Customers Repairs; Communications receivers, etc.

National HRO, Repair No. D458

 This set is an ex-WWII army receiver marked as an "R106" and is an early version using "UX" based valves rather than "International Octal" types more commonly encountered.


 The set is in near original condition and has an original external power supply unit with the angled top. The original design of the HRO has the audio output transformer mounted in the power unit with the loudspeaker. This has a significant drawback, of which valve devotees will be aware. If a power valve is operated with its anode open-circuit the screen grid will attempt to dissipate a whole anodes worth of power. This is not good for valve longevity so a common modification is to put the transformer under the chassis and to couple the low impedance output circuit through to the PSU box.

The reported problem was that the set had gone deaf over a period of time. I found three problems. Two physically very large screen grid feed resistors had gone very high in value and a small capacitor was leaky. Replacing these restored normal operation.

The HRO is not my cup of tea. I like to be able to see where I am on Short Waves and this is not easy if one has to refer to a graph; it may be dark; the perspex cover over the graph goes very yellow with age; and one's eyesight in the near-field rarely gets better with age! That and the fact that you can't change wavebands without searching through a large box of coils and then unplugging and plugging-in the new coil seems to me to be most inconvenient. Also, I suppose the fact that, ideally, one must first turn the receiver off before the unplugging/plugging operation and then turn it back on again upsets stability. For certain operations though the HRO is ideal. GCHQ operators used to use these sets together with AR88s and one feature of the HRO is resettability. An operator could whizz between different settings using the vernier dial to intercept a pair of senders talking to each other on different frequencies or check a large number of precisely logged settings for useful SIGINT. The Racal RA17 and RA117 which came later in this cloak and dagger world are better because they had resettable frequency rather than a general scale marking and didn't need plug-in coils. They didn't even need a clunky wavechange switch. Of course with digital synthesised receivers the whole problem of resettability has faded into obscurity together with warm-up drift etc....

The set has now been kindly donated to the Radio Museum by my brother-in-law Riki.

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