Mains Cable; 2 or 3 Wire

[Snappa1]

I have just been given a 1950's Bush domestic wireless, without a plug, which I haven't had time to look at yet.
However, the first thing I checked was that it does have a mains transformer, and I noticed that it doesn't have an earth wire in the mains cable; 2 wires only.
Everything these days is earthed or double insulated, and I would expect this chassis to be earthed, using the mains cable. There is a socket for earthing the chassis, alongside the aerial socket.
For safety reasons, should I replace the mains cable with a 3 wire cable ?
Thanks,
Alan
G7DCF

[evingar]

Your question suggests that you are not familiar with this technology. I would urge extreme caution. Please do not switch this set on until it has been looked at by someone familiar with this technology, or you have learned enough to be absolutely certain you know what you are doing.

A model number would be good. Earthing an isolated transformer set should be OK, but some say it stesses old tranformer insulation. There is a remote chance it's an un-isolated auto tranformer though.

[Nickthedentist]

Agree with Chris that we need a model number, but as you say it "does have a mains transformer", it's PROBABLY an isolated chassis design and can be earthed.

Many people feel that earthing such sets is un-necessary and can lead to problems, but I would agree with your sentiments and fit a 3-core lead, along with a 1A fuse in the plug, and ensure it's plugged into a RCD-protected socket.

Do get back to us with the model number, though...

Nick

[David G4EBT]

The term 'a 1950s Bush radio with a two-pin plug' together with the fact that it has no mains transformer suggests that it's odds on that it's a DAC90A - one of the most common Bush radios of the era, about which posts concerning the design of that 'live chassis' radio, safety issues and how to go about competently restoring it abound on this forum.

If it is indeed a DAC90A, then it most certainly must not be earthed. The advice in post 2 above is very sound. The fact that the question has been posed at all suggests a lack of experience and competence, but if the model number is stated and help and advice is sought, there will be no shortage of sound advice and help from experienced restorers on the forum as to how to safely go about putting the radio into safe working order.

Unfortunately, the most sound piece of advice often comes too late or is ignored: 'Don't be tempted to plug it in to see if it works before it has been tested and any faults diagnosed'.

[stuarth]

The presence of a mains transformer does not guarantee the set has an isolated chassis. Some sets had a transformer for the heaters but were otherwise live chassis.

S

[AC/HL]

Before we go any further with this we need a model number.

[Nickthedentist]

Agreed. An output transformer or a choke can sometimes look just like a mains isolating transformer... but not be!

[kalee20]

Originally Posted by David G4EBT

The term 'a 1950s Bush radio with a two-pin plug' together with the fact that it has no mains transformer suggests that it's odds on that it's a DAC90A

Not if it has an aerial and earth socket it isn't! See original post.

Model number or photos are really important here.

David G4EBT's advice, not to power it up yet, is very sound.

[David G4EBT]

Originally Posted by kalee20

Not if it has an aerial and earth socket it isn't! See original post.

Ooops, you're right - how careless of me!

[Herald1360]

That E socket could be connected to a live chassis by a low value capacitor- don't know whether Bush did that but some makers did.

[Glowing Bits!]

I don't know either whether Bush did this, other manufacturers did though.

I'll take a look at the paperwork I have to see if any Bush AC/DC sets have external antenna sockets.

The advice already given by fellow forum members should be followed when it comes to wiring up certain sets, my method is to fit an earth wire to the chassis of sets with a transformer, followed by a 2 AMP plug fuse, AC/DC 'Live Chassis' sets have a 2 core flex fitted with a 1 AMP fuse in the plug.

[Snappa1]

Sorry for the confusion, but to clarify, I said it had no plug, not a two pin plug, and a two wire mains cable.
I am very familiar with the technology. It is a Bush AC41, for which I have downloaded the schematic.
The question arose more from the "ethics" of replacing the original two wire cable with a "modern" three wire cable. I would normally earth any metal like this for safety reasons, but as I say, it is the "ethics" I was more interested in.
I haven't put a plug on the cable yet, and "No", I won't plug it in before doing a few tests, and looking at the state of it underneath.
The question is still open. Ethics vs Safety ! Your thoughts, please.
Thanks,
Alan

[Skywave]

Originally Posted by Snappa1

The question is still open. Ethics vs Safety ! Your thoughts, please.

I don't understand that implied trade-off or choice: "ethics" relates to morality and standards of personal conduct. Don't you mean "Originality vs. safety"? That is a common dilemma for any vintage radio restorer.

Al. / Oct. 17, '13 //

[evingar]

Not sure about the "ethics" of it, but looking at the circuit, I can see no reason that this set can't have its chassis earthed.

I don't see the problem with doing so with respect to "originality" because you can easily reverse the situation. In any case, in original operation the chassis is designed to be earthed via an earth stake, the plug earth is just a more convenient way of achieving an earthed chassis.

[Synchrodyne]

Originally Posted by Herald1360

That E socket could be connected to a live chassis by a low value capacitor- don't know whether Bush did that but some makers did.

Bush certainly did it this way with the EU.24 export model. The “Earth” socket was connected to the chassis via a 0.01 µF capacitor. (The earthy side of the gram input socket was connected to the chassis by a 0.1 µF capacitor.)

In this case, the earth socket was clearly intended for the connection of an RF earth only, as the capacitor would be high impedance at AC mains frequencies.

In the case of the AC41, with a mains isolating transformer and where the “Earth” socket connects directly to the chassis, then the latter could be used for both RF earthing and safety earthing purposes, although I suspect that the former was seen as primary when the model was current. Had safety earthing been a primary concern back then, then perhaps a three-core mains lead would have been fitted as standard, in order to ensure that the unit could not be plugged-in in an unearthed condition. I suppose the rationale of the era was that AC mains-only receivers were typically no riskier than the AC-DC type, assuming that they were built with similar attention to inaccessibility of live or potentially live parts.

Today of course the view is very different, and safety earthing, via the mains lead, would take priority over all else. Then safety earthing and RF earthing could be in conflict, in that the mains earth might inject more RF noise than an independent RF earth. But in some jurisdictions a completely independent earth would be against regulations; any independent or isolated earth wire must be returned to the building main earthing point, or if connected to its own earthing stake, the latter must be suitably bonded to the main earthing point to prevent large voltage differentials between the earth stakes, e.g. during lightning storms. I should imagine though, that an independent earth wire carefully routed to either the main earthing point or to a separate earthing point bonded to the main earthing point would still be quieter, in RF terms, than using the earth conductor in the mains wiring.

The question of RF earthing of the chassis itself would appear to go away if the receiver, via its aerial and earth sockets, is looking back into a closed loop, whether an actual loop aerial or the secondary of a transformer, as with the old antistatic aerial systems. In the latter case the RF earthing problem is simply shifted to the primary side of the transformer, and the same independent earthing considerations would apply. Wellbrook appears to advocate the use of an isolating transformer to separate safety and RF earths; see: http://www.wellbrook.uk.com/pdf/AFI5030A.pdf.

Cheers,

[crusher19860138]

I believe the radio should be wired how it was designed,i did however,have one radio that was tripping the earth leakage breaker in a house once,the earth of the 3 core cable doing it's job of earthing,only problem was the radio was a DAC90,and the chassis was neutral!

[jsbhavsar]

Originally Posted by Snappa1

For safety reasons, should I replace the mains cable with a 3 wire cable ?

I had similar dilemma during year 2002 when I was trying to modernise and restore my National Ekco A731 8 band valve radio in India. As the power cord had worn out, the restorer suggested that we replace it. I opted for the 3 PIN Plug wire. Then the restorer had a dilemma for sometime whether to connect the earth wire of the plug to the chassis. I believe then he connected the chassis to the earth. I recently checked by opening the Ekco radio at home and saw it was earthed through the Mains earth.