[JMS] Several more comments on the National SW-54

[JAMES HANLON]

Sherrill and Y'all,

While I would agree with most of your recommendations, it may be overkill to replace all of the capacitors and resistors in the SW-54 or any other vintage receiver.  I have restored somewhere around fifty communications receivers dating from the 30's to the early 60's, and I currently have two dozen of them on the air in my Classic Exchange station.  In only four cases did I actually find it necessary to replace capacitors because of an obvious "end of life" condition.  Those included a BC348R with its well-known capacitor problems, a Hammarlund SP-600 Super Pro that was full of "black beauty" caps, a Hallicrafters SX-43 with a bunch of wax impregnated paper caps, and a BC455 Command Set receiver made by Western Electric.  I suspect that the Command Set may have been built with a batch of sub-standard parts, because I have two others that I operate with it, a BC453 and a BC454, and both of them are still running on their original capacitors and resistors.  In all of my other radios, going from very little ones like the Hallicrafters S41's and Echophone EC-1's to the big and expensive guys like National HRO's, Hallicrafters SX-73, and Collins 75A3 and 4, I have had to replace only an occasional capacitor.  

As to the resistors, carbon resistors do tend to drift to higher resistance values with time.  But most circuits have a wide latitude to tolerate resistance change and still operate quite well.  So I take the approach of replacing a resistor only when the circuit function is impaired.  

I would recommend that you check all of the existing capacitors in the receiver with an ohm meter to see if they are shorted before you apply power.  If they are waxed paper caps, look for signs of them being too warm such as a frozen drop of wax on the bottom of the body or a drop of wax on the bottom of the cabinet underneath them.  Also look for resistors that are darker than normal, a sign that they may have been overheated.  At a minimum, keep a very careful eye on those parts when you bring up power on the radio.  

It is quite possible that the power supply filter capacitor will need to be replaced.  In almost all cases electrolytic filter caps simply loose their capacitance and the result is a loud hum in the audio output.  Occasionally they do short out, especially when they are operating in high voltage service (transmitter power supplies).  You wouldn't have much to loose since the SW54 does not have a power transformer by simply bringing the receiver up slowly on a variac as Sherrill recommends after first measuring the filter capacitors with an ohm meter to eliminate one with an obvious short.  If the result is a hum that shows no signs of going away in several minutes, it is likely that the capacitor is not going to "reform" and needs to be replaced.  

Hope this helps.  I like to keep my receivers as "original" as possible, and a wholesale replacement of all of the R's and C's violates that goal, especially when it isn't really necessary.

73,

Jim Hanlon, W8KGI