Totally agree with Chris. The possibility of residual flux. Add heat and it corrodes quickly.
I had issues with a manufacturer of wafer probes which I unfortunately was unable to solve. Took some SEM photos of the probes which turned black aft er heating at 200 C.
Solution was to boil the probes as received in baking soda and water, clean and then electroless gold plate them.
Later, the manufacturer went to 60/40 solder of the mechanical portion rath er than spot welding. they used SN96 for the needles. They were unable to go back to the old process, but would solder ours with Sn96.
I also agree with the ZNR or some type of transient supression.
If you have too, I would investigate adding some sort of forced cooling.
I had buzzing issues with new 24 VAC valves (non gas) from a reputable comp any which was traced to dust during manufacturer. All of the valves receiv ed had to be cleaned prior to use. they were used with an Inert gas as the air medium which had no lubercation.
Poor manufacturing such as dust could be causeing a higher temperature than usual.
Corona dope might be able to be used to re-insulate a repair. Be sure to c lean the flux off.
Under another note, a gas dryer at home has been operating for nearly 48 ye ars with the same electric gas valves. As you said use is 3-4 times per we ek. Preventative maintenance generally was cleaning of the ducts internal and e xternal. Painting was done rarely.
The last failure was a cascade of events where the grease in the blower bea ring transmission froze up. This cause the bearing to fail, which caused t he fan blower belt to fail and the dryer drum belt to fail.
The fixes employed at this point was:
- The set screw for the fan pulley was changed to brass tipped.
2.PM's will include looking at the transmission.
3.Special thrust washers were added to the drum tensioning pulley shaft and the fan shaft.
- A special grease was selected for the transmission.
- The drum light was changed to LED based.
- A gasket was made for the outlet stream.
- Nylon hinges (lint filter door) were replaced and painted with epoxy pai nt
- Material of the lint filter door bumpers changed.
- Always on the lookout for ignitors - I have two spares.
- Replace a lid clip.
- Replace the drum belt. 1st time in 50 years.
The to do list includes
- Characterizing the ignitors. Possibly building a capacitance welder.
- Rebuilding the lint filter.
- Possibly adding an hour meter.
- Add a dust ring to the timer shaft.
- Make the sheet metal screws - machine screws
FWIW: A company does rebuild 50's style oven thermostats with ROHS compatab le capillary tubes.
One issue I had was threading the capillary through the outer cavity and t he oven cavity double wall. I placed the sensor in a straw which was easy to thread through the double wall and then cut the straw off.
Sorry about the "noise". Hopefully it's useful to somebody.