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Old 24th Sep 2018, 6:16 pm   #21
merlinmaxwell
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Default Re: Transformer safety?

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If the lead is wired correctly, you KNOW that the chassis can only be a few volts away from ground potential.
Unless you have a neutral fault, unlikely and as all the lights go out the radio is the last thing on your mind (or hands).

PME (earth and neutral connected at the incoming mains point) is considered very unlikely to go wrong with moderate dangers if it did.
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Old 24th Sep 2018, 8:06 pm   #22
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Default Re: Transformer safety?

Some time back Chas Miller did an article in which he described how to isolate the neutral return from chassis by creating a bus bar. The chassis is connected to the bus bar using Class Y capacitors as normally you cannot isolate all components such as the tuning capacitor frame from the chassis. The same principle could be used for a set with an auto transformer. The article was published in Radiophile of course and someone will probably remember which issue. It sounds a bit complicated but for many sets it probably isn't. Many American sets were built like that in the first place.
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Old 24th Sep 2018, 8:12 pm   #23
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Default Re: Transformer safety?

Always best to check on a live chassis that the insulation is all there as intended. It only takes a loose neutral in your mains plug or multiway adapter for the chassis to lose it's reassuring tie to neutral, or indeed to turn the set off if the single pole mains switch is in the neutral, for metalwork to be hazardous to touch.
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Old 24th Sep 2018, 9:05 pm   #24
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Default Re: Transformer safety?

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Originally Posted by Nickthedentist View Post
P.S. If you want a nice-sounding, really well made, Bakelite set with VHF and a proper isolating transformer, look out for a Cossor 524. Fragile knobs aside, they're wonderful IMHO.
Another vote for the Cossor here. Great under appreciated sets. Good sound and no tuning drift on VHF.
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Old 25th Sep 2018, 4:59 am   #25
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Default Re: Transformer safety?

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Originally Posted by Nuvistor View Post
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Originally Posted by M0FYA Andy View Post
What is the reason for the mains earth connected to chassis via a 0.01uF capacitor?
Obviously not for safety reasons, so I'm guessing an 'RF earth' connection.

Andy
I can only presume that was the reason.
That approach seems to be potentially misleading, in that the presence of a three-core power lead would reasonably create the impression that the set was fitted with proper protective earthing of the chassis.

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Interesting comment in the trader sheet, early sets had the LO on the high side of the signal, this was changed to the low side, this was to prevent the second harmonic possibly interfering with Band 3 TV.
As far as I know, BREMA recommended oscillator-low for FM receivers, but that recommendation may have post-dated development of some of the early UK models.

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Originally Posted by Skywave View Post
1. Double-wound transformer: in the event of a pri. to sec'y. insulation failure, I would expect a fuse to fail or an RCD to trip. (Obviously depends on degree of insulation failure and where, in the transformer, that failure occurs. Also assumes that the chassis is connected to supply earth, as it should be)...
...but almost never is, as most radios of this era have two core mains leads. Fuses don't fail until a substantial current is flowing and not every building has RCDs (our house doesn't for example).

That's why the AC/DC method is safer, if you think about it sensibly and in practical terms. Its a lot easier to fit a few new 'Y' rated capacitors to the aerial socket (etc) than to make a 50+ year old mains transformer really safe.
The use of two-core power leads (thus no protective earthing) on equipment fitted with isolating transformers at first glance strikes me as reflecting a rather cavalier attitude towards safety. Was it done simply as cost cutting measure, or were there perhaps other reasons for it?

That aside, if AC/DC (and autotransformer AC) equipment really was much safer than the AC-type with isolating transformers, why did the UK educational authorities generally insist that only radio/TV/audio equipment fitted with mains isolating transformers be used in schools?


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Old 25th Sep 2018, 8:00 am   #26
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Default Re: Transformer safety?

I don’t have a problem with receivers with an isolated mains transformer being considered safer than live chassis sets, it’s the age of the transformer in these 50+ year old sets that is being relied upon for safety that’s the problem for me.

Perhaps I am being over cautious but I am happy to use this equipment knowing it’s potential problems, but I wouldn’t give a set to anyone who wasn’t aware.

I use RCD’s and lightly fuse the sets I have.
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Old 25th Sep 2018, 10:01 am   #27
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Default Re: Transformer safety?

I would have thought that many 50+ year old sets would not meet current “double insulated” standards, whether live chassis or not.

With a live chassis set, you need to look at and possibly modify aerial and earth connections, gram inputs where fitted, and some sets even had speaker outputs using the output transformer for mains isolation. Then there are sets such as the DAC90 where it is difficult to ensure that the chassis is always connected to neutral.

For a set with an isolation transformer, fitting a 3-core mains lead and fused plug should protect from failure of the transformer (or the wiring to the on/off switch etc).

In truth of course, we don’t necessarily choose our favourite sets by their chassis isolation, but I think it’s easier to improve the safety of a set with a double wound mains transformer than one with a live chassis.

Stuart
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Old 25th Sep 2018, 11:31 am   #28
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Default Re: Transformer safety?

The Murphy A362 is a live chassis AC only set as has been said. The later A372 is identical in most respects.

Quote:
Originally Posted by M0FYA Andy View Post
What is the reason for the mains earth connected to chassis via a 0.01uF capacitor?
Obviously not for safety reasons, so I'm guessing an 'RF earth' connection.
Andy
The snip shows the arrangement for earth and chassis. The earth connection is an RF earth and is part of the aerial / earth system. It can be seen from the circuit that as well as C26 a couple of other components need rated at mains voltage in case the mains plug is incorrectly wired.


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Not sure what type of control knobs are fitted but if they use grub screws remember to fill the holes with wax so that the brass screw cannot be touched.
The knobs are pull off types but there is an insulating sleeve over the control shafts that cannot be removed while the chassis is in the cabinet.
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Old 25th Sep 2018, 11:40 am   #29
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Default Re: Transformer safety?

There's also the Ext. LS connection to mains earth.

Lawrence.
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Old 25th Sep 2018, 5:13 pm   #30
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Default Re: Transformer safety?

..and that would be a safety earth of course.
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Old 25th Sep 2018, 11:07 pm   #31
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Default Re: Transformer safety?

There is certainly a case to be made that a mains-common chassis doesn't try to lull you into a false sense of security; you know it's not supposed to be safe to touch, and it is insulated accordingly. A set fed from a proper mains transformer by a 2-core lead with no connection to Earth is usually safe to touch anywhere with one hand, by virtue of this lack of a return path; but there will be no tripping of RCDs or blowing of fuses if a transformer-fed set develops an unwanted Earth connection, which would then render the set unsafe to touch.

Of course, there's also a case to be made that you shouldn't be treating anything as "safe to touch", at least not until you have tested it with something a bit more sophisticated than the old neon screwdriver.

I reckon an autotransformer is really the worst of all possible worlds. On first glance, it looks like a transformer, but does not actually provide isolation; while it also cannot be used on a DC supply. And an autotransformer can't really have offered much cost saving over a proper isolating transformer anyway .....
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Old 25th Sep 2018, 11:18 pm   #32
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Default Re: Transformer safety?

I don't know- a proper isolating transformer with HT and LT windings would need to be about 50VA rated whereas a heater only auto needs only to be about 20VA rated for a typical 4 or 5 + R set.
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Old 25th Sep 2018, 11:33 pm   #33
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Default Re: Transformer safety?

I don't think the un-earthed construction of transformer sets would have been cavalier at the time, they weren't meant to be held or transported while in use (unlike say a power tool, which were some of the first appliances to include an earth wire). The earth terminal was supposed to be earthed 'somehow' albeit for signal purposes, whether onto a water or gas pipe, nearby radiator or one of those chemical earths you were supposed to keep watered. Even pre-war sets would've been flash-tested on the production line several times before despatch. There's a video or two on youtube showing a uk radio factory production line in the 30's.

Since transformers insulated with paper and wax could suffer leakage 60 years on, I usually earth mine with a new 3-core flex, since we have the luxury of earthed mains sockets. Such sets of course will also have protruding chassis bolts, an external aerial wire and generally less care to prevent contact with the chassis. That said, I've never had one leak to earth, but there's always a first time.
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Old 25th Sep 2018, 11:36 pm   #34
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Default Re: Transformer safety?

Plus the heater-only autotransformer would have its cost slightly offset by the omission of a usually fairly hefty dropper section, and design eased by less heat production. There would still be an HT surge limiter section but as off-line rectified HT sets usually needed all the volts they could get for decent audio power in particular, that dropper section would be small. Makes scale bulb powering less of a knotty gamble of surge-limiting versus running brightness than with series-feed sets, too!

Crossed with McM.
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Old 26th Sep 2018, 11:42 am   #35
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Default Re: Transformer safety?

The discussions here seem to have ignored both fault and power off conditions.

Many AC/DC sets use a single pole switch to chassis that leaves the chassis live when switched off.

Even if you have a double pole switch, a fault in the switch (very common) can leave the chassis live.

Then there is the risk from a connection breaking in the mains cable giving a 50% chance of a live chassis.
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Old 26th Sep 2018, 1:09 pm   #36
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Default Re: Transformer safety?

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I don't know- a proper isolating transformer with HT and LT windings would need to be about 50VA rated whereas a heater only auto needs only to be about 20VA rated for a typical 4 or 5 + R set.
Yes!

And if the heaters are in series, then the autotransformer can be rated even less - such as the Bush DAC90a. Heater volts 116.6V at 0.1A. Operating from 240V (so losing 123.4V), the 100mA appears as 48.58mA flowing in the 116.6V winding and 51.42mA in the 123.4V winding. So the autotransformer is the size of a 5.66VA rated double-wound transformer, so can be tiny.

There is one aspect which hasn't been considered with respect to safety - and that's failure! Transformers often fail with a shorted turn or turns, getting hot, smoking, catching fire even if wax impregnated (which they often are), without drawing enough current to blow a fuse.

Dropper resistors, on the other hand, when they get old, just go open so they are fail-safe.

If you want to leave a radio running unattended, make sure it's an AC/DC one!
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Old 26th Sep 2018, 1:32 pm   #37
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Default Re: Transformer safety?

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If you want to leave a radio running unattended, make sure it's an AC/DC one!
Not the case in this incident https://vintage-radio.net/forum/showthread.php?t=77149
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Old 26th Sep 2018, 1:38 pm   #38
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Default Re: Transformer safety?

You shouldn’t leave old technology running unattended Live chassis or isolated, that’s my opinion, appreciate it’s not everyone’s.
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Old 26th Sep 2018, 9:56 pm   #39
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Default Re: Transformer safety?

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Originally Posted by kalee20 View Post

There is one aspect which hasn't been considered with respect to safety - and that's failure! Transformers often fail with a shorted turn or turns, getting hot, smoking, catching fire even if wax impregnated (which they often are), without drawing enough current to blow a fuse.

Dropper resistors, on the other hand, when they get old, just go open so they are fail-safe.

If you want to leave a radio running unattended, make sure it's an AC/DC one!
Having played with TV's since I was a kid, live chassis is, to me, the normal state if affairs. There are very few TV's with mains transformers.

Some years ago, a friend, who is very conversant with old tech, had left a 1950's woody radio on downstairs in the kitchen. He had been using it for some years and it had never given any problems. He had been upstairs for several hours when he became aware of a strange smell. Going to the top of the stairs, he could see a wall of smoke. Taking a deep breath, he plunged down in to the kitchen, to find the radio pouring acrid smoke!

The mains transformer had developed shorted turns and overheated. Fuses are usually no use in this case .

So failures do occur with old transformers! I would not leave any old tech, whether it was live chassis or transformer, operating unattended.

I'm not sure how happy I'd be giving any old mains tech to someone who I was not confident was experienced enough to understand the hazards.
We, as repairers, have the responsibility to ensure any items used by other people are as safe as reasonably practical.

A case in point was recently a friend gave me a 1920's art deco heater he had bought."Can you fit a new mains lead"? I declined on the basis that there was minimal guarding to the exposed element and no mains switch. I assessed the shock hazard as being unacceptable. He would have used it without any worries as he, like most untrained people, is not competent to assess the dangers.
All the best
Nick

Last edited by 1100 man; 26th Sep 2018 at 10:14 pm. Reason: more text added
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Old 26th Sep 2018, 10:16 pm   #40
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Default Re: Transformer safety?

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You shouldn’t leave old technology running unattended Live chassis or isolated, that’s my opinion, appreciate it’s not everyone’s.
TBH I've always rebuilt/reworked stuff so it can be worked today as it was intended to be worked when originally built.

I have no problems with running my RCA AR88D, a couple of Pye PCR receivers, a 'Collins" TCS [though outsourced to be built by Stewart-Warner] a RACAL RA217/MA323 or various Eddystone/Roberts/Bush gear (let's not mention the Barker-88) continuously and unattended.

My experience has been that a lot of 'stuff' fails when switched-on or switched-off; if you can avoid these surges it'll live a lot longer. Keeping stuff warmly powered-on also avoids damp-ingress (which leads to the dreaded greenspot corrosion of coils] and reduces oscillator-drift [I truly hate chasing SSB stations across hundreds of Hertz as a free-running local-oscillator settles-down from a cold start].
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