I was reading a 'restoration' type article about valve radios which stated that "AC/DC sets must not have their chasses earthed". Although new to this hobby, I do understand and use an isolation transformer, RCD etc, but I don't grasp the problem. Can some-one please explain .... maybe using typical circuit diagram?? Cheers, Ian

Many AC/DC sets did not come with an isolation transformer and used other methods to drop the voltage for the valve heaters. If the plug on an AC/DC set is wired the wrong way around or there is another defect inside the receiver then it will place the chassis at mains potential.

This means that if you touch the chassis in such a situation then you'll cop 240 volts through the body part that comes in contact with the chassis. or any exposed metal that is attached to the chassis.

AC/DC receivers are extremely dangerous receivers for inexperienced people to work on for this reason. Electricity is always a hidden danger.

One thing to note about external isolation transformers and RCDs - these only provide some protection and they never make a live chassis completely safe. An RCD won't work at all if you are also using an isolation transformer and your fingers form a path of least resistance between a component and the chassis of a receiver. This is because the fault to earth is taking place after the isolation transformer and not before it - therefore there is nothing for the RCD to detect.

Don't get me wrong - there is a place for safety devices in our workshops and they do make a difference but it is equally important to know their limitations.

There is only one way to avoid electrocution and/or damage to a receiver and that is to take extreme care at all times. Due to the nature of AC/DC receivers - any warnings pertaining to them should be heeded and some experience gained in fixing valve radios before attempting restoration of an AC/DC set.

Hi Brad,
Thank you, I understand your advice.
Can I clarify one point? When you talk about a "live" chassis, do you limit this to mean a chassis at positive 240 volts AC, as distinct from neutral side of 240 volts??
I seem to remember reading about old valve radios using live chasses (???). Did this really mean that positive 240 volts was connected directly to the chassis??
Either way, I never 'buy and plug-in' to see if it works. I usually spend hours checking the wiring and main components for myself before connecting 240v .... even then I check everything with a DMM, even then I do not touch anything when it is on! Cheers, Ian

Can I clarify one point? When you talk about a "live" chassis, do you limit this to mean a chassis at positive 240 volts AC, as distinct from neutral side of 240 volts??

That's about the size of it.

AC/DC radios cannot be earthed. Most of these with external aerial connections use a 0.01mfd cap as a blocker, If its paper, look out!

Unfortunately with these on the mains, there is a fifty fifty chance of the mains being on the chassis. Should the frame be earthed in this situation, the house fuses / circuit breakers, will quickly blow / trip.

These sets are lethal, If a knob comes off, there is a fair bet that the metal shaft will be "alive". Keep it away from children.

If you want to run it an external, transformer power supply is a safer option.

Marc