AV Police Squad

Fixing Someone Else’s Work: AV Witness Marks

Have you ever thought about how to fix a 200 year old clock? I’m talking about one with gears crafted from wood and pulleys that use ropes instead of chains.

There is no instruction manual to be found. The last person who serviced the clock most likely passed away before the Great Depression. Not only do you have to find the issue, but you have to reverse engineer the whole system before you can start looking for the problem. You have to use witness marks left in the inner workings over the clock’s lifetime to understand what once was.

The engineer in me finds this fascinating, almost romantic, in a weird way. The technician in me wants to reach for the nearest barf bag. That’s what a service specialist deals with day in and day out, isn’t it?

That got me thinking about the witness marks we use in the AV industry. Here are a few of my favorites.

Code Additions

Have you ever had to dig through someone else’s control system code or DSP configuration file, and they didn’t get the memo about keeping things neat, organized, documented and elegant? Signals and modules are strewn all over the place. It’s difficult to follow signal flow, much less troubleshoot anything. Even better is when you are the third (or fourth) person in there making changes.

When the code is especially bad, inevitably there will be a folder or space on the palette all the way at the bottom labelled “System Updates,” or something like that. That tells me that they got fed up with the mess as well, and couldn’t even attempt to neatly insert system changes into the existing code. Instead, they linked only the required signals to their completely separate band-aid and tacked it on at the end. This witness mark may appear lazy, but whoever left it probably did you a favor. More than likely, the problematic modules or signals can now be easily found and/or referenced in that P.S.

Tie Warps

Radio Shack used to sell white tie wraps, but professionally installed systems typically use black. So, when looking for system additions that were added in haste and most likely causing an issue…just look for the white tie wraps that some unprepared tech had to run down to “The Shack” during lunch so they could finish up the service call.

Investigating the different styles of tie wraps can tell you a lot about the history of the system. Any upgrades made to the system will most likely include different styles than what was originally used. You can quickly and easily tell original installation cabling from any later additions.


When the main installation team is onsite, they bring their solder station vices to hold connectors, and they have plenty of Teflon tubing (“spaghetti”) and heat shrink to properly protect all the system terminations.

As techs come back to troubleshoot and repair the system, they are less likely to have those tools and materials. Phoenix connectors clearly show system changes by where there is beautiful heat shrink on the original terminations, and just stripped cables on the updates.

Also, any connectors soldered after the initial installation most likely will not be as tidy as the originals. I’ve been guilty of holding an XLR connector in pliers with my mouth, to free up both hands to apply solder and heat. It’s not a pretty sight, but it did work.

The same can be said for those solder connections. (“It’s not the prettiest 3.5mm connector, but it passes audio.”) If most connectors are from one manufacturer, and a few are different, I’d check those first for a cold or loose solder joint.

The Cable

I was recently at a service call where the users were complaining of poor audio in the room. The system used cables from reputable manufacturers. However, there was a yellow mini-high-resolution (MHR) video cable connected to the output of a codec via a BNC-to-RCA adapter. The other end was stripped and twisted into a phoenix connector. That was the offender.

Obviously some tech had to upgrade the codec, ran out of audio cable, “The Shack” must have been closed, but they had plenty of MHR lying around. Replacing that video cable cleared up the hiss.

Changes in cables aren’t always so obvious, but color or manufacturer differences could give you plenty of clues, especially when trying to find where system peripherals land outside the equipment rack closet: “Input 2 on the switcher is a fuchsia coax. Find me the camera with the purple cable running to it.”

The systems we are tasked with bringing back to life most likely aren’t 200 years old, but they can easily be found in buildings that may be older than that. Lack of documentation is a huge and common problem, so we are left using whatever clues we have at our disposal to figure this out. Witness marks left by other AV technicians might not be as fascinating as uncovering a friction mark on a loose arbor in a clock from the 1800s. Knowing to look for them, however, can certainly help us do our job more effectively.

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