"The Phantom Light"

In our school auditorium we use a Lehigh, Legacy Dimming System. I don't know any of the specs from the dimmer rack, besides the fact that it is made by Lehigh.
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It all starts when we try to turn off one of the ellipsoidals. But it does not want to turn off, in fact it doesn't do anything. So we call it the "Phantom Light" because it is on even after we leave for the night. The only way it turns off is to let the dimmer cool down. (or of course hit the main breaker) Then soon after that some of our border lights started to freeze. I checked out which dimmer it was on, and it happens to be that the border lights, and the phantom light are on the same "cluster" of dimmers. So we switched one of the clusters from the house lights, with the cluster that was giving us problems, and we haven't had a problem since. But what is it about the dimmers that made them freeze?
 

Van

CBMod
CB Mods
Premium Member
Could be a bad SCR getting close to load capacity and then sticking on. Often when SCRs go bad they default to the on posistion. Perhaps when you repatched you lowered the load and the unit in question is not running as hot. As a Feild technician I would first check the scr in question to see if the heat sink were comming loose, then I'd move on to the control module.
 

JD

Well-Known Member
Agree with Van. Usually found in very old dimmers, but can happen at any point. The silicon junction can develop a crack in the subtract. Part of the junction then overheats causing a thermal runaway that keeps the device switched on until it cools. There are of course control problems that can cause these symptoms as well, but my bet would be one of the SCRs. (two found per dimmer channel)
 

gafftaper

Senior Team
Senior Team
Fight Leukemia
Me Three. You have an older product from a far from respected manufacturer in these parts. You should be able to have it fixed easily enough at your local theater dealer.
 

mspet

Member
I see that everyone has already exhausted the subject at hand, and I agree with all of the advice they've given. Quick question though. I'm suppose to help a local middle school theater reprogram their light board (also a Lehigh Legacy Series), and apparently they've "misplaced" the user manual. Are you aware of any online sources where I could find a copy?
 

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