Control/Dimming The Phantom blew out my DMX!

[AdamF1978]

Hello everyone, just joined today in an effort to get a better grasp on my own repairs. I'm a one man band, I got 10 different DMX LED par can fixtures, plugging in my guest mic in a hurry one night I accidentally plugged in my spare mic cable into my stage right dmx lighting (luckily only 4 LED par cans) when the 48V phantom was on. Half way through the night the lights went nuts flickering, blacking out, stuck on, it was almost like I had the 3 pin dmx phasing switch reverse, but too inconsistent. For weeks/a dozen shows after, I thought I can't find any info online, some night my lights work fine, others it has it's own mind, even blackout doesn't work, yet I go over and wiggle the xlr cable and it works fine for 5 seconds sometimes. I don't have the end terminated as I'm only running 40 feet maximum to each side of the stage.

Taking it apart, some of my lights are dip switch, some are LCD display, I took an dip switch model apart and it pin 2 and 3 (+/-) go through diodes inside. I tried some spare diodes since it almost seems to have ac interference. It fixed the one test fixture, but makes my lights half as bright now in the 177 pc par 64.

The part I can't seem to google is can anyone tell me what voltage/amperage dmx uses so I can shop for the proper diode to put in? I wouldn't even know where to take my par cans in to be repaired, even though I'm in Toronto. It's an amateur question but otherwise I'm looking at replacing half of my lighting rig when a $0.12 diode might be all it needs. I don't feel I need to build a terminator yet, but my lights are unpredictable (even the good ones) when 2 or more are plugged into my controller. Thank you in advance. Adam

 

[dramatech]

Most anything can be wrong, but the first thing that I would try is to see if you can identify the DMX (RS485) transceiver. Look on the circuit board and see if you can identify an 8 pin integrated circuit. It will have four pins on one side and four pins on the opposite side. Thre are several models, but the most common will have the numbers 75176. There might be other letters and numbers, but these are the most important. You can get one of these little guys for about a dollar. In the US, Mouser is one of the best suppliers, but in Eastern Canada there is one that is less expensive and many times beter equipped. The name escapes me for the moment, but I think that it is something like Advanced electronics. When replacing this circuit, remeber to make sure that the pin orientation is the same as the one that comes out. Pin 1 is identified by a dot or an indentation on the end of the chip. The best way to desolder this chip, without damaging the board, is to cut the pins close to the chip, and then heat from the back side of the board with a hemostat or small heat clamp on the pin. The weight should make the pin fall from the hole. When ever I do a desolder job of this nature, I install a socket so that I won't have to do it again. If the IC is surace mount, then you are into a different ball game altogether.

 

[traxman25]

Why not just try a terminator? It is a heck of a lot less work to try than taking apart a fixture! All you need is an XLR end and a 120 ohm resister between 2 and 3. It is a good thing to have on the rig anyway and certainly a lot easier to try!

Also do you have this issue when the few fixtures what got hit by phantom are unplugged from the rig? I really wish more manufactures would spend the $10 on components to run opto isolation, then this wouldn't be an issue for you!

 

[noderaser]

I'm not a fan of running DMX on 3-pin XLR for the exact reason you encountered, but it brings up another consideration--what kind of cable are you using to connect your DMX run, data or mic cable? Data cable has a different impedance and other characteristics that make it more suitable for wideband digital data. While mic cable may work in many situations, the impedance difference can bite you on long-ish cable runs. I'm sure there are also things like crosstalk that your standard mic cable wasn't designed to deal with.

IIRC, DMX only uses something like +/-2.5 VDC for its signaling, though decent equipment should be designed to handle "oops" situations by isolating the control electronics from the input. Assuming you haven't let the smoke out of something, I'd have a look at your data path and terminating--good pieces of equipment should be self-terminating, but many people swear by sticking a physical terminator at the end of your run anyway. Intermittent faults like you describe can be a bear to track down, so I would start with the cheaper things first, like checking your signal path.

 

[DuckJordan]

Not to mention if 40v was causing issues you would see any control the chip would be fried not malfunctioning.

 

[derekleffew]

...IIRC, DMX only uses something like +/-2.5 VDC for its signaling, ...

From the wiki entry terminator, DMX, quoting Doug Fleenor:

The standard says the signal cannot exceed 6 volts when unloaded and
cannot exceed 5 volts when loaded with 60 Ohms (One 120 Ohm terminator
at each end of the line). The standard does not state the maximum
voltage when loaded with the more common case of a single 120 Ohm
terminator. If we use the worst case of 6 volts the power dissipated
in the 120 Ohm termination resistor is 0.30 Watts, hence a 1/2 Watt
resistor is required.

Although the standard says the DMX signal cannot exceed 6 volts, there
is always the possibility of higher voltages due to miswiring or
equipment failure. Since it is difficult to determine if a terminator
has been damaged by excessive power, Doug Fleenor Design uses 2W
resistors in its terminators to maximize the chance they will survive
fault conditions.

-----

...--good pieces of equipment should be self-terminating, but many people swear by sticking a physical terminator at the end of your run anyway. ...

Actually, we learned here that there are only two pieces of gear that automatically terminate (ETC's Revolution and SmartPack).

 

[JD]

Although plugging in a 48 volt phantom line is a very bad thing, I don't think your problems are related to that. If the gate protection on one of the RS485 chips had popped, you would be having a lot of problems! In some cases, if one of the inputs survived, the differential may be enough to keep things working, but you indicate that some nights everything works fine, and others, things do not. Phantom supplies usually do not have many milliamps of current, so this may have saved you. There is a chance that the output driver on the board is compromised, but I would start by looking at this as a cabling/terminator issue. See if you can borrow some known good cables and a terminator and then see if the problem reoccurs.

 

[AdamF1978]

Although plugging in a 48 volt phantom line is a very bad thing, I don't think your problems are related to that. If the gate protection on one of the RS485 chips had popped, you would be having a lot of problems! In some cases, if one of the inputs survived, the differential may be enough to keep things working, but you indicate that some nights everything works fine, and others, things do not. Phantom supplies usually do not have many milliamps of current, so this may have saved you. There is a chance that the output driver on the board is compromised, but I would start by looking at this as a cabling/terminator issue. See if you can borrow some known good cables and a terminator and then see if the problem reoccurs.

Is the 75176 chip for a 4 channel dmx only? I finally got some on order as well as sockets. In the meantime I did build 2 terminators, one with two 270 ohm resistors parallel which gave me 135ohm. It made my lights flicker much worse. A few days ago I stumbled across a multi resistor pack at an electronics store that had a 120ohm resistor which made my lights flicker slightly more.

I did notice 80% of the time if I walk up and touch the xlr connector (even though it's ungrounded cord to the connector housing inside and out since it's touching plastic on the dmx light) If you touch it, the body grounding resets the seizure my lights are having which buys me another 3 seconds or 30 minutes sometimes. I thought since the body resistance is 300-1000ohms, I thought of maybe putting a 500+ohm resister from pin 1 (thinking it was grounded at the time to the connector housing) to the body ground on the dmx lighting, but from what I researched it seemed like I might fry something.

Sadly I am using regular mic cables, didn't know there was a difference til you said it, but it was working fine before I blasted it with phantom, I have about 120 feet of cable between the 9 present lights. It wasn't until I plugged that 9th light in that I began having problems, and that 9th light is the good one. Is is possible after things blow that they can fry other components in the good lights as well as controllers? These lights I mostly bought at www.amonstar.com which are noname lightings cans $70-100USD each. I'm using the par 64 (4 channel - 177LED) and 8 - 336LED stage wash lights (7 channel) I'm willing to bet there's no isolating circuitry.

So noderaser said dmx voltage is +/-2.5 upwards of 5v, I tried one experimentation replacing the 2 diodes for pin 2 and 3 on the board where the pins first enter the circuit board. I think I put a 5-6 volt diode. I don't know if they're zener or not, I'm guessing not. I've since removed that par can out of the chain as even though one resistor was blown, my educated guess on what size to use made the one can 1/3 as bright. I'm not even sure which lights were subjected to the phantom, I have 3 different kinds, and I'm only sure on one that was not exposed to phantom, the other 8 in the chain I don't know which half were exposed.

Thanks to everyone on the replies I'll keep you posted on the progress, waiting on parts to do anything more though. I know it's a lot of writing but I'm sure someone else will do this oops move someday... but as for now I'm the top search result when you type in "phantom power into dmx" on google... go-go-ME!

 

[Codeguy]

Adam, hi
Might I suggest renting or borrowing a DMX Opto Isolator for testing purposes.
This would permit you to isolate all of your DMX Fixtures from each other and the DMX controller.
You should be able to rent one from Steves or Long and McQuade .
A music rental store might be your best bet as they would be more likely to have an isolator with 3 pin XLRs as opposed to one of the larger lighting rental houses who would be more likely to only stock 5 pin devices...I am thinking Whites or Christie Lights.

I would suspect you have one fixture with either a TVS diode or the 485 Tranceiver with one input partialy shorted.
This can give you the sort of on again off again issues you are seeing and can effect the whole chain of connected devices.

If you are replacing the TVS diodes I would recomend one that clamps at 8 volts as the 5 volt units will start to clamp at less than 5 volts which can attenuate the DMX signal.

I would also look closely at the RS 485 Tranceivers....when in doubt...shotgun the lot...

 

[derekleffew]

... A few days ago I stumbled across a multi resistor pack at an electronics store that had a 120ohm resistor which made my lights flicker slightly more. ...

From http://www.controlbooth.com/forums/...moving-light-freakout-terminator-problem.html :

If you drop Data+ or Data- somewhere along the daisy-chained path of cables due to a broken conductor, and have a terminator in place, all the devices past the point of the break will not work correctly or at all. Seen it quite a few times and is a tricky one.

What happens here is that after the conductor break, the remaining data signal [D+ or D-] goes down to the end of the line, through the terminator and back up the other data line. This means that you no longer have correct differential data as D+ and D- are the same minus a little amplitude and a crazy amount of timing. So the light wigs out. When you remove the terminator, the light works fine because one of the data signals and data common are still in place and most receiver chips will still work with this.

A terminator at the end of line only causes problems if there are wiring issues, too many unit loads on the line or weak driver/receiver chips.

Hoping my suggestion helps.....

David

 

[JD]

Is the 75176 chip for a 4 channel dmx only?

The RS485 transceiver chip simply converts what is on the RS485 buss to voltages that look like ones and zeros. It has no idea what the stream means, so it would have no channel limitations.

NOTE: On the overall subject, this does sound more like an open conductor / bad cable issue. (See DavidNorth post)

 

[Lotos]

The RS485 transceiver chip simply converts what is on the RS485 buss to voltages that look like ones and zeros. It has no idea what the stream means, so it would have no channel limitations.

NOTE: On the overall subject, this does sound more like an open conductor / bad cable issue. (See DavidNorth post)

Yeah, this sounds exactly like what I went through that triggered the referenced post...

Sidenote: Adam, you're in my backyard... If you need someone to chat to about this stuff, hit me up.