I have been servicing electronic musical equipment for
more years than I want to admit and it never ceases to
amaze me the misconceptions people have when it comes to
troubleshooting their gear. I will present here some of
the common problems I see come across my counter and what
they actually mean.
Fuses
| can't tell you how many times, I hear this said. "I think
its just the fuse."
Well it could be only the fuse, but first let's define
what a fuse does. It is a protective device
that is in the amplifier solely for one purpose - protection.
In the event something in the amp
fails and causes it to draw more power than it's maximum
power rating the fuse will open and
keep other components that haven't failed yet from being
damaged or in the worst case scenario,
it prevents electric shock or fire. It is possible that
the fuse has opened due to an overload or power surge and
if it is replaced all will be well. Therefore the first
thing you should do is try a replacement fuse of the same
ratings. If it works fine, you're done. If it blows again
there are other problems inside the amp that need attention
and it is time to have a qualified tech look at it. I must
also add, never under any circumstances replace the fuse
with a larger rated one or the infamous Wrigley's gum wrapper
100 amp fuse. This is just asking for trouble. Additionally
if your blown fuse is the type that is made from clear
glass and the inside of it has black spattered all over
the glass, you have a short somewhere in the amp and I
guarantee you replacing the fuse is a waste of your time,
it's just going to blow again.
On a tube amp you may see more than one fuse on the back
panel. One of the fuses will be labeled main and one may
say something like H.T. This second fuse is directly in
line with the power going to the output tubes and is there
to protect the output stage circuitry including your expensive
output transformer. Output tubes can develop intermittent
shorts and blow that fuse after which the tubes will still
work but I recommend when this happens that the output
tubes be replaced. It is probably going to keep happening
and most likely even though you are hearing sound, the
tubes are no longer in spec and do not sound as well as
they should.
Transformers
Ahh, the mystique
of the transformer. It amazes me how often blame
is assessed to a transformer. I even see techs
who are not really knowledgeable (or Unscrupulous)
blame the transformer (a nice high profit repair).
I have been in servicing a long time and I can
clearly state that transformers are the least likely
components to fail in an amp. A power transformer
has three normal failure modes. A short in its
primary will cause the mains fuse to blow and
an open primary coil will cause the amp to be completely
dead leaving the main fuse intact. If your amp
is powering on and not blowing the fuse it is highly
unlikely your power transformer is at fault. A
short in the power transformer secondary may blow
the fuse or possibly start to smoke and smell. Of
course if what the transformer is connected to
is shorted it will cause the same symptoms. The
transformer must be disconnected from the circuit
it is supplying and run unloaded to actually determine
if it is bad.
The output transformer also has several failure modes. In the event of an output
tube short it can receive damage to its primary. Collateral damage here usually
produce smoke and fire so you will probably be aware something is wrong. Damage
to the secondary side of the transformer can come from overload in designs
where the transformer is not up to the task or from playing the amp loud with
either no speaker connected or an intermittent cable or jack that is quickly
working and then not working. The unloaded transformer causes spikes back into
the output stage potentially damaging many other components. A blown output
transformer may have no output or often times very little output, but it is
hard for the non technician to discern this from other failures that will yield
similar results. As I said before, I find transformers to be last on my list
of potentially failing components.
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Jacks
Some amplifiers contain as many as ten or fifteen jacks
and they are all subject to problems that will cause you
headaches. Modern amps containing printed circuit boards
that are created with automated assembly use a style of
jack that solders to the PC board. The most common failure
here is that the cable gets pulled on and breaks the jacks
solder connection to the PC board causing a no sound or
intermittent condition. The remedy for this is to re solder
the jack to the board or replace it, If the jack has sustained
physical damage. Unfortunately most of these circuit boards
are held in place by all the front panel controls and many
screws, so the few seconds of soldering needed to be done
incurs quite a bit of labor for disassembly and assembly.
A jack consists of several sections terminating in- well-
terminals. One terminal handles the grounding of the
cable plugged into it and one handles the signal coming
in but there are several other terminals that may apply
also. In the case of the input jack there is a terminal connected
to ground that will short itself to the signal terminal
of the jack when no cable is plugged into it. Its function
is to ground the input circuit and prevent noise occurring
when nothing is plugged in. If you would like to see what
would happen if that was not there unplug the cable from
the guitar and just hold it in the air. The input circuit
becomes an antenna picking up all the noise around it and
amplifying it. When your guitar is plugged in and the volume
turned to zero it grounds the signal terminal through the
guitar volume pot creating the same effect the terminal
shunt does with no cable plugged in. If the input jack
signal terminal is bent or corroded and doesn't make good
contact with the terminal that connects it to ground when
nothing is plugged in, the noise condition we discussed
above will occur.
Another common problem with jacks is when they are used
as inserts or patch points in a circuit. On a guitar
amp you will commonly see an effect loop and maybe also
a power amp in jack. These jacks have a terminal that
receives a signal from a preceding stage and passes it
on to the next stage when nothing is plugged into the
jack by touching the terminal that is normally the signal
terminal. The signal then goes from the signal terminal
to the next stage. An example of this is the output of
the pre amp going to the input of the power amp. If a
cable is inserted into the return jack the signal path
from the pre amp is interrupted and whatever signal is
coming down the cable that was plugged in now flows to
the power amp. The problem we are looking at occurs when
these jacks are NOT used. Over time the contacts of the
terminal corrode right where they touch it each other
and it interrupts or distorts the transfer of signal.
This problem can be notoriously intermittent and subject
to humidity, temperature and vibration. Occasionally
a high level signal can blast its way through the corrosion
causing it to work temporarily until the corrosion builds
up again. Several ways you can test to see if these jacks
are at fault would be to play a signal through the amp
at a very low volume and while doing so take a plug from
a spare cable and insert it into the suspect jack several
times quickly noting whether this either aggravates or
eliminates the problem. You can also try hooking a cable
between the send and return jacks to see if the problem
goes away. This bypasses the two contacts coming together
and fools the amp into thinking it is receiving an external
signal. Once the bad jack is identified my feeling is to replace it. Spraying contact cleaner into
it generally does not get it where it needs to go as the
contacts that need cleaning are pressed together when no
plug is inserted as when you are spraying into it. You
could file clean the contacts on the jack ( and potentially
damage the plating on the terminal), but to do so you need
to pull the jack out anyway so why not be safe at that
point and install a new one. A classic example of this
failure are many of the solid state Crate amps. They have
a bunch of these insert style jacks and whenever I see
one if you replace all the patch style jacks the
amp is good to go. These jacks are used in a number of
positions and they are all possible causes of signal
interruption. They can be inserts, effect loop jacks.
headphone jacks, power amp in jacks, external speaker
jacks and even foot switch jacks that use these contacts
to enable the front panel channel switching when the
foot switch is not being used.
A classic poor design using these jacks is in the Marshall
TSL series of amps. The output transformer signal feeds
the 16 ohm jack which feeds the 4/8 ohm jacks by closing
its switch contacts when the 16 ohm jack is not in use.
When these contacts bend or corrode they interrupt the
signal to the speaker leaving the output transformer
with no load which as we stated above can do lots of
damage.
Pots
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Potentiometers
commonly known as Pots are the controls on
your amps front and real panel that let
you vary functions such as EQ , Gain, Volume
etc. They have an affinity to attract dirt
which came make them noisy, scratchy or intermittent
in operation. Usually they can be sprayed out
with a professional grade contact cleaner such
as Deoxit and restored to normal operation.
Unfortunately it is not possible to spray the
cleaner in from the front of the pot. It is
necessary to get to the opening in the rear
of the pot to get the cleaner into it. Due
to modern construction methods it is often
needed to remove the circuit board to gain
access to the pots. Another common failure
mode of pots is broken solder at the pot terminals
where it is soldered to the printed circuit
board. Many manufacturers support the PC board
that the pots are soldered to from the terminals
of the pots and nothing else so as you bounce
your amp in your truck on the way to your gig
it can make those solder joints crack and cause
intermittent operation of the control. A re
solder will take care of this. The other failure
mode of pots is when they receive an impact
from the front.This serves to either push the
pot apart inside rendering it inoperable or
it fractures the part of the pot that contains
its carbon resistance trace. You can usually
crimp the pot back together but if the trace
is fractured the pot needs to be replaced. |
Intermittent Operation
How often have you had an amp that works correctly
some of the time and then all of a sudden it makes
a strange noise, some functions cease working or no
sound at all comes out? According to how often and
randomly the problem occurs can make this a hard thing
to track down. The first thing that should be done
is to try and isolate the problem, see
if any specific function, volume level etc will cause or
stop the problem. Does it only happen when the amp is first
turned on or does it take some time before it shows up. Finding
a way to be able to continually cause the problem to
happen goes a long way towards helping locate it. Take
your hand and rap on the top of the amp in different
places and see if that induces the problem. Intermittent
problems are caused by one of two things. Either a
component is failing and is working only some of the
time, maybe when it reaches a certain temperature it
goes bad, or there is a loose connection somewhere
in the amp. A guitar amp has many components inside
and each of these is going to have at least two solder
connections. It only takes one bad connection in the
right place to cause a lot of trouble. The worst offender
for intermittent problems is the combo amp. The electronics
are located directly in the vibration field of the
speaker and this will bring out any possible loose
connection more so than a head/speaker cabinet combination
that affords some degree of mechanical isolation between
the speaker and electronics.
Next month I will continue this discussion, moving on to
problems with tubes and speakers.