Few days ago i scored some 90s AEG Minikoffers to join the 2 80s ones i allready had previously. All have 2 tap 80/125W Mercury ballasts and were used at 125W

Most bowls show a browned and melty/bubbly spot where it is closest to the lamp

Most of them have a Helvar ballast, that is of thinner/longer shape than most common ballasts. The lanterns arent really designed for any other ballast form factor, though a few have VS ballasts fitted in with some additional bracket thats not part of the original design. All Helvar ballasts (both 80s and 90s) show signs of overheating - the Plastic coil covers are yellowed and crumbling. Few of the 90s ones are burned up as in having shorted turns. It looks like overheating damage, there is no short circuit in the lamp wiring, and Mercury lamps dont randomly go shorted either, so this was not an external short circuit causing the fault

Have any of you see 125W Minikoffers in the wild or have them in collection ? In what condition are they ? Are the Helvar Mercury ballasts prone to overheat on their own unrelated to the luminaire they are in ?

How big is a Minikoffer? If it's really small, I would think that there isn't breathing room for the ballast.

The ballast might be just because of heavy use. The plastic parts of helvar ballast usually become pretty brittle after years of heavy use. And if I remember right, ballast in minikoffer is mounted such way that it has very small are to conduct the heat to the body of the fixture.

It is not a tightly crammed lantern, and is made entirely of Aluminum. Not something thats expected to overheat

The covers overheated i think because it is too close to the lamp - But do they overheat elsewhere too with 125W lamps ?

The ballasts overheated i think just because of their design. The ballast is mounted on 2 tall posts between the pole entry fitting and the edge of the reflector thats far from the lamp. There isnt lot of free space around it (i really wonder why they have not installed it on the side) but still enough for air convection

It sounds to me the Minikoffer was designed for 80W and only afterwards some manager decided to offer it for 125W as well, ignoring the fact the 125W is too much...

With the bowls it indeed looks so. What about the ballasts ? The lantern is not too tight near the ballast area and its not too close to the lamp either

A couple of years ago I took down several Minikoffer (MiniQuadralux) with 125W lamps. The luminaires were made in the early seventies and were slightly different designed around the spigot and ballast fixation than later ones. The ballasts in these luminaires were made by H. Leuenberger AG and showed some browned insulating paper wrappings but all of them were still functioning! 

How were the bowls ?

Have you seen the mentioned Helvar ballasts in other luminaires ? Do they overheat there ?

I have one minikoffer that has helvar ballast but it's unused. Based on a long experience about helvar ballasts in general in all "modern style" post 80's ballast the plastic parts tend to become very brittle and fragile after years of heavy use. This happens also with ballast that do not expose to uv of the light source. If the ballast is mounted such way it can transfer lots of it's heat to the body of the fixture(like mounted directly to the body of the fixture), this will have some positive effect on that. I think the problems is the plastic that is used (well, usually it's not a problem if the ballast and wiring stays undisturbed)

Thats the condition of the 80s ballasts, the only thing remaining from the connection is Metallic tabs sticking out. But the 90s ones show actual signs of melting, as in the Plastic became liquid and deformed near the center of each cover

This is something that can be expected in a Fluorescent ballast that was left with stuck starter, but there is no equivalent fault mode with Mercury lamps

It sounds to me the Minikoffer was designed for 80W and only afterwards some manager decided to offer it for 125W as well, ignoring the fact the 125W is too much...

I could see that happening, all they care about is will it last the warranty period, after that it's not there problem..

Not the case here. From how they are constructed they are clearly meant to last many decades, except they havent. Other lanterns of the Koffer range are now into their 5th decade and still work well

Minikoffers were widely used in lighting walkways previously in my home town. Fixtures with broken bowls and lamps were very common as they were usually installed pretty low and bowl was not very vandal-resistant material and thus easely breakable (especially in winter)

I captured almost every Minikoffer at the southern pavement of Mapu street, during my way to the bus station, and most of them, which have 125W MV lamps, have a small burn mark at the center of their bowl: http://www.lighting-gallery.net/gallery/displayimage.php?pos=-134787

Not the case here. From how they are constructed they are clearly meant to last many decades, except they havent. Other lanterns of the Koffer range are now into their 5th decade and still work well

Well, engineers had that intention, but assumed just the 80W lamp. Then the managers have decided if the 125W stuff fits in, lets offer that too and dont bother asking the engineers...