dimmer issue

[dj_illusions]

hey,
had a weird issue crop up during a show tonight, never seen it happen before and was wondering if any one else had experienced it.
in the venue we run about 200 dimmer channels off a strand 520i, majority of the dimmers are jands roadpack's and a solution 12 rack.
i did a blackout on the desk (cue0 go) and everything went out except for 1 lamp, the desk said nothing was up so did a few checks on the desk etc. everything was fine. pulled dmx and rebooted etc. still the lamp was on.
went to check the dimmer, it reported no faults, so pulled the dmx there, still the lamp was on. unpatched and repatched the lamp and it remained on. power cycled the dimmer and it still stayed on, so in the end had to repatch the lamp to continue the show.
anyone know why the lamp got stuck on, or experienced this problem before? i am thinking something in the dimmer rack may be a bit iffy, possibly the processor or something on that individual dimmer channel??

cheers

[SerraAva]

Is the dimmer possibly set to be on constantly, aka hot patched? Also, try checking the console making sure the channel in question isn't parked on or the dimmer number isn't profiled to be on at full all the time. You can also try taking out the control card in the dimmer rack and making sure everything looks fine, clean the contacts a little, and then put it back in making sure its seated correctly. The last thing like you said, the dimmer may be bad. I had a dimmer once that would only dim between 0-20%. At 20% it was on full, so it made programing interesting to say the least. People would look at the console and wonder why that channel was at 15% and the others at 75%.

I know that NSI rack mount dimmers have that problem fairly often, some times the light is on full blast all the time, some times it won't come on at all but will in another dimmer. The fix is just pulling out the control card, cleaning it, and making sure its seated properly. Same thing with the old ENR Colortran racks.

[SteveB]

SCR and SSR dimmers dead-short fail in a constant on state, which is what this sounds like.

I could not find this particular Jands dimmer system on their site, but am taking a guess it's a 12 pack non-modular system ? (I.E. the dimmers are not on individual or dual modular/swappable cards). I would open up the pack (cut mains power first) and swap the dimmer card to another location to determine if the problem migrates to another DMX address. If the problem migrates, you have a blown pair of SCR's or one half of the SSR (SSR's are typically one power cube powering 2 circuits). If the problem crops up in the same output circuit, then the trouble is not in the actual SCR pairs or the SSR. Next test would be to swap control modules between packs (don't worry about DMX addressing at this point, to see of the problem migrates with the card - if not, then the problem is external control, etc.....

As David Fox at ETC told me, look for the simplest solution first.

Steve B.

[JD]

Second vote for blown SCR here. When they go, that's what you get, solid on. Plug a 100 watt houselamp into it and listen to the bulb, then plug the same lamp in another channel that is at 100%. If it's blown, you will hear a small amount of "lamp sing" from the bulb on the good dimmer, but nothing on the blown channel. Even at 100% you usually can hear some sing, but when the SCR is gone, its like plugging it into wall outlet. Nothing!

[dj_illusions]

Steve, You are right they are a non-modular dimmer, a fairly basic unit with a test function and addressing switch, nothing fancy at all. They are probably about 10 or so years old now aswell, hence not on the website.

Until I test it, I wont know however I think that you may be right in saying it is a blown SCR - will give it a go and see what the result is, if not will try swapping around the control card.

I dont believe it is a control issue as the desk was switched off, the dimmers will still hold state but then the power was cycled to the dimmer aswell which means it would loose any held dmx states, because the lamp continued to power I would agree that it is either an scr or control.

[church]

many dimmer cards allow for adjustment of the min and maximum dimmer threshold - via potentiometers. These do become dirty and I have seen dimmers come on and stay on this way, removing the control signal doesn't clear the problem. Once you have checked the SCRs then this is the next likely issue. Sometimes adjusting them solves the problem. But first inspect the pot to make sure they are not cracked or broken. Heat and dirt play havoc over time with these pots.

Most but not all dimmers using SCRs and a min max pot can be adjusted this way. If you can get an Installation Manual do so and read it carefully. If you are not sure do not attempt this. Caution is always required because there are high voltages in dimmer racks. Always remove your watch and do not wear jewelry. THe following adjustments involve low voltage electtronics but you would not want to accidently catch a metal wristband on something at 120Vac you can loose a hand.

You need to put a load on the channel typically 1kW is required, then command the channel to 15% and adjust the min pot so the light turns on to about 15% - have another channel on at this level for a comparison. Then set the channel to 90% and adjust the max pot to give approx 90% brightness. If this does not fix it then you need someone who knows electronics and has an oscilloscope. Good luck.

[David Ashton]

Right,this dimmer has a triac triggered by an opto coupler and I would bet on a faulty opto as a triac usually dies when a lamp blows.While you've got it open check the snubber capacitors as there was a bad batch of them a few years ago which blow up quite frequently.[Client rings up and says there was a bang and smoke pouring out of dimmer but now it's working fine = snubber capacitor ]

[gafftaper]

Third vote for a blown SCR. I've had it happen a couple times.

[derekleffew]

I guess I've been really lucky. Triacs yes, SCRs/SSRs no, never had one fail in either the open or closed position. But then I've used only ETC/LMI dimmers since about 1990. There, I finally said something nice about ETC! Never replaced an SCR on a Ward-Leonard or Kliegl dimmer either. I may have on a very early CD-80 pack though, now that I think about it.

[dj_illusions]

hey all, thanks for your input...

just thought i would let you know it was the SCR that caused the fault. Dimmer will be sent off to Jands for service tomorrow!

Cheers