Conservation and Restoration
Early Interceptor | HMV 4260 Radio Restoration
One of the few options available when purchasing an Early Interceptor, was the top of the range HMV 4200 & 4260 valve radios.
These expensive and beautiful push button units, were actually manufactured by Radiomobile and marketed by S.Smith & Sons Radios. In Britain they used the well known brand name HMV.
Initially the option for the Early Interceptor from 1951 was the HMV 4200 (also the option for the Jaguar XK 120). By 1954, this had been superseded by the HMV 4260 (the option for the Jaguar XK 140).
This is the unit that was fitted to the Museum’s Early Interceptor chassis ’63’. The unit was probably fitted to the car just before it went on Jensen Motors’ stand at the 1954 London Motor Show.
To give an idea as to the expense of such units, the HMV 4260 complete kit (including amplifier & speaker) was just over £40, while the average weekly wage in Great Britain was by the middle 1950s, around £8.00.
The push button Medium Wave & Long Wave unit came with a large external amplifier, along with separate speaker.
Mounting of the radio unit was under the dash board in the centre, while from the back a long connecting lead hitched up with the amplifier, which was mounted on the passenger side by the door side of the foot well.
Valve radio specialist, Adrian Gardiner, has undertaken a sympathetic restoration of the Museum’s HMV 4260 radio set-up, and tells us more about these radio units, and what a sympathetic restoration entails.
Early Interceptor | HMV 4260 Radio Restoration | Overview
“For restoration from the Jensen Museum, I received an HMV 4260 radio unit, complete with amplifier , connecting lead, and speaker.
The unit dated from the middle 1950s. Typically valve radios from this era were complex devices requiring specialist fitting and requiring a large amount of power.
Indeed, leaving the radio on without the engine running was not an option; a car radio of this type could drain a typical battery to the point where it wouldn’t start the vehicle. And, believe it or not, it could drain the battery in about an hour.
The Radiomobile set-up [ HMV 4260] from the Jensen Museum was a particularly power hungry device, as it included a more unusual Type ‘B’ amplifier. Most of the 4200 & 4260 car radios of the period, were sold with Type ‘A’ amplifiers.
Type ‘B’ units were a lot more powerful, offering around 7W of audio power (the ‘A’ units offered 3W), and was capable of driving 2 loudspeakers if required.
Apart from requiring more battery power to operate them, the Type ‘B’ units were also much more complicated internally.
Valves require high voltage electricity, between 150 & 300V typically to operate. This was a particular challenge in early car radio design, given the 12V nominal battery supply.
It is one of the reasons that these kinds of radio were typically split over 2 units; the ‘head unit’ which housed the radio tuner and controls etc, and was fitted into the dash, and the separate amplifier, which also contained the high voltage power supply.
This could be mounted in a more convenient location deep within the dash, or even in the engine bay. They were interconnected by a complex multi-way cable.
The high voltage power requirements were facilitated by a device called a vibrator. This was an electro-mechanical item, which switched the 12V on & off around 100 times a second, and these pulses were applied to a transformer, which gave a high voltage of around 300V on its output. This in turn was fed to a rectifier valve and smoothing capacitor.
Several problems exist with such arrangements. The first is the vibrator itself. These were notoriously unreliable, and had a hard and short life. They needed frequent replacement to keep the radio working.
Unfortunately, given the age, you won’t find them available with any parts suppliers today, so replacing them is now an issue. To restore the radio I received, a custom, solid state replacement, which are made by a specialist. Being solid state, this modern part doesn’t wear out, but is made to look and behave exactly as the original did.
The second problem is these power supplies produce a lot of electrical noise, which is a killer of weak medium wave radio signals. It is another one of the reasons that this part of the set-up was housed in a separate case away from the radio head end. They produce an audible buzzing sound, which again was better mounted away from the driver and passengers.
Lots of electrical filtering and smoothing components were required in the amplifier unit, to ensure this noise didn’t find its way into the radio circuitry, making these car radios far more complicated than domestic receivers of the same period.
The third problem is power. Although valves don’t require too much current in their high voltage circuits, even 120mA at 300V translates into 3A at 12V, and that’s not even allowing for losses in the power supply itself. Add in a couple of amps for the valve heaters and you can quickly see why these units were power hungry.
Outside of the radio unit, and amplifier, one must not forget the all important, multi-wire cable that connects the head unit and amplifier together. This is a heavy loom and was difficult to work with. Contained within its wall are separate cables for 12V, 300V (back to the valves in the head unit) connections to the volume control in the head unit, and IF radio signals from the head unit to amplifier.”
Early Interceptor | HMV 4260 Radio Restoration | Setting To Work
“Once I had the HMV 4260 on my work bench, I did an initial appraisal. Both the radio and amplifier were dead, the interconnecting lead between the radio and amplifier had the plug from one end missing.
Once the lids were removed, it was obvious the amplifier was in a fairly poor state. There was one small light at the end of the tunnel – the speaker was in a very good state of preservation, this wouldn’t need replacement.”
“By the time I had finished, the Museum’s HMV 4260 radio set-up was performing as well as it did when first produced in 1955. It was a very satisfying job to complete, and lovely to work with the Museum’s agenda of keeping everything as original as possible.”
Early Interceptor | HMV 4260 Radio Restoration | Completed Restoration
Early Interceptor | HMV 4260 Radio Restoration
ACKNOWLEDGEMENTS: Adrian Gardiner, Radio Restoration UK.
COPYRIGHTS: Jensen Museum | Adrian Gardiner, Radio Restoration UK.
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