[R-390] NTC Thermistors, sizing, measure twice, add once

[Tisha Hayes]

If you opt to insert an NTC Thermistor in series on the incoming AC it
should be sized to have a minimal impact when the radio is running "steady
state". If it is undersized (underrated) they will run hot and dissipate
some electrical and thermal watts.

There is a complete data sheet on the NTC Thermistors at;
http://www.thermometrics.com/assets/images/cl.pdf

A CL-90 might be a little "light duty" for a R-390/A, maybe you should have
a CL-80 or CL-70 in there instead.

The advantages;

1) catch the high inrush current to spare the transformer, micro-switch and
maybe the tube filaments a bit of abuse.

2) drop a few volts on the line input due to chronic high voltage from the
electrical utility.

Look at the resistance at 25c and then at the approx resistance at 25. 50,
75 and 100% of load (in the table).

If the NTC thermistor is running hot all the time then either your line
voltage is way higher than you expected it to be or your current it way
higher. There is a happy medium where the thermistor may be a little hot to
the touch but not glowing and smoking. They also suggest not soldering this
into a circuit as they can run hot enough to melt the solder off of the
leads. Use crimp connections into a screw block and plenty of room for air
to circulate around this miniature "space heater".

-- 
Ms. Tisha Hayes/ AA4HA
-
"Life isn't about how to survive the storm, but how to dance in the rain"