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Serviceman's Log

It's always important to gather as much information as possible from the customer before starting a repair, otherwise it's all too easy to get involved in a wild goose chase. Here's a classic example of what can happen.

by the Serviceman

Items Covered This Month

• Rockit PA/instrument amplifier

• Jansen PA/instrument amplifier

• Faulty voltage/current calibrator

• The ignorant customer and his new 500GB hard drive

*Dave Thompson, runs PC Anytime in Christchurch, NZ.

A musician friend of a friend recently contacted me, asking if I would check out two PA/instrument amplifiers that weren’t working properly. I replied that given the economic climate here in Christchurch, I’d look at anything!

Unfortunately, I wasn’t at the workshop when he dropped the amplifiers off so I didn’t get to ask my usual pointed questions. As any serviceman can tell you, asking the right questions when the gear is brought in can often save a lot of time and grief further down the track. Of course, I did get some of the background over the phone but there were quite a few questions remaining.

Click for larger image

As for the units themselves, one was a Rockit 150W 8-channel mixer/amplifier combo which his band used for the front-of-house vocal mix. The other was a Jansen 440W bass guitar amplifier. In most gigging bands, one person typically ends up doing all the PA-related set-up and in this band, the job falls to the guy who dropped the amplifiers off. He’s also the band’s bass guitarist and apparently is the only one who knows how to put it all together.

His descriptions of the amplifier faults were a little on the vague side though. He’d told me over the phone that there were problems with both amplifiers but didn’t go into great detail. When he dropped them off, he told my work colleague that the Rockit’s monitor system had stopped working and the bass amplifier sometimes squealed loudly when fired up.

In the end, I decided to call him to find out more specifically what was going on with the amplifiers. I also wanted to ask if he had tried anything to try to isolate the problems, such as trying different cables and speakers with the Rockit.

Unfortunately, after going through this process, I wasn’t much the wiser. He wasn’t sure what the monitor problem with the Rockit was other than that the drummer, parked as per usual at the back of the stage, couldn’t hear anything through his foldback wedge. They hadn’t tried any other cables or speakers and he had simply assumed it was something to do with the amplifier.

As for the bass amplifier, all he knew was that sometimes he’d arrive at a gig, plug everything in and when he turned it on, he would get an alarmingly loud squeal through his “quad” (a large bass speaker cabinet sporting four 12 or 15-inch bass drivers). Given this amplifier pumps out a hefty 440W of low-end grunt, this type of fault could pose serious consequences for the amplifier’s output stages, not to mention the speakers.

The first issue I faced was finding suitable speakers with which to test these amplifiers, as my workshop “20-watters” are a touch on the small side. With units like these, a power-on thump can blow low-wattage voice coils and/or leave the cones dangling in the frames. Even if the volume is kept to a minimum, a faulty amplifier can still send a nasty signal to the speakers and I wasn’t about to risk it.

I ended up calling the band’s drummer and he offered to drop off a foldback wedge, which sounded perfect (no pun intended!). If I was going to blow speakers, I may as well blow theirs!

Fortunately for me, the two amplifiers were both made in now-defunct Auckland factories in the mid-1980s and were pretty much standard fare. The Rockit used a transistor output stage (similar to many of the kit-set power amplifiers of the day) while the Jansen used a bank of Mosfets. Both used analog preamps stuffed with op amps like the TL071 and RC4558 and all other passive and discrete components were clearly marked.

It certainly makes it easier when parts can be easily identified, especially when circuit diagrams are hard to find.

I fired the Rockit up first. To test amplifiers, I use my trusty signal injector which I made from a SILICON CHIP project many years ago. This was housed in a cylindrical metal vitamin container and I mounted a push-to-make switch on the plastic cover. A sharpened probe, salvaged from an old multimeter lead, was mounted through a hole drilled in the opposite end, while the earth lead was brought out through a grommeted hole in the side of the tube and terminated with an insulated crocodile clip.

The result is a very useful piece of test gear that can be operated using just one hand.

Anyway, I connected a 6.3mm mono jack plug (the music world’s universal instrument input connector) with a flying lead to one of the Rockit’s eight inputs, clipped the crocodile clip of the signal injector to a ground point on the chassis and set the channel and master volumes just above the stops. I then touched the injector’s probe onto the input lead connected to the jack plug and pressed the button.

When I did this, a nice clean tone came from the speaker so the amplifier chain was working OK and the volume wasn’t high enough to frighten the daylights out of me. All I had to do now was to figure out why the monitor side of things wasn’t working.

Now whenever I get old units like this coming in for repair, I always suspect that a solder joint has degraded and formed a dry joint or perhaps a connecting wire might have come adrift. These types of amplifiers are usually very well built for the road but unlike their domestic sit-in-the-lounge cousins, they tend to suffer some terrible abuse.

Indeed, I know from my own days of touring with five other musos, travelling endlessly up and down the country in a van stuffed with audio hardware, that the gear often gets a very hard time. Unloading and reloading audio gear before and after a gig is not exactly an enjoyable exercise and the amplifiers and speakers tend to get the odd bit of “road rash” now and then. This can play havoc with physical connections and soldered joints.

Anyway, having established that the amplifier worked, I unplugged it from the mains, removed the machine screws holding the front and rear panels and slid the whole kit-n-caboodle out of the road case (I love this method of construction – everything is so easy to access). I then set the two halves up on the bench and had a good look over them, paying particular attention to interconnecting cables and solder joints.

Now although I love a good electronics detective mystery, the serviceman inside me is always looking for a quick-and-easy fix rather than a drawn-out and ultimately uneconomical repair. There was no such luck in this case – the interior of this amplifier looked pristine, without so much as a spider’s web evident. In fact, the components all looked as-new, despite pushing 30 years old and all the visible solder joints appeared shiny and electrically sound.

Oh well; you win some and you lose some.

As mentioned above, the rear section of these amplifiers usually carries all the output sockets and this one was no different. In this case, there are two main speaker outputs, as well as an echo/effects send/return loop, a monitor output for a foldback system and a “slave” output for daisy-chaining this amplifier to another power amplifier.

All these outputs are line-level outputs. In addition, the echo/effects loop and monitor outputs have separate controls on each channel to determine how much signal from the preamp stages is fed to the relevant system.

I plugged a pair of headphones into the various output sockets and all appeared to be working as they should. However, when I listened to the low-level monitor feed, alarm bells began to ring (figuratively speaking that is).

The foldback speaker I was using was one of their usual stage units. However, there was no way this line-level monitor output was going to drive this speaker at any volume without something else in the system to boost the signal. I called the bass player again and asked him if they had another monitor amplifier somewhere. He said they didn’t but I should ask the drummer because he usually plugged in his own monitor speaker (the one I had at the workshop) and the bass player wasn’t 100% certain how he plugged it into the PA system.

Now at last we were starting to get somewhere. The bane of a serviceman’s existence is misinformation and it seems that I had been getting the wrong end of the stick. A call to the drummer confirmed that he didn’t plug his monitor into the monitor output of the PA amplifier but into a bridged speaker socket on one of the front-of-house speaker bins instead!

In practice, this made both more and less sense. When wired up this way, his foldback speaker was simply an extension or slave speaker for one of the main FOH (front-of-house) speakers. This meant that he was simply hearing whatever mix was sent to FOH speakers. However, this also explained why he couldn’t alter the volume of his monitor and why he got a lot of feedback from that wedge, requiring him to position it very carefully.

It turned out that just like the band I was in, the left hand doesn’t always know what the right hand is doing and in hindsight I should have spoken to everyone concerned as to the exact system they used. This new information meant that there was nothing wrong with the Rockit amplifier at all. Instead, the problem was most likely a dud speaker lead or a broken socket in the main speaker and I decided to check that out later.

After talking with the drummer, I offered to build him an active (powered and amplified) foldback speaker. This would take the monitor output from the Rockit as intended and would enable him to control his own volume and audio mix. He replied that this was exactly what he’d been wanting for the last 10 years but didn’t know how to achieve it.

So at least something good came of the whole Rockit affair. At the same time, it also had a decent check-up and they now know it will keep pumping out the sounds for a while yet.

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