How to interpret fluor. tube colors? -- Followup

The fluorescent lamp on a Kenmore Model 250 stove (now
20 years old) has started wrecking new fluor. tubes
(Sylvania, 22-inch, 18 watts). Symptoms are:
1. Dark grey bars appear at both ends of the new tube
within 5 min. use.
2. Tube lights only at the two ends (dark in the middle
for 18 inches.) One end shows white light, the other pink light.

What does this mean?

Respondents in alt.home.repair suggest either the starter
(switch?) or the ballast has died (cf. 20 years old.) The stove
manual includes no circuit diagram. The works are concealed
by two back panels, one (40% area) over the other 95%). Removed,
the upper panel reveals only wiring and connectors and one small
transformer labelled 18w and 15w (obviously for upper fluor. tube and
stove incandescent lamp) so it would be easy to remove and replace
that -- but I suppose transformers rarely fail and the ballast and
starter are elsewhere, under the second panel.

It seems cheaper to add a new lamp fixture over the stove,
rather than dive deeper into wiring without a circuit diagram.

 

95% probability if you remove a larger access panel you will find a
wiring diagram pasted to the inside. For more complex appliances very
often you will find a full foldout paper diagram taped behind a cover or
under the bottom.

 

Does it have a glow starter (when it did work, did it flash a
few times at switch-on before staying on continuously)?

If so, the glow-starter contacts have welded closed and it
needs replacing. The failed starter has probably wrecked the
tube too if it's been left switched on like this for a long
time. Simply unplugging the starter whilst the tube ends are
glowing should cause the tube to light up properly if a welded
starter is the cause and the tube isn't dead (might have to try
it a few times).

The darkened tube ends sound like the tube's filament coatings
have been substantially lost, and the end which doesn't glow
white may have lost too much for the tube to work anymore - that
would normally result in an orange glow though, not pink.

A second possibility is that the tube has an electronic ballast.
If that's failed such that it's providing a DC supply to the tube,
it will eventually end up with the mercury moving to one end,
which will continue to glow white, whilst the rest of the tube
glows a dim pink due to the remaining argon gas discharging.

 

18W and 15W ballasts are just series inductors in one side of the
power the line between the power and one pin and one end of the bulb.
The other side of the power line goes directly to one pin at the other
end of the bulb. The second pin at each end of the bulb go to the
starter. There is a power switch between the power line and the
connections to one end of the bulb. This is high school level stuff
we're talking about here, no need to make it graduate level
electricity.