-axis and modulated RF output driving the X-axis. (for anyone unfamiliar with this, type "modulation trapezoidal display" into Google images – there are also some Youtube videos showing what you should see.)

Too strong a signal easily overloads the radio and can also cause modulation hum – but that's another story...

Anyway, good to see some experimentation, particularly getting around the Toko coil problem without drift.

Regards,

Ian

[paulsherwin]

Wouldn't it be possible to use a standard ex equipment IFT as a coil, adjusting the core to raise the frequency? An old oscillator coil might be another possibility.

Although this isn't designed as a microtransmitter, I imagine it could serve as one if connected to a suitably tuned aerial. A few metres of range would be fine for many people. The low power makes it acceptable for discussion on the forum in this mode.

Paul

[Brian R Pateman]

With a little experimentation these simple circuits will work with salvaged IFTs, oscillator coils, home made coils etcetera. Half the fun is trying things out!

The simplest one I've made consists of a BC108, three fixed caps, a variable capacitor and two resistors. The coil is a few turns of enamelled wire on a ferrite rod salvaged from a transistor radio.

[paulsherwin]

Originally Posted by Brian R Pateman

The simplest one I've made consists of a BC108, three fixed caps, a variable capacitor and two resistors. The coil is a few turns of enamelled wire on a ferrite rod salvaged from a transistor radio.

Post a circuit Brian! Just sketch it on a sheet of paper then scan it.

These very simple circuits may have limited performance, but they are great projects for beginners who may lack the confidence to build something more complex. More experienced constructors can knock them together in a few minutes to experiment with them.

Unlike similar simple valve designs, they are unlikely to produce enough output to cause interference problems.

Paul

[Brian R Pateman]

Here is a very simple circuit. I don't make any claims for originality of the design and it is not a brilliant performer. It is a starting point and about as simple as it gets.

Works off more or less any supply from 3V to about 12V. There's no chance of it interfering with the next door neighbour's radios!