[NJARC] AC/DC sets?

[Al Klase]

Mstang6T5 at aol.com wrote:

>Hi all,
> 
>Novice member here looking for a little help. I have a few ac/dc sets I  
>would like to start working on, however I'm not well versed on the safety  
>concerns involved. Can anyone tell me how the isolation transformer works  and should 
>I buy one or build my own?
> 
>
>
>  
>
Hi Dave,

In an AC/DC set there is no power transformer, and the AC cord is wired 
directly to the radio circuit.  In some sets, one side of the cord goes 
directly to the metal chassis, while in others it goes to a common 
(ground) bus isolated from the chassis.  In normal operation, everything 
is insulated so the user cannot contact any high voltage.  (You should 
see the Underwriters Lab's articulate metal test finger.)  With the 
radio opened up for service, all of this is not too bad if the set is 
plugged in so the neutral (cold side) of the AC is connected to common 
in the set, but there's a 50/50 chance the common and maybe the chassis 
will be at 117 volts.  The shock hazard is bad enough, but you can do 
serious damage to grounded test equipment if things aren't right.  
Hence, the use of an external transformer to isolate the radio from the 
AC neutral and safety ground (which are tied together in your fuse box) 
as well as the hot side of the line.  Now no appreciable current can 
flow from the secondary of the isolation transformer and attached radio 
to earth ground.

You can cobble up an effective isolation transformer.  Find two roughly 
identical transformers that will handle about 50 watts, e.g., ones with 
12-volt 4-amp secondaries.  (12 volts X 4 amps = 48 watts)  Wire the two 
secondaries in parallel.  Now you can plug in the primary of T1 and get 
isolated 117 VAC out of T2.

Hope this helps,
Al


-- 
Al Klase - N3FRQ
Flemington, NJ
http://www.skywaves.ar88.net/