Intermittents and Typical Repairs

A while back, a friend “gave” me a Mackie 12” TH-12A powered stage monitor when he was clearing out his shop of things he either didn’t want or didn’t want to spend time trying to fix. I have no reason to want a powered stage monitor, but for whatever reason I decided I wanted to take a crack at seeing if I could fix the monitor. I like Mackie gear and when it breaks unreasonably easily, I get curious. The symptom was “no power indicator or output.” Supposedly, someone renting the speaker had dropped it and it didn’t power up after that incident. My friend had already made a quick attempt at troubleshooting the speaker and confirmed the reported symptom. 

Because of the previous repair, the amp panel had already been removed from the speaker box and the wiring disconnected. My first suspicion is always “connectors and solder connections.” To remove the first likely candidate, I always check the connectors, physically, and clean the connections. A smarter person would re-connect the amp to the speakers and confirm the initial diagnosis, but I wasn’t doing this for money and I didn’t really care if the solution turned out to be overly simple. I was back in my teenage mode of just wanting to see how the product was assembled and looking at the thing out of curiosity, first, and problem-solving, second. There are advantages to being less-than-meticulous about documenting an organized repair. The first of those advantages is that I spend more effort exploring, examining, and learning than I do ticking off boxes and accounting for my time. 

A disadvantage to this process is that it is entirely possible to trip an intermittent fault back into operation and I wouldn’t know that I’d even passed over the intermittent. In my experience, at least 50% of the electronic problems I’ve been asked to troubleshoot are intermittent, which is the nature of connectors and solder connection failures. They rarely outright fail, unless there is enough current in the area of the fault to cause complete failure by burning, arcing, or in-rush current causing damage to other related components. 

What I found during my inspection was that several of the Molex connectors (see red circled connectors in the picture at right) had received enough tension from their attached wires to pull the male base up along the length of the pins; possibly removing contact from the female socket. That isn’t a problem you’d expect to see in a typical application, but Mackie had cut the speaker wires short enough that there was no strain relief possible and the weight of the wire put some strain on the connectors. One of the first things I noticed in inspecting the power amp board was that the woofer male base had slid more than half-way up the pins. Looking around the boards, I found at least two more connectors with a similar appearance. 

With that background, I disconnected, one-at-a-time, all of the Molex connectors, pressed the male header firmly against the PCB, cleaned the connectors with Caig DeoxIT, reassembled the male and female connectors, and glued the connectors to each other and the PCB with a latex adhesive. After cleaning and reattaching all of the Mackie’s connectors, I power up the speaker and found that it worked. I let the adhesive cure for a few hours and gave the amp some signal while tapping all over the amp and power supply assemble looking for intermittent operation. Not finding any evidence of intermittent operation, I “burned it in” for several hours at a variety of volume levels. With some confidence that I’d found the problem, I gave the speaker back to its owner. 

This kind of repair reminds me of the many things that were always frustrating as a repair tech. First, troubleshooting intermittent failures is miserable work. When you are "finished," at best you feel you've done everything you could to be certain you've fixed the problem(s), but you are never even close to 100% confident. Intermittent faults have a Murphy's Law mind of their own. I used to joke that about half of the problems I'd been called in to repair "fixed themselves the moment they saw a toolbox." Of course, half of that half were user error issues that often cleared themselves up before or immediately after I arrived. That kind of repair is really difficult to convince customers that my time still needed to be paid for. 

Early on in the last version of Wirebender Audio Services, I quit taking telephone calls for service work because a few customers would pretend they didn't know who called me, when the problem "fixed itself." A few of those customers learned that I never forget being stiffed and whined like spoiled children when they learned that under no circumstances would I ever do work for them again. In the case of the Mackie speaker repair, I have a couple other ideas to chase down if the intermittent operation comes back to haunt me, but since I didn't have an associated bill with the original repair I can be fairly relaxed about the whole affair.