Multicore Phantom Power issues

[Charlieg]

Hi All

 

Got a show in at the moment where every channel of our 32ch multicore is being used. The first 16 are being used for rf recievers and the remaining 16 used for band channels (mix of mics and active DIs).

Having a strange issue where a previously healthy channel (one of the band channels) is now refusing to carry phantom power to an active DI. The channel will pass audio with a dynamic mic but anything that requires phantom is dead.

I've ruled out it being a faulty DI, XLR cable or desk output and so my next conclusion is a faulty channel on the multicore.

This multi is a fixed venue install with a static rack patch at stage end and a set of tails at the other which by and large lives completely static, connected to the house desk. up until now it has been rock solid. I've put a cable tester across the channel in question, looped through several known good channels, and all three cores are showing full continuity. Still I cant get phantom power down this line.

Until the show is out next week more invasive testing wont be possible.

 

Anyone ever run in to this gremlin before?

Any suggestions appreciated!

[p.k.roberts]

If all the two cores and screen are showing continuity but phantom will not pass, then check for shorts between the screen and one of the cores. That would give the symptoms you describe.

[Charlieg]

Thanks for the response.

I should have added, no shorts on either cores or screen. 1-1, 2-2, 3-3 with no faults, but still no phantom!

[DrV]

First you should check that you can actually see the phantom voltage between each core and the screen with nothing plugged in. Then, I assume that you've tried several different mics or DI boxes on the end. If all that's ok it must be something like a strained contact in an XLR somewhere. You might well get contact with a meter probe when checking continuity but an actual XLR plugged in might not make contact. To prove this you could get an XLR extension lead (male - female) or two, plug that/those into the ends of the offending cable and buzz out the cable using those. In other words check the cable without using meter probes on the offending cable itself, just on the ends of the extensions.

[sunray]

4 hours ago, DrV said:

First you should check that you can actually see the phantom voltage between each core and the screen with nothing plugged in. Then, I assume that you've tried several different mics or DI boxes on the end. If all that's ok it must be something like a strained contact in an XLR somewhere. You might well get contact with a meter probe when checking continuity but an actual XLR plugged in might not make contact. To prove this you could get an XLR extension lead (male - female) or two, plug that/those into the ends of the offending cable and buzz out the cable using those. In other words check the cable without using meter probes on the offending cable itself, just on the ends of the extensions.

Yes good call, BNC etc and DIN it's quite common. I've never known of it with XLR but no reason it shouldn't happen.

[DrV]

1 hour ago, sunray said:

Yes good call, BNC etc and DIN it's quite common. I've never known of it with XLR but no reason it shouldn't happen.

I've seen it once with a scroller connector (4pin XLR) and once with a really cheap and nasty audio lead (3pin XLR).

[sunray]

2 hours ago, DrV said:

I've seen it once with a scroller connector (4pin XLR) and once with a really cheap and nasty audio lead (3pin XLR).

Well now you mention cheap and nasty...

I purchased some DMX RGB LED units similar to this which came with XLR3 M&F connectors. I think I dumped them after attempting to solder a lead and the pin moved.

However I'd assume (yes I know) that an in house multicore would be made with proper connecters and not Chinese 20p specials.

Now I think about it I hang my head in shame, the tie lines between DJ booth and show mixer in a nightclub I had to replace all of the female ITT XLRs due to multiple failures, I seem to remember most were fine when I tested with my cable tester and my XLR leads which weren't old and worn. The thing I recall about them being the Neutrik replacements being a little twisted to match those that had already been changed.

[Mixermend]

A couple of quick checks to carry out:

1 - swop over a pair of multicore channels with one known to carry Phantom OK from the mixing desk. Does the fault stay on the suspect channel?

2 - if not, double check that Phantom is definitely coming from the suspect channel on the mixing desk?

As you have found that the channel works with a dynamic microphone - this usually means that the screen is disconnected at some point, or no Phantom is coming from the desk on that channel.

Please report back - as we are all keen to find out what the problem actually turns out to be?!!

[Charlieg]

Hi everyone and thanks for the responses so far.

 

So, firstly, I have tested that the desk is definitely outputting phantom from that channel by plugging a di with a short xlr straight in to the desk.

Also, I've rung out that line on the multi by connecting it to another known good channel at one end with a male to male adaptor and then hooking up a cable tested to the other end. This shows solid connection on all contacts. All cables and multicore have neutrik connectors.

 

My next test will be to run that mixer input down a different multi channel. Will give it a go and report back!

[sandall]

On 11/17/2023 at 9:32 PM, DrV said:

I've seen it ....... once with a really cheap and nasty audio lead (3pin XLR).

The plastic inners in some cheap XLRs have a remarkably low melting-point (but then tend to set rock-hard when cool)🙁

[Charlieg]

Update for anyone interested.

I tried swapping over the lines patched in to the desk, ran desk channel 19 down multicore channel 18 and vice versa.

Phantom was present on multi channel 18 but not 19, suggesting to me that the problem definitely lies along the cable run and not with the desk.

I reckon the issues must be with a connector either desk, or stage end. 

It will be a while now until I have the chance to swap any connectors or take the panel apart, so for now it looks like I'll just have to avoid using that channel for a source requiring phantom. Not ideal, but manageable with our current schedule.

 

Unless anyone else has a suggestion that I can try in the meantime? 

[mackerr]

If it's passing full range audio, but not phantom power the problem is most likely a broken wire on pin 1. Just open the connector and re-solder the shield to pin 1. Phantom power is applied to pins 2&3 equally. but it is in relation to ground on pin 1. Without pin 1, no phantom, without pins 2 or 3 usually audio would be low and thin, or non-existent.

Mac

Edited November 23, 2023 by mackerr

[sunray]

5 hours ago, Charlieg said:

Update for anyone interested.

I tried swapping over the lines patched in to the desk, ran desk channel 19 down multicore channel 18 and vice versa.

Phantom was present on multi channel 18 but not 19, suggesting to me that the problem definitely lies along the cable run and not with the desk.

I reckon the issues must be with a connector either desk, or stage end. 

It will be a while now until I have the chance to swap any connectors or take the panel apart, so for now it looks like I'll just have to avoid using that channel for a source requiring phantom. Not ideal, but manageable with our current schedule.

 

Unless anyone else has a suggestion that I can try in the meantime? 

I gather you have tested end to end for continuity and short circuits, did you attempt to add a patchlead each end and test end to end of those?

[Charlieg]

To clarify, I get continuity on both cores and shield with a cable tester and with probes. This is by bridging an xlr over two of the inputs on stage end and then having the cable tester up on the desk end of the multi.

I've opened up the connector on the desk end and it's perfect visually. Haven't had the chance to open up the patch panel yet so I cant rule it out.

[richardash1981]

5 hours ago, Charlieg said:

To clarify, I get continuity on both cores and shield with a cable tester and with probes. This is by bridging an xlr over two of the inputs on stage end and then having the cable tester up on the desk end of the multi.

I've opened up the connector on the desk end and it's perfect visually. Haven't had the chance to open up the patch panel yet so I cant rule it out.

What cable tester are you using? Unfortunately, I've met more than a few over the years which had blind spots - certain faults they couldn't see! I suspect you are looking for a short to ground, and maybe your tester can't see that.
Another thought - do you have a short to mains earth from one of the lines, but with the multi unplugged from the desk there is no path to mains earth from the tester? Hard to see how it could happen, unless a core is pinched in the patch box somewhere.