Over Zealous!

[tiv48]

good point joe which is why some sets have only single pole switches.
rob t

[Old Sparky]

Maybe it's because I spent 23 years working as a radio service engineer, but I usually replace faulty components. Seldom do I open up 50 year old pots. to fix them, unless they are of a special type or unusual value/track law, etc. Others may disagree with me, but, unless the radio(or whatever)is a particularly rare one, I don't see a problem replacing an old metal-shafted control with a modern pot. having a plastic spindle, other than the fact that most modern ones have 6mm shafts, rather than 0.25" ones. Usually if a double pole switch was present, I replace like with like, though I take Joe's point regarding the use of a single pole switch.

[Bushbaby]

Thanks for the advice guys but you will have to be a wee bit more explicit for an electronics numpty like myself. I've replaced the pot with a new 470kR Log switch pot but now you have made me concerned about the switching.

Regards and thanks

Rob

[Joe]

As you know, Rob, one side of the mains goes straight to the chassis on these sets. You'll have made sure that this is the neutral.

If you connect both sides of the switch and for some reason the neutral side switched off but the live side didn't the chassis would no longer be connected to neutral. Electricity would come up the live wire, through the electronics and to the chassis. It would lurk there awaiting a finger to complete its journey to earth.

This is why I prefer to not switch the neutral. It's not likely to be a problem with a new switch and disconnecting both halves is safer PROVIDING they do both disconnect.

As so often it isn't about Right and Wrong so much as what you feel most comfortable with.

- Joe

[pwdrive]

I can see a couple of scenarios, O/C neutral connection in the mains plug or lead or mains plug/socket connection, if the neutral switch is discarded ie: neutral connected direct to chassis then the chassis will still be live if the live switch does not switch off.

If the set somehow ends up with someone else who is non tech: live switch does not switch off + pilot bulb u/s, set remains on but for how long..long enough and it could overheat..with consequences.

My advice is that when servicing ac/dc live chassis stuff always run the set via a suitably rated mains isolating transformer and connect the chassis to the mains earth with a seperate earth lead.

Lawrence.

[Joe]

An isolating transformer is the only really safe solution as however well you repair a radio you can't be sure that it will never be connected to a socket where the live and neutral are reversed.

That said, providing the case, knobs and back are in good order it shouldn't matter if the chassis IS live during normal use, the danger is when fiddling about. These sets were made at a time when two-pin plugs and lampholders were still common so it was fifty/fifty anyway and a double pole switch was indeed better.

It doesn't keep me awake at night as I only repair things for my own use and know to be careful.

- Joe

[pwdrive]

Hi Joe, I learnt my trade on chassis that had the potential to become live, both radio, record players and tv's, mainly tv's. When I first started work at the first firm the workshops saftey equipment consisted of a cartoon picture of an angel going up to heaven carrying a service techs tool box pinned up on the wall at the back of the bench, a 40w bulb with one end connected to earth and the other end connected to a fly lead, that was it. I soon progressed to another firm that took a more progressive view, every bench had it's own mains isolating transformer fitted.
I can only speak from my experience in the trade concerning some of the bodge ups/frigs etc that others had inflicted on various tv's, radio's etc, many were downright lethal from a shock hazzard and fire point of view.
My interest in radios is at the moment limited to AC transformer only stuff, even so, everything is strapped to the mains earth including test gear.

Lawrence.

[Nick]

Regarding switching, some sets don't have polarised mains connectors, thus chassis can be live or neutral.

If you are using a DPST mains switch and are worried about live but not neutral switching, then connect a neon (e.g. NE2 + dropper) between the chassis and a known neutral (unswitched side of mains). If, as in the case of the DAC90A, you don't necessarily know which side is neutral, use two NE2s from chassis, one to each side of the incoming mains. If either is glowing, the chassis is still connected to one side of the mains (you can determine which one by labelling the neons)

If a neon glows at any time, the chassis is potentially (!) live! If the chassis is really isolated, then neither neon should strike...

[Skywave]

My proposed solution to the 'possibly live chassis problem' & safety issues arising.

These days, a lot of mains leads for equipment is of the two-core with a non-removable 3-pin plug. Take one of these leads and use it as a replacement mains lead. Fit it to the item so that makes a direct, non-removable connection to the internals of the item: no intermediate plug / socket connections allowed! Fit that replacement lead so that the blue wire goes to chassis, the brown wire to a SPST switch - and thus to the internals of the set. Fit a 2-amp fuse in the 3-pin plug.
For an extra measure of safety, if you think that the item might pass into someone else's hands, add a warning label at the back of the item which states:
"Warning: Mains lead must only be replaced by a qualified electrical technician".

Al.

[pwdrive]

I would be interested to know from a law point of view what happens if the original mains input circuit has been modified or otherwise altered from it's original wiring configuration ie: from a customer/consumer perspective if anything untoward happened.
If anyone bypassed one pole of an immersion heater switch or bathroom fan isolating switch then clearly in the event of a serious mishap they could be in for the high jump, personally I cannot see any difference between that and a domestic consumer item situation.

I realise that different folks have different ideas etc but in the context of servicing/repairing potential live chassis stuff then an isolating transformer is the best way forward.

Lawrence.