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This month we take a look at something
that gets a lot of attention when things go wrong with
an electronic device- the FUSE. The #1 thing people say
at our service counter when leaving an item for repair
is some variation of " It's
probably just the fuse". Well there could be just
a blown fuse, but most of the time the fuse has been a
victim of another problem in the device and has blown because
of it.
Let me digress a bit from the fuse and talk about what
can make a fuse blow as it will help you understand better
when we come back to fuses. Probably just as often or accompanied
by "I think it's just a fuse" is the statement "It
has a short". Right away this tells me the person
has virtually no knowledge of anything electronic. Let's
look at what a short is. By definition a short circuit
is the connection of two parts of an electronic circuit
that are not supposed to directly connect together. This
could be due to the failure of a component in the circuit
or two parts of the circuit touching each other because
something has physically moved. These are certainly possibilities
but only comprise half the failure possibilities. What
people lump in with a short is an "open" circuit.
This is where two parts of the circuit that are connected
by design for whatever reason no longer are. All it takes
is for a wire to come loose or a component lead to come
unsoldered due to vibration or thermal cycling. I can tell
you from my many years of repairing equipment that you
are way more likely to have an open circuit than a short
circuit. An open circuit most of the time renders all or
part of a piece of gear to not work. The short circuit
is more likely to cause some visible smoke or fire damage
especially if it occurs in the power supply or output stage.
So what does a fuse actually do? It is there for one purpose
only - to turn off the unit if it begins to draw more power
than it is rated for. When a manufacturer designs a piece
of electronic gear they calculate how much amperage it
will draw under its full on operating condition. They then
install a fuse that is a slightly higher amperage than
the maximum rated current draw. Therefore if something
happens inside the unit that causes the current draw to
exceed the rating of the fuse, the fuse will blow shutting
off power to the unit. This prevents damage to other parts
of the circuit that are still intact and also can prevent
a fire from starting.
The following is a list of things fuses do not do:
- Create tone in guitar amps
- Are a part of the circuit function of any unit as
a resistor or capacitor would be.
- Go bad over time and need replacement. I had a customer
the other day whose unit blew a fuse and he made a big
point that he had installed a "fresh fuse".
I myself have never seen an expiration date on a pack
of fuses.
Let's look at some points about fuses
to help if you find
a blown fuse in your gear.
FUSE RATINGS
There are two ratings that are on any fuse- voltage and
amperage
.
The voltage rating indicates how much voltage the fuse
can withstand between its two ends before power will arc
from one end to another. The normal ratings you see are
32 volt, 120 volt, 250 volt. It is OK to use a larger value
but not smaller. For instance a 250 volt fuse will work
on a 120 volt circuit but a 32 volt will not. Here if the
fuse has blown, the 120 volts will be able to arc between
the ends of the 32 volt fuse.
The current rating indicates how much current can pass
before the fuse element blows. This is the most important
rating. NEVER INSTALL A FUSE WITH A HIGHER CURRENT RATING
THAN THE ORIGINAL. I can not stress this strongly enough
but I have pulled many a 20 amp fuse out of a fuse holder
requiring a 1 amp fuse. I can remember a guy with a Boogie
Simulclass. It probably had one bad tube, but after installing
a twenty amp fuse he had burned circuit boards and destroyed
transformers. Do not succumb to this temptation to do this.
If you are at a gig and this is your only amp you will
probably only make matters worse. The amp still won't play
and you have potentially upped your repair bill.
People do not understand the function of the fuse as a
protective device only. It is already rated to pass more
than the maximum current that should be flowing through
the unit, so even if by some chance the unit will operate
with a larger fuse it is a BAD idea. Remember most branch
circuits you plug into are rated at 20 amps in the U.S.
so a 20 amp fuse affords no protection at all. You are
just as likely to blow the circuit breaker in the panel
as you are the fuse. And worse than this is what I call
the Wrigley's 100 amp fuse. This is where someone, probably
at a gig, blew a fuse and had no spares, so they wrapped
their blown fuse in the foil that chewing gum comes in.
This is the equivalent of the penny in the fuse box for
those of you old enough to remember back to the days before
circuit breaker panels in your home.
FUSE SIZES
Generally in musical
equipment we see two sizes of fuses either 20mm (M025)
or 1 1/4" (3AG).Both sizes have
exactly the same functionality if their ratings are the
same. The larger fuses were common in American equipment
and the smaller in European in earlier times, but now you
see more of the smaller type used everywhere.
FUSE PACKAGES
Fuses are manufactured in hundreds
of different types of packages, but for musical gear
it is almost always the cylindrical 20mm or 1 1/4".
The package is usually a glass tube with metal ends.
The fusing element spans across the glass tube and is
attached on each end to the metal caps. You will also
occasionally see ceramic material used instead of glass
and sometimes these are sand filled to prevent arcing.
SLOW AND FAST
You have probably heard of fuses being referred to as
fast blow or slow blow. This is an important consideration
that can be confusing. For example if we have a fuse that
is rated at three amps shouldn't it blow if the three amps
are exceeded? How can it blow faster or slower? The answer
is that some units are more dynamic in their power draw
and some more static. A static unit will work fine with
a fast blow fuse as its power draw doesn't vary much. But
some units have a wider swing of current draw. A good example
of this is a tube guitar amp. If you turn it on even with
the standby switch engaged the first thing that happens
is a rush of current into the low resistance cold filaments
(heaters) of the tubes. As the tubes warm up the filament
resistance increases drawing less current. Next throw the
standby switch. Power is now delivered to the discharged
filter caps which present a very low resistance, drawing
much current until they charge up and reduce their current
draw. Or possibly as I said before, the current draw of
the unit in operation is very dynamic briefly drawing more
current than the fuse is rated for. All of these conditions
create a spike of current over a brief period of time.
If a fast blow fuse is used, rated to the continuous current
draw of the unit, it will not be able to handle the spike
and blow. The slow blow fuse is more tolerant of the spikes
and will not blow, but if its current rating is exceeded
in a continuous manner it will blow
WHAT TO DO IN THE EVENT YOUR EQUIPMENT BLOWS A FUSE
The first thing, especially if you are gigging, is to
have prepared in advance and have a selection on hand of
the exact fuses your equipment requires. This prevents
you from going into panic mode and doing something that
will make matters worse. Remember also if there is time
and a Radio-Shack nearby they carry a nice selection of
fuses.
If you experience a blown fuse, pull it out and look at
it. What you see can tell you what happened. If you see
a big splash of black on the inside of the glass it is
almost a sure sign that something has failed inside the
unit and is drawing enough current to explode the fuse
element. It is very unlikely that replacing this fuse with
an identical one will do anything but provide another blown
fuse. You can stop right there because you need to have
a technician service the unit.
If the fuse element has melted this is an indication of
an overload condition and replacing the fuse may bring
the unit back to life. Occasionally a power surge from
the power line can blow the fuse also, so a good rule of
thumb is to replace a blown fuse with a correct one and
see what happens.This should not hurt anything and may
get you going again. If it blows then you know the fuse
did its job and protected against further damage to the
unit.
Also a word about surge protectors. They are not the protection
you think they are. The ones that mount in racks and have
multiple outlets on the back and lamps on the front are
no more than glorified outlet strips. People tend to confuse
these with line voltage regulators and they are far from
it. A line voltage regulator is an expensive device that
will keep the voltage at 120 volts under almost any condition.
This means it will actively pull the voltage up or down.
It can fit in the same space as a surge protector, hence
the confusion. A surge protector works by inserting a device
called a metal oxide varistor (MOV) into the power line.
If the voltage increases above a certain level the device
will absorb the over voltage protecting anything operating
on that line. It has two short comings though. First, it
often does not respond fast enough to absorb the whole
spike and second it has the tendency to wear out over time
from absorbing spikes. These devices in my opinion provide
a false sense of security.
One last thing to do in the event of experiencing a blown
fuse. Look carefully at the fuse and make sure that it
matches the ratings printed on the back of your unit. It's
possible that at some point a fast blow fuse was installed
where a slow blow was required and the blown fuse is because
it couldn't handle the inrush current even though it worked
other times. Make sure you are reading the fuse value appropriate
to your local voltage on multi-voltage equipment (as in
120 volts or 240 volts).
Another thing on modern equipment that many people miss
is the fuse holder itself. We are all used to the round
fuse cap we push in and turn to gain access to the fuse.
But many people are unaware that often the fuse is integrated
into the connector where the power wire plugs in. If you
remove the power wire you can see a cover that can be snapped
out to access the fuse. It is done like this as a safety
measure. Since the power cable must be disconnected in
order to access the cover there is no chance of an electric
shock occurring while the fuse is being removed. Remember
the fuse holder terminals the fuse inserts into are at
line voltage.
Sometimes a unit will not have a fuse but a circuit breaker
instead. For example, QSC amplifiers use them. There are
pros and cons to this. The circuit breaker is resettable
so you can find out right away if the unit will come back
to life or needs servicing. Make sure you wait a bit until
the breaker cools off or it will not reset as it trips
after a certain temperature is reached. It is not possible
to substitute a wrong value fuse so the possibility of
damage being caused from that is eliminated. The con is
the people who try to keep resetting the breaker when it
trips, especially if it is tripping right when the power
switch is turned on. This is providing power to the failed
components and if you keep doing it may damage other components
in the unit similar to putting too high a value of fuse
in. If you are finding that you tried to power on the unit
and the breaker trips, try it a few times and stop. The
unit is not going to magically start working, it needs
to be serviced.
Next month I will cover another extremely misunderstood
topic - the third wire A/C plug ground and the infamous "Death
Cap".
Steve
Weiss is the owner and main technician of Steve Weiss
Electronics Inc. He is experienced in the repair
of analog and digital musical equipment. This includes
everything from Vintage Tube Amps and Pro Audio equipment
to Digital Keyboards There is also a guitar repair
shop staffed by some of the areas top guitar repair
techs. He is authorized for warranty work on most
major brands. Steve Weiss Electronics is located
inside of Sam Ash Music at 5460 West Sample Road
Margate, FL 33073 954-975-3390 Ext 272. Steve has
also spent 25 years on the road as a performing guitarist
and is the designer of Primal Guitar amps that can
be seen at Primal
Audio.com Steve can also be reached at stevew@metromusicmayhem.com |
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