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Heating Blanket Controller: Soldering QC

A friend reported that three of the four heating blankets he’s bought over the last several years have failed, so he sent the lot to me for teardown and maybe repair.

Looking inside one controller showed some obviously bad solder joints:

Blanket controller - bad joints

Blanket controller – bad joints

Hitting the joints with the soldering iron improved their outlook on life, but the controller remained dead; they weren’t really bad joints, they just looked that way.

If the “lot number” labels on the controllers mean anything, they’ve tried three different triac mounts over the years:

  • A through-hole triac screwed to the board with no heatsink
  • An SMD triac using the PCB copper as a heatsink
  • A through-hole triac with a big aluminum heatsink

That’s in order of ascending lot number, suggesting the triac caused some reliability problems.

I’m still trying to figure out how to probe the circuitry without killing myself. An isolation transformer comes to mind, because the blanket dissipates only 85 W.

Surely the triacs have snubbers…

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  1. #1 by Cuno on 2014-11-22 - 14:53

    Hi Ed,
    I have a Tektronix THS 720 and a Scopemeter 123 for those jobs. The tek is ideal.
    They ar relatively cheap on ebay. 2 isolated scope inputs.

    • #2 by Ed on 2014-11-22 - 19:55

      relatively cheap on ebay

      “Relatively”, indeed: can’t justify dropping that much for my very occasional needs, but if one should drift by, I’ll definitely grab it.

      Thanks for the heads-up!