Eddystone 770R VHF Communications Receiver

A 19 valve receiver designed for 19 to 165 MHz (see below for details)

 The set introduced in the early 1950s weighs in at 60lb or about 27kgm and receives AM and FM signals. It is old-school and has only a single conversion circuit design so that image problems abound. The oscillator is always on the high side of the selected band so that on Range 2 where there are many strong broadcast signals, setting the local oscillator to 93.7MHz, for example, will pull in stations on both 88.5 and 98.9MHz. Unless its primary requirement was for listening on a very narrow band of frequencies via a narrow-band yagi the use of single conversion with a relatively low IF was a very poor choice by the designers.

Still, as far as the mechanical design is concerned it's certainly in the same mould as other Eddystone offerings. Modern sets, with their digital displays and tiny push buttons may produce better actual results but you just can't beat a nice large linear dial and flywheel tuning for relaxing band searching, unless of course there had been a small reference display showing the exact frequency (although, in 1952 this extra would have occupied a box of similar size to the old Eddystone so we'll have to put up with the little window of the centre rotary display and its mainly-hidden plate-sized dial).

 

Valve line-up:

V1 RF amplifier, 6AK5 or EF95; V2 Mixer, 6AK5 or EF95; V3 Local oscillator, 6AK5 or EF95

V4 1st IF amplifier, 6BA6 or EF93; V5 2nd IF amplifier, 6BA6 or EF93; V6 3rd IF amplifier, 6BA6 or EF93

V7 4th IF amplifier, 6BA6 or EF93; V8 FM limiter, 6AU6 or EF94; V9 Discriminator, 6AL5 or EB91

V10 AGC rectifier and noise limiter, 6AL5 or EB91; V11 Meter control 6AU6; V12 BFO 6BA6 or EF93

V13 Noise amplifier, 6AU6 or EF94; V14 Muting control and 1st AF amplifier, 12AU7 or ECC82

V15 2nd AF amplifier and Phase Splitter, 12AU7 or ECC82; V16 and V17 Push-pull audio output 6AM5 or EL91

V18 Stabilizer 0D3 or VR150/30; V19 HT rectifier 5Z4G or U50

D1 AM Detector; D2 and D3 Noise rectifier all type GEX34

Wavebands

Range 1 114 to 165 MHz

Range 2 78 to 114 MHz

Range 3 54 to 78 MHz

Range 4 39 to 54 MHz

Range 5 27 to 39 MHz

Range 6 19 to 27 MHz

IF Frequency: 5.2 MHz

Audio output is provided by terminals for a 2.5ohm loudspeaker, a 600ohm feed and a headphone jack socket

These sets have suffered more than most over the years. A type of capacitor used in the construction has deteriorated to such an extent that most are entirely useless ranging from just plain leaky to low impedance resistors. As many of these capacitors are used in the RF front end, where they are used, not only for decoupling and tuning, but also, because of the peculiar design of the front-end, as mechanical mounting components.

At the rear of the underside of the chassis is a detector/audio strip. The design of this unit is incredibly poor, demanding that it be completely disconnected from the chassis wiring and removed in order to measure or replace components. Suffice it to say, there are many of the bad brown-moulded capacitors lurking in this unit.

A further poor feature is the design of the turret tuner mechanism. The precise position of the turret depends on balls locating in holes in a tufnol or bakelite disk. With a lot of use the holes wear and their resulting increased diameter means that the turret may come to rest within an eighth of an inch from its proper place, upsetting the dail calibration. A side effect of this is to worsen the already poor image performance resulting in the highest frequency band having twice as many signals present as there should be.

See the technical manual

Another example of a 770R, acquired August 2004, waiting for alignment

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