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Serviceman's Log

The old adage that a little knowledge is a dangerous thing certainly applies when it comes to mains-powered equipment. This home-made amplifier was so dangerous and the standard of assembly so dodgy that I really shouldn?t have taken it on.

by the Serviceman

Items Covered This Month

• The dodgy, dangerous home-made stereo amplifier

• Intermittent MIDI keyboard

• Water pressure pump controller

• The freezer that really froze

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

Every now and then, something lands on the workbench that causes us to shake our heads in wonder. This has happened to me quite a few times over the years and it never ceases to amaze me what some people do. Sometimes they are simply rank amateurs blundering about with something they don’t understand and shouldn’t touch. At other times, it’s so-called professionals who just do shoddy repair work.

Good workmanship is especially critical with mains-powered equipment. Aside from any performance or usability issues arising from shoddy work, there is the very real danger of killing someone, so it pays to know what you are doing when dealing with such equipment.

One memorable example was a home-made stereo amplifier that an acquaintance asked me to have a look at. He’d been given the system as part payment for some deal or other and he’d used it for a few months until one day the left channel stopped working.

I asked him to bring the whole kit and caboodle in because I wanted to test the speakers and the leads as well. After all, many a serviceman has jumped into an amplifier repair only to find that the cause was literally outside the box. In my case, I learned a long time ago to start with the easy stuff and work my way up from there.

The speakers were surprisingly well-made and it looked like they had been made from a professional kit (either that or the woodworking skills of the builder far outshone his or her electronics abilities). I tested them for continuity before removing the backs to check that the crossovers and driver units all looked and measured as they should.

There were no fuses inside the speakers, so that immediately ruled out one possible source of trouble. The crossovers were decent units and appeared to be professionally assembled. However, I didn’t recognise the speakers and there were no brand names to give me a clue.

That said, they looked like a nice piece of kit. The bass drivers were beefy 150mm units and the tweeters were expensive ribbon models which looked very similar (but not the same as) a couple I purchased from Jaycar a while back. In short, it all looked good so I buttoned everything back up and connected the speakers to my workshop test amplifier using the supplied cables and gave them good thrashing
. . . err, I mean workout.

They performed flawlessly and so, having ruled out the speakers and cables, I shifted my focus to the amplifier. I connected the speakers to it and this indeed confirmed that there was no sound from the left channel. In fact, there wasn’t even a power “pop” when the switch was turned on or off.

Click for larger image

There was, however, a considerable thump from the right channel when power was applied. And just to prove a point, I then swapped the speakers over but the fault remained in the left channel. There was also obviously no switch-on muting or speaker protection, which was hardly surprising given that this was an older home-built amplifier and as I discovered, was pretty much a bare-bones deal.

Everything was mounted inside a standard rack-mount case (the type with handles on each side at the front). I removed the four case screws holding the cover, lifted it clear and was immediately struck by how amateurish (read “extremely dodgy”) the wiring looked. For starters, the power cable passed loosely through a badly-worn rubber grommet and (unbelievably) had a knot tied in it for strain relief!

In fact, I could see metal through the grommet where the cable had rubbed against it and the chassis over time. If the amplifier hadn’t failed at this point in time (due to some other fault), it would almost certainly have made a nice “bang” at some point in the future.

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