Lamp Auto Tranformer troubleshooting and repair

Thread Starter


Joined Mar 2, 2019
I have an autotransformer for a lamp that immediately, and without being connected to the bulb, starts to hum and smoke when connected to the socket. When i connect the bulb, the bulb lights up, but the tranformer continues to smoke. As pictured, the transformer takes either 220V or 125V, and steps it down to 125 V @ 300W. Does anybody have any advice in what's the best course of action for troubleshooting and repairing the transformer? As you can probably gather, i have very little experience and knowledge in electronics repair, but i am tying to learn and i do have access to the maker space at my university where i believe i can access the tools and materials i need to troubleshoot and repair the transformer.
Last edited:


Joined Jan 29, 2010
hi ed,
Welcome to AAC.
My first thought would be the transformer has 'shorted turns'.
Which means the insulation between the copper coils adjacent turns has failed.
It needs rewinding, which is a major job.


Joined Jul 18, 2013
Strange that appears to be a very large Va transformer for that kind of load, which looks like it is dedicated for that lamp.
As Eric mentions it is no mean feat to rewind, I did it a long time ago when I had more time than cash.
The E I lamination's have to be driven out to reveal the spool and bobbin, which has to be rewound and then the arduous task of replacing the lamination's.
What is the technology of the lamp itself?