Electrovert Econopak Wave Solder Pot Temperature too much variability/Swing

We have an electrovert 16" econopak wave solder machine with two identical pots (exchange between Tin Lead and Lead Free). When the pots are exchanged the same thermocouple is used, and the same controller is used - with the set point adjusted. The Sn/Pb pot maintains its temperature within about 8 degrees F or better. The Lead Free pot temperature swings are much greater closer to +/-25 F resulting in poor process repeatability. A second thermo couple suggests that the controllers TC is accurate, as the temperature away from the machines TC tends to agree with the secondary tester TC. All the heaters appear to be operating, and we are going to inspect if they have adequate contact with the tank, and or if the are actually switching on and off appropriately. Posting here to see if anyone else has ever had a similar problem. One yet to be proven hypothesis is that a failed or failing/intermittent heater means that the others end up overshooting the set point before shutting off - but this is still a theory only.

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