I've had two pieces of gear come in this past week, both failing on gigs for working musicians (which is just the worst). One turned out to be an easy fix...the other one is being a bit elusive.
The first contestant this week was a Carvin B2000 bass amp. Paul, the owner, described the problem as "it would get real quiet, then loud". Immediately, that sounded like a connection problem somewhere in the front end of the amp. We got it on the bench, connected up a speaker and a signal source, and fired it up. Sure enough, it wasn't anywhere near the volume it should be, and what was there had a lot of noise and static.
The problem was in a ribbon cable. "What's that?" you ask. Once only found in computers and other high-tech gear, ribbon cables are used by pretty much every electronics manufacturer out there now. They're an inexpensive way to shuttle a lot of signals around, and it makes assembling a product faster.
There are two ways to implement them...solder them in (preferred), and using push-on connectors. Carvin (and most amp manufacturers) use the connectorized version like so:
Those black cables running between boards....that's them. Here's a close up of a ribbon cable with connectors:
If you're dealing with what's known as "logic level signals" (which are anywhere from 3.3 to 5 volts) these things will work for years without a problem. However, it you're dealing with low level signals (usually a half volt or less), then things sometimes start going haywire.
These connectors can oxidize over time. This gets into a little bit of heady science, but if there's enough current flowing through the connection there's an electrolysis effect that takes place. No, it doesn't remove hair but it does "clean" the contact. Low level signals...not so much. That's where you have to intervene with a good contact cleaner. My favorite is still DeOxit that I mentioned previously. Just remove the connector, spray the pins lightly, and then you want to plug/unplug/plug several times to distribute the cleaner and work the corrosion off.
We did just this with the bass amp, and it's rockin' again. Oh...how did we find it? That's where knowing where you can touch and where you shouldn't in a live amp comes in handy. And actually, I used a pencil and tapped on all the ribbon connections while the amp was output a signal to the amp. When we found it, it was very obvious (it got really loud). Unless you know what you're looking at and understand the risks, I can't condone someone that doesn't have some electronics training doing this in the field.
The next piece of gear that came in was a mid 2000's Crown power amp for a PA. The client had it on a road trip (in a rack, as it should be) the previous weekend. It worked Friday night, but after driving the next day to the Saturday gig it did not make any sound.
Now, let me introduce you to the 7th corollary of Murphy's Law, which states:
"When a piece of equipment is in a position to be diagnosed, it will perform flawlessly".
And that's exact what happened...it passed signal on both channels. I couldn't blame ribbon cable connectors this time. They're used in a couple of locations, but they're soldered in and secured with adhesive.
So...the amp works on the bench but not at the gig. What then? Well...it could be speakers, cables, or drive gear but the guy who brought the amp in has been a road dog as long as I have. When he says he checked those things, I believe him.
One thing that we can deduce by the behaviour is that this is likely a physical/mechanical problem, and not a failing component. Very few electronic components "heal" themselves (but there are actually a couple...nothing in this amp though).
So how do we find the problem? Electronic techs have long had two low-tech tools that we use to sometimes solve some high-tech problem...cold and heat. By subjecting circuit boards and components to rapid temperature swings, we can expose marginal connections. You know, the ones that make connection at room temperature, but when the equipment warms up (and things start expanding) the connection goes away.
OK, heat is easy...we use a hair dryer or heat gun. But cold...how do you do that? We'll there's an app for that...or should I say a can.
So...we're going to leave it at this point. I'll be working on this today, and will have the diagnosis next week. While I have the amp apart, I'm going to give it a good cleaning too. Because that's the way that Gus Rummell taught me in my Advanced Troubleshooting class at Central Texas College oh so many years ago.
Until then, keep the meters out of the red.
Ken
Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com
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