Armstrong Micro FM Pulse Counting Receiver Kit.

It was quite a surprise whilst browsing eBay for radio kits to see this. The old Sinclair Micro FM has been reborn - well, almost! There is now a modern version available as a kit.
At the very low price it was for sale at (~$20), it was impossible to resist. In fact, it would be hard to build it with all new parts for that. In the eBay ad there was some information on the kit and a circuit included, although without component values. From this I could see that it was certainly a genuine pulse counting receiver. So, the order was placed...


Very professionally packaged. Assembly was quick and easy.

It didn't take very long to arrive and the excellent quality of the kit really stood out. I've built a few kits over the years and this is certainly very professional. The packaging and instructions were as they should be; I really couldn't fault anything. I had it built in about two and half hours.


Note the 1.5V operation and the loop aerial.

It went together very easily. In fact, as an example of the quality, the resistor leads were already preformed. The leads needs be cut off close to the PCB because it rests right against the plastic case. If the PCB sits too high, the earphone socket and volume control won't line up with the holes. The earphone socket has small protrusions that need to be removed for it to sit flush against the PCB.



The Circuit.
At this point I would recommend learning about my own solid state pulse counting receiver, if you are not familiar with this method of FM reception.

I've redrawn the circuit with component values to make it easier to understand. Two things stand out compared to the original Sinclair design. First is the operating voltage. It's only 1.5V! The Sinclair used an obscure 9V battery. This, incidentally, is the first pulse counting receiver I've seen operating at such a low voltage.
Secondly, the aerial is quite novel, using just a stretched out  oscillator/aerial coil for signal pickup. In this regard it's just like a ferrite loopstick aerial used in a MW set, except it's for VHF. There is neither a telescopic aerial (like the Sinclair), or provision to use the headphone lead as an aerial. Experiments in the past have confirmed that this kind of aerial is actually quite legitimate, and it's surprising it hasn't been used more often for VHF receivers.

Apart from that, the design is largely conventional. An autodyne frequency converter of the Colpitts type converts the VHF signal to an IF of 80kHz. The two following stages determine the gain and selectivity in the usual way; i.e., the coupling and collector bypass capacitors determine the frequency response of the IF amplifier. The next transistor performs as a limiter by means of the low bias, and the diode from base to earth. The squared pulses from the collector feed the usual pulse counter circuit, consisting of two germanium diodes as a charge pump. The manual states these are 1N43a types. I think that's a typo and they're really 1N34a.
The time constant of the .022uF and 22K volume control pot average out the pulse frequency so it becomes clean audio.
After the volume control, the signal feeds a single class A amplifier transistor which drives the earphones. These are 32 ohms each, but are connected in series to present a 64 ohm load which is easier to drive. Note that the Sinclair did not include a volume control.

As with the Sinclair, plugging in the earphones turns on the reciever. However, here an electronic switch is used, instead of the modified earphone socket used by Sinclair. There is a transistor in series with the negative end of the cell (an AAA type). When earphones are plugged in, base current flows, saturating the transistor, and completing the circuit.
Note that the earphones must have DC continuity for this to work. A crystal earphone won't work, and nor will a feed to an external audio amplifier, unless a suitably low resistor is connected in parallel.

It is pointed out in the notes that by operating the receiver at only 1.5V, radiation from the oscillating aerial coil is minimised. The supply to the mixer/oscillator is regulated by another transistor in a shunt regulator circuit, so that a stable 1.1V is provided to reduce drift.

Like the Sinclair, AFC is provided by taking the DC from the detector output and using it to control the bias of the oscillator, which then alters the frequency. This is done by the 220K resistor and .1uF condenser. Mention is made of frequency compression like the Philips TDA7000 IC uses. Essentially, by modulating the local oscillator frequency with the demodulated audio, it is possible to use an IF of less than the maximum frequency deviation of the FM signal. The notes claim an IF of 80kHz. However, I fail to see how this could work effectively with the long time constant of the 220K and .1uF. That would surely filter out most of the higher frequency audio components. I must point out I haven't actually measured its effectiveness.


Shown for comparision with the original Sinclair design.

Performance.
Alignment was straight forward, but having a R&S SMS signal generator helped. Like other VHF transistor autodyne converters, the bias is important, and here a 100K preset adjusts the optimum level. It is a compromise because performance varies from one end of the band to other. Like other receivers I've experimented with using this type of converter, it was found best to set it for best sensitivity at the low end of the band.

And the aerial? Yes, it actually works very well. At 80km from the main Sydney stations, and further from the Central Coast stations, there is no problem receiving them. Like any loop aerial it is directional. The sensitivity is less than my own design with a telescopic aerial and RF amplifier, but that's to be expected obviously. It is certainly not a DX receiver, but it receives all the stations in the intended broadcast area very well.

The sound quality is more bassy than I would like, and this appears to be due mainly to the supplied earphones. I tried different types of conventional design and found the frequency response somewhat better. Incidentally, the hole in the side of the case is too small to fit anything but a very narrow plug, so will have to be enlarged to use other types of headphones.

All in all, I highly recommend this kit for anyone interested in pulse counting FM receivers. It works well, and the price is unbeatable.



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