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Old 3rd Apr 2020, 8:15 am   #1
Paul Mathew
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Default Radio chassis earthing?

Hi, I have recently completed a restoration of a RCA D8-28 radio chassis. I have a question that is much spoken about. Can the earth of the mains supply cable be connected to the chassis for safety? It's not a live chassis, but there is a voltage divider network from the negative of main smoothing /reservoir caps C35 & 36, providing a negative voltage to chassis of some -6V for use on various tube grids. i.e. a positive voltage of +6V when measuring DC from chassis to C35/36 common negative point. I'm assuming the chassis is 0V with respect to ground, but confirmation would be great!
Thanks.
PS. For those interested:
Direct download 29.8MB→ https://www.americanradiohistory.com/AR ... s-1936.pdf
pdf-page-80 onward.

From here... https://www.americanradiohistory.com/RC ... ervice.htm
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Old 3rd Apr 2020, 1:55 pm   #2
Chris55000
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Default Re: Radio chassis earthing?

Hi!

The unconventional style of ancient American radio set circuit–diagrams makes answers a bit difficult to fathom out by inspection, but the answer to your question is yes, it's fine to earth the chassis of this set directly!

As far as voltage reference points go, chassis metalwork is treated as 0V in exactly the same as a British set with conventional cathode bias resistors!

Chris Williams
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Old 3rd Apr 2020, 2:14 pm   #3
AdrianH
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Default Re: Radio chassis earthing?

The links do not work, at least for me looks like there is sections missing.

Where do the two wires go, that go off bottom right from the mains primary?

Adrian
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Old 3rd Apr 2020, 2:24 pm   #4
AC/HL
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Default Re: Radio chassis earthing?

Try this: https://www.americanradiohistory.com...Notes-1936.pdf
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Old 3rd Apr 2020, 2:43 pm   #5
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Default Re: Radio chassis earthing?

Agree- chassis gets called 0V by convention and supplies to the circuitry end up having positive or negative polarity with respective to it according as to the PSU arrangements, irrespective as to what the chassis is connected to supply-wise. It's the fact that it's a large chunk of metal that decides that connecting supply earth to it is wise!

Quote:
Originally Posted by Chris55000 View Post
The unconventional style of ancient American radio set circuit–diagrams makes answers a bit difficult to fathom out by inspection
I suppose it's a case of what people have been accustomed to working with- Americans probably look at our circuits and mutter, "Gee, what is it that the Limeys smoke to make them draw schematics out like that?"
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Old 3rd Apr 2020, 2:43 pm   #6
AdrianH
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Default Re: Radio chassis earthing?

Thanks Bill

Off to the turntable motor that took me a while to follow!

I agree with Chris the chassis can be earthed as all else should be isolated through the mains transformer.
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Old 3rd Apr 2020, 6:09 pm   #7
Paul Mathew
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Default Re: Radio chassis earthing?

Great. Thanks all for the confirmation. Sorry about the link. Not sure what happened there, but Bill has supplied the correct one.
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Old 3rd Apr 2020, 6:14 pm   #8
Paul Mathew
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Default Re: Radio chassis earthing?

Here's a link to 12 volumes of RCA service notes: https://www.americanradiohistory.com...or-Service.htm
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Old 3rd Apr 2020, 7:01 pm   #9
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Default Re: Radio chassis earthing?

Thanks for the links, plenty of reading material!

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