Chris is quite right. Sorry, the light bulb rig was a bad idea (I have
never used it but it was mentioned to me once upon a time) and I agree
the isolation transformer is definitely the ONLY way to go. Regarding
trusting a spec sheet or schematic, I wouldn't. Many sets I've come
across have been modified, often in unconventional ways.
Marty
______________________________ Reply Separator
_________________________________
Subject: Re: AC/DC radio precautions, was Old Radios
Author: classiccmp_at_u.washington.edu at internet
Date: 12/28/98 3:31 PM
At 12:15 12/28/98 -0500, you wrote:
>
>
>
>______________________________ Reply Separator
>_________________________________
>Subject: Re: Old Radios
>Author: classiccmp_at_u.washington.edu at internet
>Date: 12/23/98 6:44 PM
>
>
> >
> > Come now comrade Griffiths, an old radio never hurt anyone.
>
> >>>What, not even if you touch the chassis of an AA5 with wet hands?
> I stand corrected. This is a good time to mention to our list members
> that if they find an old tube radio, especially an AC/DC set (you can
> tell this by the absence of a power transformer and a tube complement
> which will equal 120 volts for total filament voltage, some even have
> line cord resistors), THESE SETS ARE EXTREMELY DANGEROUS AND LETHAL,
> often having the 120 hot wire soldered to the chassis. If you touch
> the chassis on one of these sets wired with the hot to chassis, then
> ground yourself, you'll electrocute yourself. Always use one hand only
> so you don't provide a path for the current (likewise be certain you
> have insulated shoes, not providing a current path anywhere on your
> body). If you need to work on one of these sets, use an isolation
These points are valid . . .
> transformer or wire a light bulb between the set and house current
> (the light bulb filiment should open before you fry, thus breaking the
> circuit).
Hold on there! An isolation transformer is the *only* way to go to
provide isolation from the line for a transformerless radio or TV.
The light bulb will NOT open (like a fuse, as the above statement infers)
but will simply pass current up to at least the wattage rating of the
filament. (100W bulb = approx 0.9 Amperes _at_ 115V.) A few microamperes could
be enough to kill a person under many conditions at this voltage.
The concern for inexperienced folks handling a bare chassis of a
transformerless set is nonetheless well placed and the part about the
chassis being hot (115VAC in North Amer. or 230VAC in most of the other
parts of Earth) is particularly important to be aware of. A chassis could
be hot even if the line cord is correctly plugged into the wall receptacle.
This is because a commonly used signal bypass capacitor from the hot side
to chassis often is quite leaky (simply, it has relatively low DC
resistance) or even shorted from age. I've been hit too many times while
handling these sets over the past 33 or 34 years.
As with any of us troubleshooting computer power supplies while the power
is on, *always* stuff one hand into your pocket while probing around in the
circuit.
Enough of this off topic radio stuff --although I had to post this to
correct a seriously incorrect piece of advice given above. On topic part
could be to note that using an isolation transformer while poking around in
a computer's switching power supply could make accidentally touching a few
parts in the line-side of the PSU somewhat safer.
Just being careful and focusing on where one's hands are at absolutely all
times is the safer way to go while having to work on live high voltage
circuits.
Regards, Chris
-- --
Christian Fandt, Electronic/Electrical Historian
Jamestown, NY USA cfandt_at_netsync.net
Member of Antique Wireless Association
URL:
http://www.ggw.org/freenet/a/awa/
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Subject: Re: AC/DC radio precautions, was Old Radios
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