DMX Input Not Working

ironambrose

Member
Joined
Mar 22, 2021
Location
Australia
Hi all,

I recently got a second hand PAR light and have realised the DMX is not working. After opening it up I can see that there are a couple of zener diodes which have blown close to the DMX line which I am guessing is the issue here. Furthermore one of the traces looks to be popped out somehow (the wire sticking up in the air in the photo). Unfortunately I cannot track down schematics and have no idea what voltage the zener diodes are. Is it even worthwhile trying to replace given the damage. Any insights would be much appreciated.

IMG-9135.PNG


Thanks
 

Mac Hosehead

Well-Known Member
Joined
Sep 10, 2014
Location
Shark Tank
This is a situation of limited information where one can only really guess or give an opinion. My guess is that zeners are there only for protection on DMX line and replacing them wouldn't necessarily restore DMX function. They look as if they have been intentionally removed maybe in attempt to get DMX to work. The wire sticking up is odd and does not look like a trace but something added. I tend to think that your only hope would be if you can follow the DMX connections from the input connector to the chip on the circuit board with a meter.
 

StradivariusBone

Custom Title
Fight Leukemia
Joined
Aug 23, 2013
Location
Space Coast, FL
If it's socketed, it might be worth throwing a new DMX transceiver on it to see what that does. Maybe trace out where the diodes are in relation to the data + - coming in. I agree with Mac that the diodes in are probably for some sort of reverse current protection and wouldn't necessarily be required for a functioning DMX interface. If that's the case, you could just bridge those terminals to test. That missing pad will be a thing, but the way I see it the light's already broken, what do you have to lose?

Another random thought- Blizzard is really great about sending out replacement boards and their designs from what I've seen are all pretty modular (eg power board, DMX board, LED driver board, etc). I don't know the manufacturer here, but it's worth a shot to reach out to see if they can ship you a replacement DMX board. Blizzard is very cost-economical on that front and I've saved a few hundred dollars over the years doing repairs that way.
 
  • Like
Reactions: RonHebbard

ironambrose

Member
Joined
Mar 22, 2021
Location
Australia
Thanks for the responses :) Just as an additional input the light is an Acme / Lightmoves multipar 12FC (http://www.lightmoves.com.au/downloads/Documentation/Litecraft/ILED-MP-12F5-020_3.pdf). The light itself works with all internal programs (e.g. colour fade, fixed colours, sound activation etc) working fine however its just the DMX which is out. In the event I cannot get DMX to work I will just used it as a fixed colour however would love to get the DMX working.

I have done some further testing and tried connecting the DMX signal directly to the board (bypassing the DMX input and board which goes into the main board), however this does not help so definitely believe its something to do with the initial image sent through. I might try bridging each of the terminals and see whether I get a signal and then try backwards from there. Hopefully it wont blow anything up..

Thanks again all
 

almorton

Well-Known Member
Joined
Dec 17, 2014
Location
Caterham, Surrey, UK
My guess is that the zeners were in parallel with the incoming DMX and are intended to conduct on overvoltage (usual usage of a zener). If you bridge them, you'll just short the DMX signal to ground. I'd hazard a guess that they were 12V or thereabouts. See if you can spot a 75176 chip somewhere. That's the differential bus transceiver that @StradivariusBone alluded to, and if that is blown you won't get any DMX going anywhere. If it's socketed or through hole mounted it's an easy replacement. If it's SMD then it's a little trickier to replace but still doable with a fine enough tip, a magnifier and a (very) steady hand.
 

JD

Well-Known Member
Joined
Jan 1, 2005
Location
North Wales PA
If the zeners were pulled because they failed, then the unit probably was hit with a very high over-voltage at the DMX port. That can happen if another fixture fails in a way as to introduce line voltage on it's circuit ground. Since most all units use switch-mode power supplies, high-pot testing and QA is critical in manufacturing to insure line voltage does not end up in the secondary. Unfortunately, this testing is very questionable in some of the low-end stuff from China. Usually, the fixture will fry itself if properly grounded.
I am amazed at how many cheap phone chargers end up outputting line voltage on the USB line. Same type of power supply, but QA on the transformer is pretty bad.
 

almorton

Well-Known Member
Joined
Dec 17, 2014
Location
Caterham, Surrey, UK
If the zeners were pulled because they failed, then the unit probably was hit with a very high over-voltage at the DMX port. That can happen if another fixture fails in a way as to introduce line voltage on it's circuit ground. Since most all units use switch-mode power supplies, high-pot testing and QA is critical in manufacturing to insure line voltage does not end up in the secondary. Unfortunately, this testing is very questionable in some of the low-end stuff from China. Usually, the fixture will fry itself if properly grounded.
I am amazed at how many cheap phone chargers end up outputting line voltage on the USB line. Same type of power supply, but QA on the transformer is pretty bad.
You should see some of the teardowns of USB supplies that big clive has done. They range from very well engineered (e.g. Apple, Ikea) to appallingly lethal (no name bargains).
 

mar Jori J. A

New Member
Joined
Mar 27, 2021
Location
NY
Hi all,

I recently got a second hand PAR light and have realised the DMX is not working. After opening it up I can see that there are a couple of zener diodes which have blown close to the DMX line which I am guessing is the issue here. Furthermore one of the traces looks to be popped out somehow (the wire sticking up in the air in the photo). Unfortunately I cannot track down schematics and have no idea what voltage the zener diodes are. Is it even worthwhile trying to replace given the damage. Any insights would be much appreciated.

View attachment 21724

Thanks
My opinion, no not worth it. A par can is so close to obsoleteville (except in weddings and theatres) the repair may be cost the price of an hour not a day rate.
 

JD

Well-Known Member
Joined
Jan 1, 2005
Location
North Wales PA
You should see some of the teardowns of USB supplies that big clive has done. They range from very well engineered (e.g. Apple, Ikea) to appallingly lethal (no name bargains).
Big fan of Big Clive. I have seen the videos. Most people do not realize that all that lies between them and line voltage is a thin film of tape that often is applied wrong, or did not separate the primary and secondary lead wires.
I have run into more than one case where the output of an ac adapter is at line voltage due to isolation failure. If that adapter is powering something that is then connected to something else that finds a ground path, bang!
 
Last edited:

Crisp image

Well-Known Member
Joined
Jun 18, 2017
Location
Eastern Victoria Australia
  • Like
Reactions: StradivariusBone

Users who are viewing this thread