Permanent Install Mic Cabling Puzzle

[allanr]

I've just finished some basic fault finding and mic cable tracing on a local church PA system. The work is completed, but whilst tracing several floor mounted mic XLR sockets back to the mixer/amp I found puzzling (at least to me) continuity measurements. I'd appreciate comments from those experienced with installations as to whether this is a normal situation.

 

Details

- Berhinger CT100 Cable Tester used in "Installed Cable" test mode gave ambiguous results with only 5 lit LEDs instead of the normal 6 for a good balanced connection (or 2 or 4 if any open-circuits). This suggest pins 2-3 issue.

- Continuity tester showed pins 1-3 as ok, but 1-3 and 2-3 as very high resistance (weak LED pulsing).

- Multimeter measured pins 1-3 as 3 ohms, and 1-3 and 2-3 in the 20MOhm region.

- Each of the four XLR floor mounted sockets gave similar results, but they do function with SM58 mics. Also, the XLR sockets are wired correctly on the back with the cable disappearing underfloor enroute to the mixer/amp.

 

The only explanation I can think of is that there may be a capacitor in series with pin 2, possibly within the TSR jack at the mixer/amp end. To test this theory, bench testing back in the workshop using a 0.1uF capacitor did replicate the 5 lit LEDs on the CT100, the weak indicator flashes on the continuity tester and the MOhm readings on a multimeter.

 

Is this a plausible explanation and why would such a capacitor be used on an installed system since:

1. It knocks quite a chunk of the low fequency response, e.g. not suited for connecting keyboards, bass etc.

2. It would prevent condensor mics being used.

3. Generally interferes with the "balanced" aspect of the connection.

 

PS Just noticed a comment in another thread which might lend some support to this theory?

 

Link: http://www.blue-room...showtopic=59775

 

Not wishing to hyjack.... but presumably it would be straight forward to make a short 'phantom blocker' lead? I would guess a small capacitor inline with PIN 2 would do the job?

 

You would need a pair of caps in series with pin 2 and pin 3. This is exactly what a desk without an input transformer does. The caps are quite large values though, as Tim as noted; something like 10uF is common.

[Lamplighter]

You can do a further test with the CT100 if you have phantom power available. When switched to tone mode it will indicate phantom on both both pin 2 and 3 if all is well. A capacitor will clearly cause one of the indicators to remain unlit.

Brian

[allanr]

You can do a further test with the CT100 if you have phantom power available. When switched to tone mode it will indicate phantom on both both pin 2 and 3 if all is well. A capacitor will clearly cause one of the indicators to remain unlit.

Brian

That's a good idea. Wish I'd thought of that when I was on-site. I'll be going back in a week or so and will do that check (using a jack to XLR converter to allow phantom to be connected temporarily). Also have a peek inside the TSR jacks to see what's in there!

[allanr]

Well, I finally returned to the church and took apart the 4 balanced 6.3mm jack plugs at the mixer input (each wired to a fixed mic position), and can confirm each jack plug did contain a 0.1uF 63V capacitor in series with pin 2.

Glad I'm not totally losing it!

 

On a side note, the dead TOA 100V line mixer-amp was replaced with a Swissonic SA65 60W 4-channel equivalent from Thomann and it is performing very well for the modest price of £79 + p/p.