25-09-2012, 01:30 PM
(25-09-2012, 09:37 AM)Old Sparky Wrote: With all due respect to both Mark and Lawrence, the Westminster, like many other record players(and other things)of that day was originally fitted with a two-core(non 'polarised') mains lead, which could have led to the amplifier metalwork being 'live' if the plug was wired incorrectly. The owner's son had, he told me, removed the original lead, connected a 3-core one, using terminal blocks to join the live & neutral(the correct way round0, then attached the earth lead to the record deck metalwork rather crudely, by just baring the end, twisting the strands together and fastening them via the screw securing a metal cable clamp. IMHO the player would DEFINITELY have failed a PAT test under those circumstances, as well as emitting a very loud hum. In this player the mains neutral is common to the DC Negative line, the only thing to do was to use a 2-core mains lead, correctly wired and soldered to the on-off switch contacts, which I did by cutting off the earth lead inside the player. I don't know of many that were fitted with isolating capacitors when new, as shown in the diagram posted by Joe(Of what make/model is that the circuit, as a matter of interest?) I should add that, although one side of the P.U. cartridge wiring is connected to the common negative, and thus to mains neutral, which is also the chassis of the AMPLIFIER, it is isolated from the record deck metalwork. The Volume & Tone Controls have plastic spindles, and no part of the amplifier metalwork is touchable when the player is reassembled,
As a matter of interest, many sets, especially AC/DC types would surely fail a PAT test as originally manufactured?
Hi,
I take your points, but I'm afraid none of what you say reassures me.
Yes, in original form, the unit had a two-core lead, and yes, it was probably safe enough for the safety standards that prevailed at the time. And, truth be told, it might even have passed a PAT test.
The only problem with it in its original state is that it relied on the isolation between the screen of the PU lead and the desk. Perhaps not a problem in practice, but might not meet modern regulations for creepage, etc...
With the added 3-core cable, there was an electrical connection to the metalwork of the deck. A person carrying out a PAT test would recognise that there was a 3-core lead, making the unit a Class 1 device, and so would measure the continuity between the earth pin and the exposed metalwork of the deck, and would have measured close to zero Ohms - that's a "pass". The PAT tester will also measure leakage from L/N to earth, and assuming that PU cable was in good order, it would have passed this aspect too. So, based on information provided, and despite your assertion to the contrary, I can't immediately see how this would have failed the PAT test. Remember, the PAT test does not involve any internal inspection of the equipment - although it should involve an inspection of the plug itself (and the fuse).
Now that you've disconnected the earth internally, this is definitely going to cause it to fail. A tester would note the 3-core cable, and attempt to test it as a Class 1 device, but will be unable to find earth continuity with any exposed metalwork.
If you want to remove the earth from the record player, then you must replace the cable with a 2-core cable. Apart from anything else, a partly-technical user seeing a 3-core cable would assume that the deck is earthed, and this would give a false sense of security.
By leaving the 3-core cable in place after you've worked on it, you're responsible for this, and you could be blamed for modifying this unit. If you are charging for work on this unit, you could end up in trouble should the worse happens.
In your position, I'd do one of two things:
1. Return the wiring exactly back to the original condition, and get the customer to sign a disclaimer. Instruct them to operate it via an RCD (or buy a plug-top version to replace the mains plug). Perhaps buy a cheap figure-of-eight mains lead with a sealed mains plug, and chop off the figure-of-eight plug and hard-wire the lead to the deck, thus ensuring no future live-neutral swaps...
2. Install an isolating transformer, and firmly earth the deck and any exposed metalwork, and the amplifier chassis. Make up a sticker detailing exactly what you've done and put it on the back.
Having done either, get it PAT tested.
Once you've done one of these 2 things, then you can think about the hum problem, if it still remains.
I work in professional audio, and I am well aware that people do "strange" things with the earths in audio systems in an attempt to deal with hum loops and similar issues. However, these measures, even if temporary, must not be allowed if they compromise safety in any way. I've seen the consequences of this, and it ain't pretty - I make no apologies for being so firm about this.
In BBC studios, we PAT test any equipment that a band brings in, and we often power equipment from isolation transformers - even if it passed the PAT test.
I'm trying to help.
Mark
