I just finished restoring an Atwater Kent model 72. It is the first AK that I’ve worked on.

The power transformer was bad and I was able to find a similar one online from another AK model but it had an extra filament winding.

Somewhere back in my days at tech school in the 60’s I seem to remember it not being a good idea to leave a secondary winding unloaded on a power transformer.

I looked around on the web but could not really find the theory behind my foggy memory.

I loaded the unused winding with a resistor and things are working fine. Can anyone shed light on my foggy memory?

Also, as usual, a number of the resistors were open or way off value. They have a ceramic body with metal end caps and what I assume is a carbon thread through the center of the ceramic body. When I unsoldered the wires from the resistors, the end caps melted along with the solder. I’ve never seen resistors like this before. It seems like the resistor endcaps are made of solder or have a very low melt point. Were the resistors made this way so solder didn’t need to be applied during production or is there some other reason?

:I just finished restoring an Atwater Kent model 72. It is the first AK that I’ve worked on.
:
:The power transformer was bad and I was able to find a similar one online from another AK model but it had an extra filament winding.
:
:Somewhere back in my days at tech school in the 60’s I seem to remember it not being a good idea to leave a secondary winding unloaded on a power transformer.
:
:I looked around on the web but could not really find the theory behind my foggy memory.
:
:I loaded the unused winding with a resistor and things are working fine. Can anyone shed light on my foggy memory?
:
:Also, as usual, a number of the resistors were open or way off value. They have a ceramic body with metal end caps and what I assume is a carbon thread through the center of the ceramic body. When I unsoldered the wires from the resistors, the end caps melted along with the solder. I’ve never seen resistors like this before. It seems like the resistor endcaps are made of solder or have a very low melt point. Were the resistors made this way so solder didn’t need to be applied during production or is there some other reason?
:
:
:
I've never heard of that "guideline". It should not damage anything to leave a heater winding unloaded, as long as the end terminals are properly insulated. As far as the primary winding is concerned, an open secondary just represents a "no load" condition so energy won't be coupled into that winding. So, the transformer ends up being oversized/underutilized but no damage is done.

The resistors were probably just a style that the A-K component-purchasing agent got a sweet deal on. Sounds like they would require a needle-nose pliers to heatsink them for proper installation (so that the leads wouldn't melt off the resistor body). Odd, but not particularly mysterious. The A-K production floor probably had a detailed writeup (assembly procedure) for installing them so that newbie assemblers wouldn't be melting the leads off the parts as they soldered them in place.

Hi

Don't need to load unused power transformer windings. It just wastes power.

An audio output transformer does require a speaker or resistor load. Without any load voltage can be very high.

Some AK resistors had molded solder on each end. Not really a problem when it melted as you changed resistors.

Norm

::I just finished restoring an Atwater Kent model 72. It is the first AK that I�ve worked on.
::
::The power transformer was bad and I was able to find a similar one online from another AK model but it had an extra filament winding.
::
::Somewhere back in my days at tech school in the 60�s I seem to remember it not being a good idea to leave a secondary winding unloaded on a power transformer.
::
::I looked around on the web but could not really find the theory behind my foggy memory.
::
::I loaded the unused winding with a resistor and things are working fine. Can anyone shed light on my foggy memory?
::
::Also, as usual, a number of the resistors were open or way off value. They have a ceramic body with metal end caps and what I assume is a carbon thread through the center of the ceramic body. When I unsoldered the wires from the resistors, the end caps melted along with the solder. I�ve never seen resistors like this before. It seems like the resistor endcaps are made of solder or have a very low melt point. Were the resistors made this way so solder didn�t need to be applied during production or is there some other reason?
::
::
::
:I've never heard of that "guideline". It should not damage anything to leave a heater winding unloaded, as long as the end terminals are properly insulated. As far as the primary winding is concerned, an open secondary just represents a "no load" condition so energy won't be coupled into that winding. So, the transformer ends up being oversized/underutilized but no damage is done.
:
:The resistors were probably just a style that the A-K component-purchasing agent got a sweet deal on. Sounds like they would require a needle-nose pliers to heatsink them for proper installation (so that the leads wouldn't melt off the resistor body). Odd, but not particularly mysterious. The A-K production floor probably had a detailed writeup (assembly procedure) for installing them so that newbie assemblers wouldn't be melting the leads off the parts as they soldered them in place.
: