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Yura Keychain Radio
Measuring just 60x70x25mm these little key-ring radios were advertised during the mid Sixties in the British radio journal Practical Wireless by Electronics (Croydon) Ltd - later to become Bull Electrical. Most of these sets were labelled "Microsonic" and marked "Made in Hong Kong" despite being of obviously Soviet manufacture. This probably had something to do with the Cold War which was going through one of it's more hostile phases at the time. Furthermore Hong Kong was a British possession and enjoyed very favourable tax concessions on it's exports to the UK. A few did, however, actually bear their maker's name - Yura - and were marked "Made in USSR". At the time these sets were described as complete but corrosion damaged as they had been stored with batteries fitted. Thirty years on I have been fortunate to acquire what I understand to be the last of this parcel of sets available in the UK. Despite the corrosion, obviously worse after all this time, a lot of tender loving care has restored many of these little sets to life.
The circuit is a Medium Wave only single conversion superhet covering 530 - 1650 kHz using seven germanium transistors and one diode. Line-up is 1 off self oscillating mixer, 2 off IF amplifier, 1 off germanium diode detector, 2 off AF amplifier and 2 off output. The push pull output stage utilises driver and output transformers. The circuit is unusual in having a band pass IF filter, and some examples also use negative feedback in the audio section. The tuning capacitor is of the solid dielectric type. Two IF frequencies seem to have been adopted: 455 kHz and 465 kHz. The nominally 2.4 volt power comes from two NiCad button cells that were meant to be recharged in an accompanying plug top type charger. Unfortunately these cells are of a non standard size and I haven't to date found suitable replacements, however these radios can be operated via external batteries using a small pair of croc clips. A socket is provided for an external aerial, together with a miniature 2-pin socket for an earpiece. This is designed to fit a "deaf-aid" type plug commonly used in the Fifties and Sixties.
When refurbishing these sets, corrosion was removed using Safewash 2000 - a water based PCB cleaner - and plenty of hot water. After a thorough drying the PCBs were inspected under 10X magnification and any damaged components replaced. All electrolytic capacitors were replaced as a matter of course with new subminiature types. Tuning capacitors and volume controls were treated with Electrolube No1 fluid. PCBs were then protected by an application of PCB varnish. Finally, refurbished sets have been checked for alignment, and realigned when necessary, then soak tested. When finished I should have about 25 completed sets and will make some available for sale.