AC/ DC radios live chassis.

[tin0gauge]

Hello, on an AC / DC chassis like the Marconi T18DA which has a live chassis should the neutral go to the chassis? The existing mains wire is not colour coded.

[PJL]

AC/DC sets are not very safe under any circumstance but the T18DA does have a double pole mains switch so the side that switches to the chassis should be connected to Neutral. Many of the cheap imported USA sets have a single pole switch that disconnects the connection to chassis and if you connect this to neutral then the heater chain will make the chassis LIVE when switched off (only solution is to change the switch for double pole).

[kalee20]

Yes, the neutral should go to chassis if possible. (If you're operating it on DC and the positive happens to be grounded, you don't have a choice of course!)

[cathoderay57]

Ideally, operate it using an isolating mains transformer. Jerry

[Nickthedentist]

Lots of useful info here: https://www.vintage-radio.com/repair...ly-stages.html

[Techman]

Originally Posted by cathoderay57

Ideally, operate it using an isolating mains transformer. Jerry

Absolutely, that's the ultimate answer if you're worried about safety.

As for switching, the best method is a single pole switch in the LIVE only, with the neutral permanently hard wired to chassis for best safety.

[Nickthedentist]

Originally Posted by Techman

As for switching, the best method is a single pole switch in the LIVE only, with the neutral permanently hard wired to chassis for best safety.

Or simply switch off by pulling out the mains plug, which also saves wear and tear on the radio's mains switch and volume/tone pot if they're combined with it.

[ms660]

With the mains Neutral connected to/switched through to chassis a Neutral interrupt could surprise the unwary

Lawrence.

[G6Tanuki]

It was somewhat normal for the switch to be in the neutral wire though, because this means that when the radio is switched on the power wiring adjacent to the volume control wiring is at chassis potential, meaning less chance of AC hum getting across and into the audio circuit.
Putting the switch in the live wire means you have 250Volts passing rather close to the volume control..

[Nickthedentist]

Yes, I have several AC/DC sets where the switch is single-pole and in the neutral/chassis connection, e.g. Roberts P5A.

[emeritus]

In the days when 2 pin plugs, or a BC adaptor in a light socket, were commonly used to power domestic appliances, it was a 50:50 chance which conductor would be "hot".

[usradcoll1]

Originally Posted by PJL

AC/DC sets are not very safe under any circumstance but the T18DA does have a double pole mains switch so the side that switches to the chassis should be connected to Neutral. Many of the cheap imported USA sets have a single pole switch that disconnects the connection to chassis and if you connect this to neutral then the heater chain will make the chassis LIVE when switched off (only solution is to change the switch for double pole).

In the US, we're a little casual in our design of electrical products because of our low mains voltage.
Most of the radios exported to 220/240 volt countries, generally were the inexpensive makes using a resistive mains lead, which isn't very impressive.
Our listing agencies, such as U/L would not approve this design.
Dave, USradcoll1.

[winston_1]

With radios with a line cord (resistive dropper in the lead) the switch has to go in the neutral.

[lesmw0sec]

Originally Posted by emeritus

In the days when 2 pin plugs, or a BC adaptor in a light socket, were commonly used to power domestic appliances, it was a 50:50 chance which conductor would be "hot".

Indeed. In my apprentice days, i was advised to see if the chassis was live by gently drawing the back of the hand across it and detecting a 'buzz'!

[Herald1360]

Originally Posted by winston_1

With radios with a line cord (resistive dropper in the lead) the switch has to go in the neutral.

Why?

Most line cords were simply a resistor in series with the whole set, at any rate for US imports. The position of a switch in a simple series circuit has no effect on its switching function.

There were some UK designs with tapped line cords which allowed higher HT voltage to which this could apply I guess though a single switch in the heater feed branch would still work and would have the advantage of leaving the chassis at neutral with fewer parts still live than with switched neutral. And no worse off than any other AC/DC with reversed mains in and single pole switching either.

[usradcoll1]

Originally Posted by Herald1360

Why?

Most line cords were simply a resistor in series with the whole set, at any rate for US imports. The position of a switch in a simple series circuit has no effect on its switching function.

There were some UK designs with tapped line cords which allowed higher HT voltage to which this could apply I guess though a single switch in the heater feed branch would still work and would have the advantage of leaving the chassis at neutral with fewer parts still live than with switched neutral. And no worse off than any other AC/DC with reversed mains in and single pole switching either.

Some of the US makes used to offer an extension lead that contained a resistor for that particular model, intended for export to area with higher voltage mains.
Dave, USradcoll1

[broadgage]

Yes, the neutral side of the AC mains should go to chassis.
With DC mains, the user got no choice, and with non polarised plugs on AC it was pot luck.

But these days with DC mains being effectively extinct, and polarised plugs being almost universal, then neutral to chassis.

For best safety turn the set off at the socket outlet when not being used.

[Sideband]

Originally Posted by PJL

AC/DC sets are not very safe under any circumstance

They can't be that unsafe! TV's used the same technology for a lot longer and I repaired hundreds of them without the hint of any danger. The workshop I started in didn't have isolation transformers and the benches were fitted with two-pin plugs so worked on the sets (radio's and TV's) on raw mains. All I did was check the chassis with a neon screwdriver to see if it was live or not.

An isolation transformer doesn't really make the sets safer. You can still get a nasty belt if you touch anything live. All the transformer does is enable you to connect earthed test equipment.

[ms660]

Re: Isolating transformer, see Post#8.

Lawrence.

[Hybrid tellies]

Originally Posted by Sideband

They can't be that unsafe! TV's used the same technology for a lot longer and I repaired hundreds of them without the hint of any danger. The workshop I started in didn't have isolation transformers and the benches were fitted with two-pin plugs so worked on the sets (radio's and TV's) on raw mains. All I did was check the chassis with a neon screwdriver to see if it was live or not.

An isolation transformer doesn't really make the sets safer. You can still get a nasty belt if you touch anything live. All the transformer does is enable you to connect earthed test equipment.

Totally agree. My first few years after leaving school being employed as a trannie TV engineer all the television sets we worked on were AC/DC and there was no isolation transformer in the workshop or in customers houses. It was down to the good training we received from other engineers and of course the college that kept us and our customers safe.