THERMALLY FUSED TRANSFORMER

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My 2 cents...
I have replaced many transformer thermal fuses, and as ! said above, these will fail from time to time due to simple thermal cycling. I usually use a rule of thumb that if the transformer is smaller than 2" x2" x 2", replace it; it's not worth the effort. If it's bigger, or more of a vintage (you have to pay for it with your first born), you have nothing to lose by trying.

Before going to the tearing apart, visually inspect it and make sure it didn't blow BECAUSE of overheating. If the tape wrap or insulation looks discolored, replace the transformer. If you can temporarily bypass the the thermal fuse, do it and monitor the input current without any load on the secondaries. If it's pulling more than 1 or 2 watts, replace the transformer. Leave it connected for an hour or so and then see how hot it's getting. If it's more than warm to the touch, replace the transformer.

If it passed the above tests with flying colors, go ahead and replace the thermal fuse, put the transformer back in circuit, and let the unit run for several hours. If it feels normal temp (you can keep the back of your hand on it for more than 10 seconds, you're probably good and you've saved yourself some $ by replacing a $1.05 component.
 
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