[mikethedruid]

I have been working on a very interesting National SW-54. As many of you probably know, this set has a chassis which is connected to one side of the line. It also has a steel housing which is connected to that chassis. This can pose a serious shock hazard. Someone has isolated the chassis from line on this unit using two 120V to 12V filament transformers. The first steps the line down from 120V to 12V for the filaments of the four remaining tubes in the set and the dial lamp. The second steps the 12V back up tp 120V, which then goes through a diode bridge to provide the B voltage to the set. The 35Z5 is eliminated, and the 50C5 us changed to a 12CU5/12C5. All four remaining tubes filament circuits are rewired in parallel as well as the dial lamp. Whoever did the work did an excellent job, very neat, with the filament wiring twisted and dressed right next to the chassis. I replaced the big firecracker four section electrolytic with four modern electrolytics of higher voltage rating and slightly higher capacity, and I replaced all the paper condensers. I then aligned the set. It actually works very well. It is a little deaf on the upper band, but that is fairly normal on such a set. I am quite pleased with it, and am going to keep it since it is such a rare bird. Here are some pictures of the underside of the chassis and the chassis top, as well as the schematics. Compare the pictures to how the set normally looks using the manual which you can find here   http://bama.edebris.com/manuals/national/sw54/   
[Image: https://64.media.tumblr.com/f5cbfcef0dee...1e7776.jpg]

[Image: https://64.media.tumblr.com/4cabd5eb4cc3...076d97.jpg]

[Image: https://64.media.tumblr.com/b62d3c6001b7...f40541.jpg]

[Image: https://64.media.tumblr.com/dac8ad2e4131...a6621f.jpg]

[Paul Philco322]

Looks very neat, a clean set though not sure if you were the guy doing cleaning .

Hope you can pick up WPRO when you are done. Nice to see your post and hope you are well.

Paul

[Radioroslyn]

Gee I wonder how a 12BA7 would get a long in there instead of the 12BE6.
More gain and less noise.
Maybe was a ham that wanted to use it w/other stuff (transmitter) that was
@ earth ground. I would have bought an isolation transformer but I'm no
fun!
Get a load of those diodes! 1965 again.

[David]

The sch looks a lot like a Halli S-38E. The first filter cap is a 40UF which is correct for that 35Z5 tube.

[Arran]

They must have had a reason for adding transformers other then to eliminate the hot chassis problem, switching to a floating B- would have eliminated most of the problem, and all it takes is some times, and some wire, so they needed the set isolated for some other reason, maybe to connect it to other equipment. Usually when I see things like this my temptation is to put it back to stock as 9 times out of 10 it doesn't work right, if at all, you must have got lucky in this case. What I further don't understand is why not leave the series string tubes and just use the isolation transformer for the B+? Some factory built sets were like this, Rogers and Electrohome made a few models this way, in fact I think that the tube heaters were operating from the transformer secondary in those sets, but I will have to check.
Regards
Arran

[David]

A number of years ago when in Europe spotted a halli S38D with a step down transformer added. Found out later that it was factory.

[Phlogiston]

Might want to add a safety cap, .01UF, to the chassis after the power switch. It might make your antewnna coil more effective.