Safe use of an old valve radio

[joolsmy]

Apologies if I'm posting this question in the wrong forum. I have an old valve radio that has been rewired internally so is basically sound. The problem is it has no provision for a protective earth, the internal chassis is wired to neutral (was live before rewiring!) and it is far from double insulated. The control knobs are on brass rods which obviously protrude from the wooden case, so pose a risk (they are mostly concealed by the bakelite knobs but you could slide a nail file behind them and contact the rods). The grub screws in the bakelite knobs are recessed and have been covered with a non-conducting wax-like substance but a determined poke with a sewing needle would probably break through to contact them.

Given all the above plus the ventilation holes in the back cover, would a permanently wired RCD plug provide sufficient protection?

 

[Specialist]

I do use the radio and I don't think of it as dangerous, I just wanted to ensure I was using it in the safest way possible. As it happens it's fitted with a 3 amp fuse so I'll take your advice regarding a 2 amp fuse. Thanks.

I'm glad to hear that your getting use out of your radio and hopefully enjoying it, If you want to seal the grub screw holes in the knobs & fit felt washers behind them, a good local craft shop should be able to get Sealing wax and Brown felt quite reasonably. Or if you have something like the Range anywhere near you, they sell both items. At a push you could leave the 3a fuse in. I prefer a 2a as that's what we were always taught to use.

 

[D00DY_T00DY]

I'd pick a valve radio over transistor any day, love the sound quality & they're much more fun to restore.

I would rather like a valve amplifier however don't quite have stacks of cash laying about!

 

[D00DY_T00DY]

What concerns me in threads like this is that people seem to think that an RCD will prevent an electric shock, it does not.
An RCD reduces the duration of an electric shock if you receive one, this reduces the amount of current which flows through your body to a level which won't be deadly to the average perfectly healthy adult.
You will still get a shock from an RCD protected circuit, and from experience I can tell you that the time it takes to trip feels like forever and it hurts like hell.

Like falling down the stairs. Took about a week if I recall correctly.

 

[Lucien Nunes]

Powering the set from an isolating transformer separates the entire circuit from the mains, preventing the possibility of a shock to earth from an exposed metal part to which the insulation has broken down. My opinion is that any but the best-insulated AC/DC sets with mains-connected chassis, when operated by unskilled people, should preferably be fed from one. There are certainly some models that I would not like to let the family use directly on the mains due to the insulation being pretty flimsy in places. There can be sneaky things like metal speaker grilles in contact with the speaker basket, with the output transformer bolted to it where the HT rail (rectified mains) is only insulated from the frame by 1/16" of paxolin. OK when new but over time the insulation can track over and cause leakage. Or on record players, the cartridge leads may be accessible to the touch and not robustly insulated from the live bits (this sometimes applies to the whole autochanger chassis too). Other sets are very sound and probably as safe as anything modern.

When running without a transformer, an oddity to watch out for is that some sets have a single-pole switch in the chassis lead, in which case with the plug wired one way the chassis is live when on, and the other way it's live when off (through the set, plenty low enough resistance to provide a fatal shock.) One would hope that anyone overhauling a set and giving it a recap, and in this case changing the wirewound dropper to capacitive, would have swapped the switch over if it had been like that. Of course, the chassis can still swing up to line volts if the neutral becomes disconnected elsewhere, which is why ensuring the mains lead is the right way round is not a cast-iron guarantee of the chassis being near earth potential.

That might sound stupid but it's easy to forget, in these AC-only times, that on DC mains half the houses would have a positive live and the others a negative live. Since the rectifier anode has to be positive and the chassis negative, on a negative live service the chassis definitely had to be live (not neutral) otherwise the set would not work. Hence the reversible 2-pin plug on the back of many universal sets; plug it in, if the heaters come on but no HT, flip the domino plug to reverse the polarity. So, there was no point in the manufacturers aiming to make sure the chassis was always at neutral, because that would only work for half of DC users.

On the subject of an always-live chassis, anyone going in the back of transformerless solid-state tellies should remember that the chassis is often at the negative of fullwave-rectified mains (unlike valve sets which were normally half-wave) and therefore the chassis is always live with rectified mains whichever way round you connect the mains lead.

 

[marconi]

Apologies if I'm posting this question in the wrong forum. I have an old valve radio that has been rewired internally so is basically sound. The problem is it has no provision for a protective earth, the internal chassis is wired to neutral (was live before rewiring!) and it is far from double insulated. The control knobs are on brass rods which obviously protrude from the wooden case, so pose a risk (they are mostly concealed by the bakelite knobs but you could slide a nail file behind them and contact the rods). The grub screws in the bakelite knobs are recessed and have been covered with a non-conducting wax-like substance but a determined poke with a sewing needle would probably break through to contact them.

Given all the above plus the ventilation holes in the back cover, would a permanently wired RCD plug provide sufficient protection?

I reckon the wrong type of potentiometers have been used to rewire the radio - or are they the originals? I remember when I cannabalised these radios for components as a teenager the the shafts were plastic.

My wife bought me for Christmas 20 years ago a 'refurbished radio' from a chap at an antiques centre who had worked for Bush. Much to her disappointment I refused to plug it in and indeed cut the cord off it. Once she got fed up polishing and dusting it I got my way and binned it. My fears were it causing electric shock and fire. Just not worth the risk even if you supply it via an isolation transformer.

 

[Lucien Nunes]

On the older sets the pot shaft(s) were metal with knobs designed to insulate the user from them. E.g. by extending the knob through the hole in the cabinet so that the screw was inside, or by having it deeply recessed with the hole plugged. I can't recall when the first nylon-shafted pots came out but this was towards the end of the valve radio era. There was a kind of pot with with integral bakelite knob that could be presented through the cabinet of a TV as a small finger-twist or screwdriver-slot adjustment point, for functions like line and frame hold. But bakelite was too brittle to make a shaft for a full size control knob to fit on, for key functions like volume and brightness.

I think binning any kind of radio is a bit extreme although I can understand not allowing people to use it. An isolating transformer almost eliminates the risk of shock from most sets. You would need to take the back off, or with a card-backed set break through it, and contact both the chassis and some other point (HT+ or the other pole of the supply), but a top-of-head risk assessment suggests this is much less likely to happen than many other kinds of equally serious accident in the home. There is indeed a small risk of fire, but even the most precariously designed radio perhaps no more likely to burst into flames than a frying pan. I would not generally leave a vintage set on unattended, indeed there's not much point if you're not listening to it, so you would be on top of the situation in the unlikely event of a catastrophic fault. Vintage equipment that I expect or want to run for long periods or without supervision, I tend to attach a thermal fuse to the transformer (or the dropper of a universal) which both dramatically reduces the fire risk and avoids the transformer being destroyed by a simple fault such as a shorted reservoir cap.

 

[marconi]

On the older sets the pot shaft(s) were metal with knobs designed to insulate the user from them. E.g. by extending the knob through the hole in the cabinet so that the screw was inside, or by having it deeply recessed with the hole plugged. I can't recall when the first nylon-shafted pots came out but this was towards the end of the valve radio era. There was a kind of pot with with integral bakelite knob that could be presented through the cabinet of a TV as a small finger-twist or screwdriver-slot adjustment point, for functions like line and frame hold. But bakelite was too brittle to make a shaft for a full size control knob to fit on, for key functions like volume and brightness.

I think binning any kind of radio is a bit extreme although I can understand not allowing people to use it. An isolating transformer almost eliminates the risk of shock from most sets. You would need to take the back off, or with a card-backed set break through it, and contact both the chassis and some other point (HT+ or the other pole of the supply), but a top-of-head risk assessment suggests this is much less likely to happen than many other kinds of equally serious accident in the home. There is indeed a small risk of fire, but even the most precariously designed radio perhaps no more likely to burst into flames than a frying pan. I would not generally leave a vintage set on unattended, indeed there's not much point if you're not listening to it, so you would be on top of the situation in the unlikely event of a catastrophic fault. Vintage equipment that I expect or want to run for long periods or without supervision, I tend to attach a thermal fuse to the transformer (or the dropper of a universal) which both dramatically reduces the fire risk and avoids the transformer being destroyed by a simple fault such as a shorted reservoir cap.

We have different appetites for bearing risk with old electrical equipment.

 

[Specialist]

We have different appetites for bearing risk with old electrical equipment.

Why would a correctly restored Vintage set be any more likely to catch fire or give you an electric shock ?
Do you have any instances of this happening to go on ?

 

[marconi]

No I cannot provide you with instances of restored vintage sets catching fire or giving someone an electric shock or causing death by electrocution. Nor could I find, unsurprisingly, any data from google searches. My decision not to use the radio was based on it lacking any CAT 1 or 2 qualification, it would need a safety isolating transformer to make it safer (by floating the radio wiring with respect to terra firma and the house earthing system), I'd need a special plug to prevent it being plugged into a wall socket directly and it had no IP grading. And since it used valves, power resistors and a transformer which all run warm it required natural convection cooling through ventilation holes; these can become blocked by dust or by objects covering them and risk overheating. The case was varnished wood and not treated in any way to be fire retardent. Inside there are often components which have chemicals considered toxic or which produce toxic products when they burn. It did not come with any statement on what had been done to 'correctly restore it' - so 'let the buyer - me - beware', whatever is meant by correctly restored - to the standards of its era or to today's standards?

This is my thinking and reasoning to assert that the risks of shock and fire from use of this old radio equipment were best avoided in my own home.

But others will assess these risks differently - they will have a different risk appetite to mine.

Would you use one in your own home?

 

[Specialist]

A vintage radio does not need to comply with Class 1 or 2 classification, they were never intended to. Your trying to apply modern rules to a vintage item.
Vintage radios don't need an Isolation transformer to make them safe, if they were actually a fire hazard then an Isolation transformer would do absolutely nothing to change that.
As for your question of would I use one in my own home, Yes I would and do on a Virtually daily basis. After many years of doing so, I'm still here to tell the tale & have to report that my house is still standing & has not been burned to the ground by any of the vintage radios that have passed through it's doors.
You may think I'm biased as I Repair / Restore / Use Vintage radios on a regular basis, guilty I'm afraid.
I also have a hatred of seeing any part of our Technological history being destroyed especially Vintage radios as that's where I cut my teeth on repairs, they weren't Vintage in those days though they were everyday items in most homes.

 

[Lucien Nunes]

Would you use one in your own home?

Yes indeed, and I also use a gas hob, which has exposed parts running at nearly 2000° C that will set fire in seconds to anything combustible that falls on it; emits the asphyxiant carbon dioxide all the time it is in use and can potentially emit deadly carbon monoxide; but worst of all can cause a devastating explosion capable of demolishing a building, simply through misoperation of one control knob. Would you let any member of your family use one of those?

 

[marconi]

Yes indeed, and I also use a gas hob, which has exposed parts running at nearly 2000° C that will set fire in seconds to anything combustible that falls on it; emits the asphyxiant carbon dioxide all the time it is in use and can potentially emit deadly carbon monoxide; but worst of all can cause a devastating explosion capable of demolishing a building, simply through misoperation of one control knob. Would you let any member of your family use one of those?

Yes. Actually more like 1000C see; http://www.npl.co.uk/upload/pdf/temperature_answers.pdf

 

[Specialist]

Yes. Actually more like 1000C see; http://www.npl.co.uk/upload/pdf/temperature_answers.pdf

Have you actually looked up the amount of modern appliances that have caught fire etc ? Not nice reading, or the appliances imported into this country & having passed safety classification but were found to actually Incorporate Selenium rectifiers.

 

[marconi]

Have you actually looked up the amount of modern appliances that have caught fire etc ? Not nice reading, or the appliances imported into this country & having passed safety classification but were found to actually Incorporate Selenium rectifiers.

Yes. The figure I turned up was 12% of fires in the home were caused by electrical equipment. see page 24 of 63 of:
https://www.gov.uk/government/uploa...tics_Great_Britain_2013-14___PDF_Version_.pdf

(I know this is for 2013/2014) but I leave it to you find more recent data.

 

[Specialist]

Thanks for that, but to be honest I didn't want to read the government lie sheet.
You won't get a true picture from the government figures, there's too much money involved.
What I was actually hoping was, that you'd look and compare the safety record of modern equipment that you may well have in your home & in daily use against the item you wouldn't use due to your perception that being Vintage it was unsafe to use.