Bodging my way through life

I'm a fan of bright, high-quality light for reading and crafts, so I figured I'd try the fancy Microsun lamps. These are 68W metal halide lamps, and provide 5300-6000 lumens with a CRI of 72-90, depending on who you believe. The lamps are expensive, but nicely made. I bought two of 'em, a floor lamp (for crafts) and a table lamp (for reading). They work well, and several people admired the quality of light they produce. However, after two years of fairly light use, the reading lamp started to flicker and then died. I tried swapping bulbs, but the problem remained with the lamp.

People who know me will not be surprised by my next move. The ballast (called a "gear pack" by the seller) is clearly marked "DO NOT OPEN. NO USER SERVICEABLE PARTS INSIDE." maugorn and others might remember me reciting the litany "Do not open, no user serviceable parts inside, pilot lamps soldered in place, danger high voltage, warranty void if opened, etc." as I tore into things.

Well, I found out some things. The manufacturer claims that you can't just make your own lamp by buying a gear pack, because the gear pack doesn't include "the transformer". I had initially believed there was a transformer tucked into the base of the lamp, as the lamp was sufficiently large and heavy. But there isn't one. The AC leads go straight to the gear pack. You could easily retrofit any ordinary threaded-tube lamp by buying a $60 gear pack and screwing it on top.

The ballast is patented. This means I can download the schematic. It's not quite the same as the one used in the lamp, but it's pretty close. It's just a current-regulated DC power supply with a transformer-coupled starting pulse network, run off a voltage doubler. Armed with that info, and the failure mode, I deduced that it isn't the starting circuitry (the lamp failed when it was already started), and it probably isn't the current regulator (those generally work right or not at all). That leaves the voltage doubler. Not much to it, an inrush current limiter, a pair of diodes, and two big electrolytic capacitors. And electrolytic capacitors are famous for drying out and failing. So I read the specs off 'em, measured the relevant dimensions, and found equivalent units on Digikey. The inrush limiter was a house-numbered part, but a little research convinced me that it was a 10Ω, 1.7A unit or thereabouts, and such things aren't critical (it's just there to make sure that the discharged capacitors don't pull too much current when first turned on). I chose a similar unit that would physically fit and ordered it too.

While I waited for the parts to show up in the mail, I considered my other options. These days, low-wattage metal halide bulbs aren't too hard to find, and ballasts can be had from Fulham (a manufacturer whose products I like) and cheaper manufacturers. If it dies again, I can always re-use the lamp body and nice, high-temperature ceramic socket with a 70W (or more!) metal halide bulb and ballast.