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Old 22nd February 2008, 02:34 PM   #1
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Default Bench Testing Positive Ground AM Radio

Hi Guys,

I picked up an old '40's am radio that is positive ground. I've never tested a positive ground radio. If negative ground radios ground to the chassis of the radio what does the positive ground radio ground too?

Thanks for your help!
Steve
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Old 22nd February 2008, 02:47 PM   #2
jol50 is offline jol50  United States
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As I recall the positive is ground instead. So you power it with the negative and the whole case is positive compared to modern ones.
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Old 22nd February 2008, 03:26 PM   #3
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Thanks for your reply and info.!

Does that mean that the positive from the power supply isn't connected?
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Old 24th February 2008, 05:16 AM   #4
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The positive of your power supply connects to the chassis of the radio. And connect the wire of the radio to the negative terminal of the power supply. To be safe, remember to put a 5 amp fuse in either wire.
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Old 24th February 2008, 05:57 AM   #5
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Now that sounds about right. I appreciate your information (especially about the fuse!). Thanks!
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Old 24th February 2008, 03:07 PM   #6
jol50 is offline jol50  United States
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Also make sure your bench or anything the radio could touch is not grounded.
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Old 24th February 2008, 03:55 PM   #7
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Default Why?

Quote:
Originally posted by jol50
Also make sure your bench or anything the radio could touch is not grounded.

The radio chassis will be grounded, and powered from the negative side of the supply.

There is no need to worry about isolation.

Perhaps you are thinking of a situation using a supply with the negative already grounded? Or putting the radio in a vehicle with a negative ground battery? THEN you will have problems!
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Old 24th February 2008, 06:13 PM   #8
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Default Re: Why?

Quote:
Originally posted by cliff



The radio chassis will be grounded, and powered from the negative side of the supply.

There is no need to worry about isolation.

Perhaps you are thinking of a situation using a supply with the negative already grounded? Or putting the radio in a vehicle with a negative ground battery? THEN you will have problems!
See that's what I'm not sure of. Whether or not the cars back then had negative or positive ground batteries. I almost want to say that being a positive ground radio the battery would almost have to be positive ground?

Anyway.....so I think I have this straight (hopefully). Neg of power supply to power wire w/inline fuse, pos of power supply to chassis, power up and close my eyes.
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Old 24th February 2008, 06:43 PM   #9
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Yes, you've got it all right.

A long time ago cars were mostly Postive earth - but not all.

'caused havoc trying to fit any electronics!


There is nothing magic about "earth" or "ground".

Any circuit will quite happily work without any ground connection, just + to + and - to -.

However when connecting 2 or more things together, and for electrical safety, it becomes important.
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Old 24th February 2008, 07:09 PM   #10
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Ooooh. I've got it now.

Thus the invention of the three pronged plug so my house doesn't burn down. Makes sense to me.

I really appreciate your help!

I would imagine there were more '20's and '30's cars with pos. ground than since.
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