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Doctor Mains

Breakdowns...

...and how to avoid them. Or at least, how to tell what's gone wrong, quickly.


Fast ways to find faulty fuses. Adrian Legg has the tips.

IF YOUR operation is pretty big deal and you have a roadie with an I.Q., or you are a totally acoustic Morris team, then you probably don't need this piece.

Most of us, though, are likely to suffer from an amp breakdown onstage sooner or later, and will have to sort it ourselves. A speedy diagnosis is essential, and a fix or ditch decision has to he made before the promoter decides to dock the money.

There are two main problem categories — horrible loud scrunches or dead silence.

Scrunches are usually associated with guitar leads, and the first check is to unplug the suspect and see if the amp still does it. Nine times out of ten the cause is a dirty jack plug and/or socket on the amp. You cure this by switching off and squirting the plug and socket with an aerosol contact cleaner. Do the same at the instrument end, and to any fx en route. You get this stuff from Tandy, Maplin, or any other decent electrical parts shop. Ask for one without grease in it, and watch out for the Archer one; it is high pressure but doesn't actually seem to have much stuff in it.

The tenth time is either a break in the lead screen, or a loose socket/dry joint in the guitar and/or something worse in the amp. There are two schools of thought on leads. One is to buy cheap and ditch at the first sign of trouble, the other to buy more expensive ones. I've tried both approaches and now make my own.

I use high quality thick twin core screened mike cable, and parallel one core with the shield. This cuts out screen break problems and means that at a push I can use any guitar lead to run an extension speaker. I replace the most heavily used ones every couple of years or when they start to look a bit kinked, and have had no problems with core to core/screen capacitance losses or shorting.

But, back at the scrunchy amp. If it's still doing it without the lead plugged in, listen carefully and see if the scrunches are coming through on any of the other gear. Go to the plugboard and pull out the plugs one at a time. If the noise stops, take apart the one you have in your hand and check that the contact screws are tight. Check that the board itself has not deformed and is clean. Check that the plug prongs are free from traces of Guinness, Castrol GTX, gaffa glue, or any other combination of the usual gigging grime. Check the contacts in the mains board plug as well.

If the problem still refuses to go away, it now seems likely that it is only going to do so when threatened with big pliers by a qualified electrician, and you must either substitute spare gear, or make your excuses.

There are, of course, scrunches that relate to specific controls on the guitar or amp. Frequently, grime gets into the pots and between the wiper and the track. Periodic attention with an aerosol cleaner and lubricant will keep these at bay until the time when an old pot really must retire. Try to squirt the stuff into the pot through the gap by the terminals. Use it sparingly, and mind your eyes. If the pot is sealed, then it must be replaced.

In the dead silence category, the first check may sound puerile, but is the standby on? Checking that the speaker lead is plugged in comes into the same area.

But what if the little light doesn't even come on? There could be three or four fuses between you and the wall, and you could spend 15 minutes trying to find a dead one even if you had remembered to bring a spare.

Quick tracing comes with neons.

You can get a neon suitable for use on 240 volts from a decent electrical supplier; usually it will be a low voltage light incorporating a 270k resistor in series.

It will have two wires coming out of it. Attach these to the plug along with the mains cable — one with the live and one with the neutral. Tape down the excess wire and the light to the cable casing. When power is on, it will light up, and when the fuse goes, it will go out. If you do this to your amp lead, extension lead, and plugboard lead, you can see at a glance where a duff fuse is.

Except for the one in the amplifier.

You may prefer to have an electrician do the next bit if you are even slightly all wobbly with technical things. Inside the amp chassis you should see the body of the fuse holder, if you have the usual plug-in-and-twist or screw-up type. There are two solder points on the holder, one on the mains side of the fuse, the other on the amp side. Drill a hole in a space in the chassis, being extra careful to clear up swarf afterwards, and mount the neon. Attach one wire to the mains side solder point, and the other to the amp side solder point, being careful to see that all the solder melts properly, and the mains wires maintain a good connection. When the fuse goes, enough current will pass through the neon to light it up, but not enough to power the amp (so it's the reverse of the off-is-broken system above).

Most times a fuse goes it will be the one in the amp, as this is usually the only one anywhere near the correct value. Check it out and get spares. Thirteen amp plugs usually come with 13 amp fuses, which is way too high for most amps. Your amp's power consumption should be marked somewhere on the chassis in watts. To tell what this means in amps, divide it by the supply voltage. So, my old Twin's 360 watts consumption divided by the 240 volts supply equals 1.5 amps... certainly doesn't need any more than a 2 amp fuse in the plug.

Blowing a fuse once may not indicate a serious problem. Amp fuses can go for simple mechanical reasons like getting bashed about or vibrated in transit. I used to carry a small Loco to gigs on a motorbike in a tank bag. It stopped blowing fuses when I put a piece of foam in the bottom of the bag. Valve amps have been known to blow fuses just because of a sudden abnormal spike during operation — or sheer bloody-mindedness if you like.

But blowing several fuses on the trot indicates a need for professional attention. Don't ever replace fuses with higher values, silver paper, bits of wire or guitar strings — your executor and estate could end up with a bigger repair bill, and the gig money might be forgotten in the rush to hospital.

Most promoters will accept sudden death as a reasonable excuse for not being able to complete the gig, but it can be avoided with care. A Martindale ring mains tester that will tell you about any homicidal tendencies in your mains supply can be had from a decent electrical shop for a few quid, and you can even get plugs with built in earth leak mains trips in Argos now; 30 milliamps for 30 milliseconds going to earth and off goes the supply. Sneakier headliners with too clever supports may care to note the blue test button on the top of the unit; it must be unplugged to be reset, and that detail is likely to be overlooked in the panic of a power failure.



Previous Article in this issue

Synth Sense

Next article in this issue

Light Show


Making Music - Copyright: Track Record Publishing Ltd, Nexus Media Ltd.

 

Making Music - Nov 1986

Feature by Adrian Legg

Previous article in this issue:

> Synth Sense

Next article in this issue:

> Light Show


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