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Old 25th Sep 2018, 4:59 am   #25
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,241
Default Re: Transformer safety?

Originally Posted by Nuvistor View Post
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.

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.

Originally Posted by Nuvistor View Post
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.

Originally Posted by Studio263 View Post
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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