Mains polarity of live-chassis radios. - UK Vintage Radio Repair and Restoration Discussion Forum

PJL · 18th Jan 2007, 11:19 pm

I just started to work on a Ferguson 909U and was a little alarmed to see the mains switch is single pole and connects the mains to the chassis. So if I make this neutral, when switched off the chassis is LIVE through the heater chain.

I can rewire the switch to be on the live and connect the chassis to neutral? What have others done?

Overall this set has to be described as SERIOUSLY dangerous. I have replaced the mains dropper lead with a capacitor but unlike later AC/DC sets, the knobs are screw fit and the shafts protrude out of the case and there is very little care to prevent fingers touching the chassis through the back.

From 1940, this was built in the early part of the war so I guess electrical safety was a low priority...Peter

Paul Stenning · 18th Jan 2007, 11:31 pm

This is fairly common in many AC/DC sets, not just the Fergie you are working on.

I think there were two reasons for doing it this way:

1. Ease of wiring. Neutral to the switch then a short link to the chassis; live directly to the dropper or voltage selector.

2. Reduce the risk of hum getting onto the audio. If the live is switched, you have 230V AC wiring close to small signal high impedance audio wiring. That could require care in positioning the wires to avoid hum pickup, which is probably not practical in hand-wired production.

I always rewire it so that the switch switches the live instead of the neutral. Sometimes the wires have to be tied away from the signal wiring and nearby components to avoid pickup, but it makes it safer. Note "safer" not "safe" - it still won't comply with current standards, but at least the chassis isn't live (unless the plug or socket are wired incorrectly).

Ideally the pot should be replaced with one having a double pole switch, but I would only do this if the original is faulty. Most modern ones have plastic shafts which are not as rigid as the originals which can result in the knob feeling a bit wobbly. Also the shafts on modern ones are often not quite long enough.

Radio_Dave · 18th Jan 2007, 11:43 pm

Quote:

Originally Posted by Paul Stenning

Ideally the pot should be replaced with one having a double pole switch, but I would only do this if the original is faulty.

You could always fit an inline double pole switch into the mains lead. It would, also, save wear to the volume/tone control track http://www.maplin.co.uk/Module.aspx?...=SEO&U=strat15

David

paulsherwin · 19th Jan 2007, 1:10 am

As Paul says, this is a very common arrangement with live chassis sets from the 40s and 50s. There's a third possible reason in addition to those he states. Switching the connection to the chassis means that, *if* the plug is wired the wrong way or inserted the wrong way round (2 pin plugs remember) the chassis will be isolated when the radio is switched off. This seems a pretty mad approach to safety from our cosy world of fused polarized mains plugs and RCDs, but it would have made a lot more sense in the 40s and 50s. My parents' house had nonpolaraized 2 pin plugs until it was rewired in the mid 60s.

Paul

ppppenguin · 19th Jan 2007, 9:55 am

If you are going to use early radios etc in the home it does no harm to fit RCD plugs to them. Alternatively ensure that the circuits they are plugged into have RCD protection at the consumer unit.

In my opinion it is always an acceptable modification to rewire a single pole mains switch from the chasssis side to the "hot" side.

PJL · 19th Jan 2007, 1:01 pm

Thanks all...it dawned on me that it would have needed an extra way on the switch due to the cord dropper. In the interests of safety I am going to make the changes to put the switch on live and will twist the leads in the hope of preventing induced hum.

Paul, I used your excellent article on capacitor droppers and it came out right on the nail - 3uF motor run capacitor and 50ohms. It has a very gentle start which I suspect is inherent in the design - when the load resistance is low, the phase changes so less voltage is produced?

G8DLG · 19th Jan 2007, 8:37 pm

Quote:

Originally Posted by PJL

I just started to work on this set and was a little alarmed to see the mains switch is single pole and connects the mains to the chassis. So if I make this neutral, when switched off the chassis is LIVE through the heater chain.

A large no of sets had this arrangement,it can be argued that it is safer to switch the chassis connection since this could be connected to the live side of the mains,or neutral.if the neutral is connected,the chassis will be live at switch off,but only through the heater line which limits the current.
This idea is used on automobiles,etc the earth side is switched.
Personally,I would not change the wiring,but ensure that there is no electrical access to any external parts,and maybe as been suggested fit a through lead switch,or else fit a double pole switch as the later sets had.

PJL · 19th Jan 2007, 10:42 pm

In this set it is pretty much impossible to prevent accidental touching of the chassis under reasonable fault conditions. It would only take a knob to come off to expose the shaft or someone to use a wet hand to cause contact with the grub screw so I think I will make the change anyway.

It only had one original capacitor and I gutted the electrolytic to fit the capacitor dropper inside (no other space available in these sets!) so it is only going to look original above the chassis anyway.

So far the other 40's AC/DC sets I have worked on have all had recessed shafts and have taken a bit more care with the design of the back panel.

18th Jan 2007, 11:19 pm	#1
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Mains polarity of live-chassis radios. I just started to work on a Ferguson 909U and was a little alarmed to see the mains switch is single pole and connects the mains to the chassis. So if I make this neutral, when switched off the chassis is LIVE through the heater chain. I can rewire the switch to be on the live and connect the chassis to neutral? What have others done? Overall this set has to be described as SERIOUSLY dangerous. I have replaced the mains dropper lead with a capacitor but unlike later AC/DC sets, the knobs are screw fit and the shafts protrude out of the case and there is very little care to prevent fingers touching the chassis through the back. From 1940, this was built in the early part of the war so I guess electrical safety was a low priority...Peter Last edited by Darren-UK; 12th Aug 2007 at 12:04 am. Reason: Model number added to text due to title editing.

18th Jan 2007, 11:31 pm	#2
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: Ferguson 909U safety issues This is fairly common in many AC/DC sets, not just the Fergie you are working on. I think there were two reasons for doing it this way: 1. Ease of wiring. Neutral to the switch then a short link to the chassis; live directly to the dropper or voltage selector. 2. Reduce the risk of hum getting onto the audio. If the live is switched, you have 230V AC wiring close to small signal high impedance audio wiring. That could require care in positioning the wires to avoid hum pickup, which is probably not practical in hand-wired production. I always rewire it so that the switch switches the live instead of the neutral. Sometimes the wires have to be tied away from the signal wiring and nearby components to avoid pickup, but it makes it safer. Note "safer" not "safe" - it still won't comply with current standards, but at least the chassis isn't live (unless the plug or socket are wired incorrectly). Ideally the pot should be replaced with one having a double pole switch, but I would only do this if the original is faulty. Most modern ones have plastic shafts which are not as rigid as the originals which can result in the knob feeling a bit wobbly. Also the shafts on modern ones are often not quite long enough. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

19th Jan 2007, 1:10 am	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: Ferguson 909U safety issues As Paul says, this is a very common arrangement with live chassis sets from the 40s and 50s. There's a third possible reason in addition to those he states. Switching the connection to the chassis means that, if the plug is wired the wrong way or inserted the wrong way round (2 pin plugs remember) the chassis will be isolated when the radio is switched off. This seems a pretty mad approach to safety from our cosy world of fused polarized mains plugs and RCDs, but it would have made a lot more sense in the 40s and 50s. My parents' house had nonpolaraized 2 pin plugs until it was rewired in the mid 60s. Paul

19th Jan 2007, 9:55 am	#5
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Ferguson 909U safety issues If you are going to use early radios etc in the home it does no harm to fit RCD plugs to them. Alternatively ensure that the circuits they are plugged into have RCD protection at the consumer unit. In my opinion it is always an acceptable modification to rewire a single pole mains switch from the chasssis side to the "hot" side. Last edited by Darren-UK; 12th Aug 2007 at 12:00 am. Reason: General tidying.

19th Jan 2007, 1:01 pm	#6
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Ferguson 909U safety issues Thanks all...it dawned on me that it would have needed an extra way on the switch due to the cord dropper. In the interests of safety I am going to make the changes to put the switch on live and will twist the leads in the hope of preventing induced hum. Paul, I used your excellent article on capacitor droppers and it came out right on the nail - 3uF motor run capacitor and 50ohms. It has a very gentle start which I suspect is inherent in the design - when the load resistance is low, the phase changes so less voltage is produced?

19th Jan 2007, 10:42 pm	#8
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Ferguson 909U safety issues In this set it is pretty much impossible to prevent accidental touching of the chassis under reasonable fault conditions. It would only take a knob to come off to expose the shaft or someone to use a wet hand to cause contact with the grub screw so I think I will make the change anyway. It only had one original capacitor and I gutted the electrolytic to fit the capacitor dropper inside (no other space available in these sets!) so it is only going to look original above the chassis anyway. So far the other 40's AC/DC sets I have worked on have all had recessed shafts and have taken a bit more care with the design of the back panel.