On Sat, 10 Sep 2016 10:25:09 -0400, Ralph Mowery
wrote:

WE had hundreds of that brand where I worked and some were pulsed every
30 seconds or so and some were almost always on. Hardly any problems
with the coils.

Ralph, should I presume that was with pre-ROHS vintage coils? As I
mentioned in my O/P, the original pair (which were pre-ROHS) both
lasted two decades. Subsequent ones were 15-18 months AND the oven
gets less use nowadays as the kids migrate outwards. Not a
statistically significant sampling for some, but sufficiently damning
from where I stand/sit.

Most problems were either the rubber disk wearing out or the plunger
enlarging due to all the beating they take or the spring wearing out.
The plunger is enclosed so no way for it to touch the coil form.

We haven't had the moving parts show any issues, just coils.

While it could be something in the coil, I would think that as many that he
changed out it must be something external.

Nothing external has changed, except our supply voltage has been
"harmonised" with rest-of-Oz i.e. reduced from the original 254V rms
down to 240V rms.

One other thing, is the coil all the way seated on where the plunger is
? If not it could be drawing enough current to burn out the wire. We
often left the coil hooked up to the conduit and just changed out the
mechanical part. Mainly did that to keep from having to go to another
floor and find the power source for the coil. Found that we needed to
stick a large screwdriver or other item in the core of the coil or a
fuse would blow or the coil would burn out.

Appreciate the question. It is seated down properly. The frame which
sits over the coil/valve assembly as part of it is actually a snug fit
(you need to slide the coil into it and align the holes, it's a press
fit) and then the frame is held down over the valve plunger "cylinder"
and screwed down. No unintended magnetic/air gaps.

There is also a "spacer" pressed into each end of the coil bobbin the
ensure it is located correctly (radially).

Pics of the valve assemblies in situ:

http://imgur.com/xGlio51
http://imgur.com/3NB4ALX

and of the coil post-Dremel (*):

http://imgur.com/An9XCk8
http://imgur.com/wH0l3eP

(*) I needed effective bobbin length/diameters to calculate turns and
hence deduce a credible wire gauge to match the known DC resistance.