DMX problem

[sunray]

17 hours ago, sunray said:

As a follow up to this I've done a little bit of playing. I hadn't intended to repair the broken lead but the thread pointed to different things, I soldered the broken screen ie I didn't reterminate the whole end, just removed the airgap. putting it back into the original location it performed correctly. To verify, while the system was running through a simple colour changing scene, I applied the soldering iron the the repair and straight away the flickering started and equally resoldering corrected it. At that I moved the lead (open circuit) to all of the other parts of the small system, (meticulously retaining the position of all other cables) it was the only position it caused problems. I also tried another cable (Canford DFT) about 200mm shorter and the flickering started as soon as I cut the screen.

I checked earth to metalwork is sound, also QTX dimmer pack(fitting 2) has earth continuity to pin 1 of XLR, the other dimmer packs (different universe) don't have this so I swapped 2 over with no apparent effect.

 

As much as I'd like to have a different answer I think this has concluded the problem was the broken screen.

Thinking about this some more it only mattered where it is in the system and it's a very small system. Earlier on yesterday I derigged those fittings and my playing was done on the floor but the fault was totally reproducable. Returning after dinner I had another play.

Original system, fauly cable & flickering light:

With yellow in any other position - no problem.

Swapping fittings the flickering stayed in that physical position including if a dimmer pack (any of the 3 present, one earthed pin 1).

Swapping black and green had no effect but replacing the green with a 6m DMX cable the flickering stopped, various combinations of different cables totalling 10m in place of black and green (and a essentially a fitting had to be included somewhere in that) created the flickering... I concluded it is most likely to be a reflection type problem but the whole systen has to be a critical length within ~300mm (presumably further experimenting will find other critical lengths too).

Adding more fittings leaves theflickering at the same point as long as the black/green region is ~10m and the red region is ~2m.

I have not been able to find another position where a fitting flickers but I'm sure there will be somewhere.

The presence of a terminator may possibly slow the flickering but I'm not sure.

It has been fun playing with a 'strange' DMX fault, until now any faults have been found, fixed, move on, just as this would have been if I hadn't started thinking about it.

Edited August 4, 2022 by sunray

[richardash1981]

I'm interested that the last post has (on page 3 of the thread) introduced another wildcard, in the form of the Starquad lead (green). My understanding is that because the two pairs of conductors are much better coupled than a twisted pair, the characteristic impedance is much lower. This is certainly borne out by a data sheet for Belden 8424 online which gives the characteristic impedance as 52 ohms - about half of the 100 ish ohms of most twisted pair cable.

This miss-match has no effect at DC, but at high frequencies it is putting a big reflection generator at each end of the starquad length, with about 50% of the incoming signal being reflected back ...

[sunray]

2 hours ago, richardash1981 said:

I'm interested that the last post has (on page 3 of the thread) introduced another wildcard, in the form of the Starquad lead (green). My understanding is that because the two pairs of conductors are much better coupled than a twisted pair, the characteristic impedance is much lower. This is certainly borne out by a data sheet for Belden 8424 online which gives the characteristic impedance as 52 ohms - about half of the 100 ish ohms of most twisted pair cable.

This miss-match has no effect at DC, but at high frequencies it is putting a big reflection generator at each end of the starquad length, with about 50% of the incoming signal being reflected back ...

Funny thing is as soon as I realised it's starquad I had the same thought but replacing with similar length mic cable seems to make no difference.

Do remember though this is a stupidly small system with only 14m of cable.

The really crazy thing, and I really struggle with the fact this is so reproducable, is this:

using a different desk and LED fittings onthe table at home also gives the flickering when the DFT has the screen cut at the XLRm even with no fittings or terminator after the flickering fitting, the first LED can be in any of the first 3 joins but it has to be there. With DFT screen connected it's fine.

This selection of cables is in a bag awaiting the time I can put a scope on it.

[sandall]

Maybe it's time to bin the offending bit of cable & accept that the gremlins will always win?

[sunray]

55 minutes ago, sandall said:

Maybe it's time to bin the offending bit of cable

Which is exactly what I was going to do, except...

55 minutes ago, sandall said:

accept that the gremlins will always win?

That is the case as I've reproduced the fault with another cable.

In fact I was unable to reproduce the fault at home initially with the yellow at the end of the line with the different selection of cables due to not finding the sweet spot. The DFT lead is slightly shorter than the yellow. Effectively  I've been moving the position of the 'faulty' fitting around to find the sweet spot with varying lengths of cables.

All dimensions are approximate arms length measurements.