Mac 250 bad DMX

[Amy Worrall]

Got a Mac 250 that has started misbehaving. Was fine yesterday but this morning, every time this Mac 250 was on the chain, the other fixtures were flickering and flashing. I’m guessing the Mac is spurting out bad DMX. 

Is this a common or known fault? Is there a fix?

[Brian]

Well, there is an option to disable DMX TX as a diagnostic.

 

[pjrails]

Have you got a DMX terminator at the end of the run? this will normally sort out any flickering. See the attached image.

[Amy Worrall]

It’s not a termination issue. (Got a source4 with internal terminator fwiw). This isn’t occasional flickering, this is loads of bad data causing strobe channels etc to fire. 

[pjrails]

The only other thing could be the  DMX Cables you could have pins 2 and 3 swapped in one of them this some times happens if you are mixing 5 / 3 XLR's even in purchsed ones so meter them out if you can. Other than that it might be a case of contacting a Mac supplier. Or you could try Encore ( Hawthorn's that was ) on 01664 821111 and ask one of the service teams.

[Robin D]

I would still try a terminator first. One of my dimmer packs is supposed to internally terminate but doesn't. It was at the end of a line and fitting a terminator in the DMX output solved the flicker from upstream LED PAR's. 

[Amy Worrall]

On 8/22/2022 at 3:31 PM, pjrails said:

The only other thing could be the  DMX Cables you could have pins 2 and 3 swapped in one of them this some times happens if you are mixing 5 / 3 XLR's even in purchsed ones so meter them out if you can. Other than that it might be a case of contacting a Mac supplier. Or you could try Encore ( Hawthorn's that was ) on 01664 821111 and ask one of the service teams.

Doubt it’s that, it was working ok one day then not the next.

[DrV]

Does the 250 work ok using the menus? If so then it's likely the DMX transceiver chip (a 75176 etc) has failed. You might be able to verify this by measuring the resistance between pins 2&3 and between each of them and pin 1, then comparing what you find with a good fixture (all with the power off). You might well find it is much lower than it should be. Can't give you suggested values because it is very dependent on your meter and what voltage it outputs when measuring resistance but the values for two good fixtures will be in the same ballpark.

[Amy Worrall]

I'll need to get it out again to check the menus (have packed away that event now). I'm pretty sure it did work ok, but I only tried changing the DMX address since the issue happened.

DMX transceiver sounds likely. Do you know if it's fairly easy to access the pcb to get to it? I've opened the movers recently to change gobo and lenses, but not dug far enough into them to see a pcb!

Thanks for the troubleshooting guidance, I'll do that when I next have some time 🙂