Diaries of a Guitar Tech: Sleuth Against The Clock

When the techy saves the day!   24-Aug-13

SonicState's resident guitar tech blogger Nev Ward isn't just a guitar tech, he's an 'everything' tech, and he recently saved the day for a major music venue and a major band by sleuthing his way to solving a problem which was a major pain in the arse...

A popular music venue in Brighton had a distortion problem. It appeared at medium volume and was not gross, but severe enough to be unpleasant. It was coming from the mid - top cabinets but it wasn't immediately obvious from the sound whether this was an amp problem or a speaker problem, or even a mixer problem, but it had to be addressed immediately as there was a big gig in two days time and, depending on the cause, it could well get worse.

The mid – tops were flown above and in front of the stage, three each side, about five metres above the floor, so it wasn't possible to tell if the distortion was coming from one cabinet or all three. The strange nature of the distortion meant that it wasn't easy to tell if it was coming from the left of right set of speakers.

Speaker repairs involved hiring a crane to get them down and shipping them to France for replacement drivers to be hand matched into the cabinets, then shipping them back and more crane hire to get them back up. Obviously, this couldn't happen in the time frame, so if it was the speakers, a whole new system would need to be hired in. Needless to say, the manager and sound engineer were rather anxious.

After swapping some cables, the mixer was ruled out and after more swapping of cables, complicated by the way the active mid - top split was shared across the amp rack, the speakers were ruled out, to visible sighs of relief all round. We finally got it down to one side of one amp. I took it out of the rack and back to my workshop.

On the bench, the amp initially performed perfectly. Connecting a dummy load brought a tiny amount of distortion, but I had to lash up a simulation of a speaker to provoke it into revealing its true nastiness. It looked like a stability problem, with the amplifier breaking into brief bursts of high ultrasonic oscillation every time the output voltage swung past a certain value.

Although the amplifier was modern and sophisticated, containing hundreds of components, this sort of problem generally comes down to one of a handful of components located at key points in the circuit.

As I was in touch with the company who specified and installed the system I had access to the full technical information on the amplifier, and was able to locate all these components with ease. Unfortunately, although the amplifier was easy to dismantle, it was impossible to operate while in pieces, making impossible the probing of key points while running. This meant removing and testing the relevant components one by one, so I looked to my long experience to narrow down the field and pick the most likely first.