The Wanderer expressed precisely :
On Fri, 22 Jan 2010 09:09:49 -0800 (PST), Dan Gravell wrote:

I have four halogen LV downlighters in my kitchen, controlled by one
dimmer switch. All of them use dimmer transformers. I once had to
change the chocolate box on one of them that melted, but other than
that they have given three years of trouble free operation.

Recently they have been failing to turn on. This affects all of the
lights, not just one. I noticed that if I turned the dimmer down (made
the light more dim) they would then come on, and turning the dimmer
back up would turn them off again.

It's rather intermittent, sometimes they turn on fine at full power,
other times not.

I took a look in the switch unit but all the connections seem fine
there.

Any other ideas?

Have several 12v downlighters in my kitchen, two have been troublesome, not
always coming on, though I don't use a dimmer. The bulb connectors have
proved to be a bit problematical, I found the cables need to come out of
the bulb connector more or less straight up. If the cables are bent at any
sort of angle, the lights don't always come on. Very often, just the action
of removing the bulb retaining ring from the fitting and easing the light
down would make it light, then it would go out again when the bulb was
secured back into the fitting.

I have a bungalow, and dressing the wires into a different position from
the loft rather than trying to bend the wires from below through the
fitting seems to have cured the problem

Most likely the lamp-holder to lamp connections. They run a lot of
current and with a lot of heat. The lamp pins and lamp-holder
connections tend to burn and become high resistance. Look at the lamp
pins, they should be clean and polished, if not fit new lamp holders
and lamp as a pair - they are not expensive.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk