Eddystone 840A power-socket.

[G6Tanuki]

The Eddystone 840A I'm currently working on [bought at this year's Newbury radio-rally] has, amongst other mutilations, had its standard power-socket removed and a length of orange 2-core lawnmower-cable wired direct to the two fuseholders.

My question is - what to do to replace this. The space available is rather too narrow to fit my power-socket-of-preference, the 3-pin "Plessey Mk.4" as used on loads of 1950s/1960s test gear.

I'm inclined to cut out the web between the two holes, and fabricate a plate from a chunk of scrap PCB then install an IEC "C8" chassis fitting so to take a traditional figure-of-8 'cassette-player' type lead.

Other suggestions about how to fill the hole are welcome.

[Mans Veldman]

I faced the same problem with my 840. It was missing the original connector so I replaced it with a standard kettle connector. This is a dutch version, but I believe in the UK you have almost the same.

Also note the incorrectly installed rear, it should of course fall to the inside of the metal side. I later corrected this.

[G6Tanuki]

Originally Posted by Mans Veldman

I faced the same problem with my 840. It was missing the original connector so I replaced it with a standard kettle connector. This is a dutch version, but I believe in the UK you have almost the same.

I must investigate this. Do you Nederlanders have an "IEC" code for your kettle connector?

[Mans Veldman]

I don't know if IEC coding was already used in those day's. We normally referred to it as Toaster or Vacuum cleaner plug This type is no longer in production and was succeeded by IEC C14 or C16.

The pins are spaced 19mm apart and thickness of the pins is 5,9mm

[G6Tanuki]

Originally Posted by Mans Veldman

I don't know if IEC was already used in those day's. We normally referred to it as Toaster or Vacuum cleaner plug
The pins are spaced 19mm apart and thickness of the pins is 5,9mm

Thanks for the info - though I doubt these connectors are going to be easy to buy here in the UK.

We had a range of non-uniform plugs used for kettles in the 1950s and 1960s - but they were usually circular.

See http://www.amherst.co.uk/round-kettle-socket.jpg

[Silicon]

Do you intend to earth the metal case?

If you do, it may be better to used a fixed 3 core flex.

[Kala_12]

Or an IEC C13 type Socket Lead?
Stuart.

[turretslug]

Or the IEC C5/C6 combination ("laptop", "cloverleaf")- the C13/14 is a proven and readily available stalwart but the panel plug (inlet) is quite large in close-packed/compact kit.

[G6Tanuki]

The laptop type cloverleaf is definitely an option - it would fit without too much rework. Non-reversible, too. [my plan is to cut out the 'web' between the two existing holes, then use a PCB scrap to make up a new mounting to ake whatever connector I standardise on.

I've already determined that the small round 3-pin Bulgin-type sockets are too bulky to be a sensible option.

Given that the 840A is a 'potentially-live-chassis' design I'm wondering what to do about earthing. My initial thought was to use a suitable common-mode mains-filter at the input, with the 'earth' from this connected to the mains earth only.

[turretslug]

The C6 panel plug is a good, minimum-butchery alternative for many unobtainable older types- it's available from the likes of Farnell for less than £1 and the C5 moulded line socket is ubiquitous now- I dumped a bundle of NOS ones on the table at the last Harpenden in the hope that someone might be interested but we've probably all got a few kicking around!

There is an in-line (but still polarised) re-arrangement of the 3-pin 2.5A C5 that would be ideal for many Eddystones (other than the 840 types) but, as far as I can gather, it seems to be a Dell OEM "special".

[G6Tanuki]

I likewise have a cache of C5 moulded line-sockets, and am also familiar with the Dell connectors [Dell laptops are the default issue for one of my business clients]. Like you say, the C5/C6 is probably going to be the easiest option.

The Eddystone 840A in question has already been ;got at; in various ways and will never be a pristine example of the genre, so I have no qualms about irreversibly modifying it if the mods are subtle and improve overall usability. I've got a replacement UY41 rectifier on order (to replace the tagstrip-wired BY127 diode fitted by a previous owner) and a new UL41 too (existing one has significant grid-leakage). The missing HT choke [P.O. replaced it by a totem-pole of wirewound dropper resistors] will probably be replaced by the primary winding of a small mains-transformer with the E and I laminations re-stacked to provide an air-gap.

[Synchrodyne]

Originally Posted by G6Tanuki

Given that the 840A is a 'potentially-live-chassis' design I'm wondering what to do about earthing. My initial thought was to use a suitable common-mode mains-filter at the input, with the 'earth' from this connected to the mains earth only.

My understanding is that the Eddystone AC/DC receivers were intended to be operated with a protective earth connection to their outer cases, logically the mains earth. With a common mode mains filter upstream, it would appear that the Eddystone protective earth would need to be connected to the filter earth, in turn connected to the mains earth, in order to obtain the desired result in protective terms.

Doing the protective earthing properly might though make it difficult to do the functional earthing (for unbalanced aerials) in the optimum way. At least from the Eddystone 840A schematic, it looks as if once the protective earth connecting plug is in place, then it also connects to the functional earth. So one would be stuck with using the mains earth as the RF earth. Regulations vary by territory, but generally protective and functional earthing systems should be kept completely separate except for being joined at one point, such as at the main switchboard or the main earth stake. In the US, bonding of functional earths to the main protective earthing system is mandatory; here in NZ I think it might be optional. Where an earthing system is both protective and functional, then the protective requirements take precedence.

Actually, with valve era radio receivers, one has the impression that the makers were not always clued up about good earthing practice. Hence one found AC-only receivers whose chassis were earthed via the mains lead, but which also had earth terminals connected directly to the chassis, with the recommendation that a direct earth connection, to a ground stake, water pipe, etc., be used to improve reception and/or reduce noise. Thus protective and functional earths would be bonded at the chassis, quite contrary to regulatory requirements and good practice. One might envisage spitting the two earths by a suitable network, say a choke (capable of carrying fault current) on the protective side and a capacitor (whose only possible failure mode was open-circuit) on the functional side, but I am not at all sure that such would be acceptable in a regulatory sense.

I suppose the most elegant solution is what in the USA is known as an “isolated ground”. Here the designated outlet receptacle has an earth connection that gets back to the switchboard earth bus or to the earthing stake by a separate pathway designed to avoid pickup of noise and interference. It thus serves both as a protective earth and a low-noise functional earth. But that is probably well beyond what might be reasonable for the receiver at interest.

An isolating transformer between the common mode filter (which would have a protective earth) and the Eddystone would be another option. This way the Eddystone earth would be purely functional, and so could be configured for the best RF results. Whether it would affect filter operation I am not sure. Alternatively, having the isolating transformer upstream of the filter means that there would be two appliances connected to the same transformer, which is out-of-bounds unless they are connected by suitable equipotential bonding, such as the normal protective earth conductor in their respective three-core cables. Then having the filter and the receiver share a common functional earth might not be too desirable.

So, no easy path to earthdom.

Cheers,