We're having problems with defective (or damaged in reflow) audio transformers and I'm not buying what the manufacturer is telling us. A significant number are falling out during manufacturing with what we originally thought was a low DCR (12-15ohms vs 19ohms nominal) but the turns ratio doesn't look altered significantly as measured with a 1kHz tone. I've also found that the inductance drops through the floor (600uH, perhaps vs. 1.8H, as indicated by an LR time constant measurement).
The vendor is telling us that the RoHS reflow process is melting the winding insulation but I'm not sure I'm buying it. Shorting a few turns (even a third) doesn't seem like it should change the inductance by over three orders of magnitude and it should also show up as a significant change in the turns ratio, no? Also, I usually see a DCR change but in a failing transformer both the primary and secondary
*always* have low inductance. The LR decay of our measurement isn't a normal exponential, either. My guess is that something (heat) is damaging the core somehow (turns ratio OK, L missing three zeros, unnatural LR decay - no explanation for low DCR, though). The vendor has agreed to fix the problem but their fix is high temperature wire, which seems like a waste of time, though I'm certainly no transformer expert. Any thoughts?