[R-390] Variacs and solas: additional thoughts

[Roy Morgan]

At 04:44 PM 5/1/2005, Gene Dathe wrote:
>Nice thread.  It's great to read the tug-of-war on various issues;

Gene,

Yup. that's one reason I'm still here, (after all these years).

>I want to drop my household voltage down to 115 to run the R390A 
>continuously.  Where can I get a good bucking transformer?

That depends on your the depth of your junque boxe (or that of your 
friends'), and whether you want to build or not.  For continuous operation, 
I suggest a line bucking transformer with a current rating about twice what 
the radio draws.  That will be quite small: the R-390A draws an amp plus 
some.. so a 2 amp transformer will be enough if you run the radio only on it.

>  Any circuit suggestions?

Yes:
http://www.r-390a.net/ (read the whole site.)
but for the bucking transformer setup in particular, see "Technical" then
"<http://209.35.120.129/faq-HiVolt.htm>Compensating for High Line Voltage"
or: <http://209.35.120.129/faq-HiVolt.htm>

Also, "Voltage Reducer For BoatAnchor Gear" from Ed Richards, K6UUZ:
http://bama.sbc.edu/voltagereducer.htm

This page has parts list, instructions and drawings.  For a transformer, It 
suggests "the 120 VAC primary, 12 VAC ct secondary at 3 amperes,  Radio 
Shack #273-1511 or equivalent." This is just fine if your load is going to 
be 2 amps or less (Radio Shack transformers are known to  go into 
saturation at rated load!) and if the reduction you want to make in the 
line voltage is either 6 or 12 volts. (You could add a switch to select the 
voltage change.)

The added drawing at the bottom of that page is both left to right reversed 
and has the connections crossing over each other all up-side-down-like. But 
it will work.  It calls for a 10 amp transformer which is fine if you have 
one and have need for 10 amps of load current.
"Holy Amperes, Batman, we can run the whole shack on that!"

The astute and curious worker may note that if the transformer at hand is 
rated at 115 volts, it can be arranged to be powered by the OUTput of the 
system, with the low voltage winding reducing the line voltage before the 
transformer.  The Phase would be arranged backwards from the arrangements 
shown on the websites above.

If you want to buy something and plug it in, see:
http://www.toddsystems.com/newindex.html
You may need to sit down before finding out the prices on these.

(There are likely many other sources, too.)

A rather elegant solution to this situation is the General Radio Automatic 
Line Voltage Regulator, which will actively correct line voltage changes 
for loads up to  60 amperes.  However, this thing is both very, very heavy 
and hard to find since they have not been made for a long time now.  It has 
a few tubes inside, a voltage sensing system, and a small motor that is 
driven to move a center-tapped variac which runs a boost-buck transformer 
to correct the output, all  within a part of a second.  It's pretty quiet, too.
"... And the music goes round and round.. and comes out here.."

Roy



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
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