AMX --> DMX, old TTI dimmers, and flickering - oh my

Uncle Dirtnap

Active Member
Hello all!

Moved to a new city 2 weeks before covid, and I'm finally getting set up to volunteer at the local community theatre. Like most small theatres, they have a mix of donated equiptment; kinda a 'lighting through the ages' museum exhibit. The system is basically a 90s ETC board going into a Dove DMX/AMX converter feeding racks of classic TTI modular dimmers.

Recently they started having a problem with flickering. When they dim a few lights, occasionally all lights will occasionally all flicker together. It doesn't happen all the time, and never seems to happen when lights are on full. We thought it may be isolated to a specific dimmer pack, but some experimentation has killed that theory. When it happens, its like someone is jiggling the master - all lights are affected. It will usually stop in a few seconds but may happen again if the lights are still dimmed. Between the pars and the small space, it seems that leaving the lights on full as much as possible has been the stopgap measure.

There is no power conditioning to speak of, but since it happens directly in response to dimming I am not sure it relates.

Any ideas on what I can start looking for? I feel pretty old, but I think I was born the same year the dimmers were made.... this is outside my experience.

-RJ
 
Oh, by modular I meant 6 dimmers per pack (and the name "modular dimming system"- models are mostly MDS-1-7) Attaching pictures

--RJ
 

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I would have an electrician come in and tighten/verify the NEUTRAL connection to the dimmer rack... ah hell, ALL the power feed connections to the rack. At each end of the wire, too.
 
Assuming your ETC console is supplying DMX to the Dove converter, the converter is then supplying a 0-10vdc analog control signal to each channel of the dimmer packs.
If you know how to use a VOM, you could probe the outputs of the Dove unit and see if the control signal is fluctuating in time with the fixtures. So the changing control signal is telling the
dimmer to follow it.
Based on the age of things, I would first suspect a bad capacitor in the power supply of the Dove converter. Look for one that has a domed instead of flat end. See photo for an example.
 

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Last edited:
Assuming your ETC console is supplying DMX to the Dove converter, the converter is then supplying a 0-10vdc analog control signal to each channel of the dimmer packs.
If you know how to use a VOM, you could probe the outputs of the Dove unit and see if the control signal is fluctuating in time with the fixtures. So the changing control signal is telling the
dimmer to follow it.
Based on the age of things, I would first suspect a bad capacitor in the power supply of the Dove converter. Look for one that has a domed instead of flat end. See photo for an example.
Awesome! So excited hing within my wheelhouse- I have replaced entirely too many capacitors in my day- :)

-RJ
 
Microstar is probably on the right track with the filter caps, given the age.

Some ETC consoles have a DMX speed adjustment. If the DMX is faster than the Dove can handle, it could cause weirdness like you are having.

I would also check the DMX interface. Make sure the cable type is made for DMX, that pins 1-3 have continuity, that pins 2-3 are a twisted pair of conductors, and that the last DMX device is terminated. The Dove has an internal termination, if it is turned on.

https://www.manualsdir.com/manuals/764289/dove-systems-mtx-de-48.html
 
One thing to check. trunrunning your light board and the dmx to analog box on the same circuit. I have seen problems when they were on different circuits
 
It's still being investigated. Power company came out and validated our setup, and the converter has checked out fine. We are tearing into the dimmers and checking connections on that end today-. I will let everyone know when we get a solution!
 
We found a handful of loose connections on the breakers, and one of the main legs was also shockingly (ha!) Loose as well- testing the lights this week. It's an intermittent problem, so it will be a bit before I will confirm a fix!
 
We found a handful of loose connections on the breakers, and one of the main legs was also shockingly (ha!) Loose as well- testing the lights this week. It's an intermittent problem, so it will be a bit before I will confirm a fix!
@Uncle Dirtnap Regarding the loose connection on one of your main legs.
WITH THE POWER OFF loosen the connection(s) remove the conductors from the lug(s) then thoroughly clean the conductors and lugs prior to reconnecting and tightening them.
Fine emery cloth / paper is useful for cleaning. Carefully reinspect visually and tighten one more time as they may have loosened slightly due to the conductors migrating from the pressure point(s).
Finally re-power and test thoroughly.
Consider powering down and measuring / feeling for any warm connections.
Toodleoo!
Ron Hebbard
 
Thermal imaging at full load will tell you exactly where the problem connection(s) are. Better to use fine sandpaper for contact cleaning as emery is aluminum oxide which is a non-conductor.
 
Thermal imaging at full load will tell you exactly where the problem connection(s) are. Better to use fine sandpaper for contact cleaning as emery is aluminum oxide which is a non-conductor.
A brass-bristled "toothbrush" from the auto parts store works great too (learned from my industrial electrician friend)

Ideally if there's available length just lop off the oxidized (or charred) end bit and re-strip (especially with larger-gauge AL feeders). You may have to change the lug and/or breaker if the lug is damaged and can't be separated from it.

That panel with what looks like stab-down breakers mounted on a water-damaged wall might be a clue to possible oxidation issues. Have the breaker-to-buss connections checked too -if they aren't bolt-down types then that's also an issue with what appears to be a consumer-grade panel serving large branch loads. They are buried beneath the breakers so the thermal imaging JonCarter mentioned really helps.

The electrical contractor I partner with offers low special rates (or free) to community theaters for such work as it's a necessary but often overlooked part of using large load dimmed conventional. So it doesn't hurt to ask -maybe offer a program ad in exchange?
 
!!!!!!!!!!!!!FIXED!!!!!!!!!

The tightening of all the connections was the key. Apparently the hot leg was 1.5 turns out from tight, and the others not much better. All is good and beautiful. Thank everyone for their help! Moral of the story: check your fundamentals. It's far too easy to go in circles looking for an exotic solution to a simple problem. There are at least 3 show runs I know of that were affected by the flickering, and probably a lot more.

I'm sure a lot of people - myself included - would be slightly uncomfortable checking this myself because it's been a long time since I did it regularly and electrocution is so final. I need to make a point to get a refresher in safety and add this kind of basic checking into my debugging skillset.

-RJ
 
!!!!!!!!!!!!!FIXED!!!!!!!!!

The tightening of all the connections was the key. Apparently the hot leg was 1.5 turns out from tight, and the others not much better. All is good and beautiful. Thank everyone for their help! Moral of the story: check your fundamentals. It's far too easy to go in circles looking for an exotic solution to a simple problem. There are at least 3 show runs I know of that were affected by the flickering, and probably a lot more.

I'm sure a lot of people - myself included - would be slightly uncomfortable checking this myself because it's been a long time since I did it regularly and electrocution is so final. I need to make a point to get a refresher in safety and add this kind of basic checking into my debugging skillset.

-RJ
Glad to hear the problem is solved.

An item of note from the old days: When I would come upon one of our techs at Production Arts in an hours-deep chip-changing repair without success, I would always ask the question:

"Does it have food and water?"

Food and water equated to correct power and a valid clock signal. It was not unusual for the problem to be solved after looking into that question.

Wisdom from the old fart department: Always look first for valid power. And when you hear hoof beats, never assume zebras.

ST
 

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