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The following is a collection of notes on discharging Filter Capacitors
DISCHARGING CAPACITORS – The filter
capacitors in an amp will retain a charge for quite
some time after the amp is shut off and unplugged.
It is a good idea to make sure your amp is safe
to work on. One way is to take a wire with an
INSULATED alligator clip on each end, clip one
end to a good ground point, and the other to one
of the plate leads for the first stage preamp tube.
Another method would be to put a 100K, 5W or
more resistor in line of these two clips by attaching
one end to ground (first), then the other to the filter
caps themselves. NEVER SHORT THEM OUT
DIRECTLY!
BE THE ONE-ARMED TECHNICIAN – There
will come a time when you have to work on a
chassis that is running. When doing so, if possible
clip one of your meter's test leads to ground, and
use only one hand to probe the circuit, keeping
the other in your pocket, etc. That way if you DO
take a shock, your heart is not in line of the path
of least resistance – in one arm and out the other.
When you ABSOLUTELY must use two hands, be
EXTREMELY CAREFUL, and use common sense,
but PLEASE try to avoid these situations!
"Filter" caps can store fatal amounts of electrical
current. The caps are connected near the rectifier
and are part of the power supply, and aid in
converting AC to DC. In fact, they are a standard
component in any power supply.
If you're completely lost, and don't understand this,
DON'T MODIFY YOUR AMP. You haven't enough
knowledge to work on high voltage/current circuits
safely.
Several ways to discharge caps:
FIRST, UNPLUG THE AMP! (But that doesn't
make it safe!)
THEN, take a screwdriver or a jumper and short
the capacitors leads.
56
This project and its documentation is the result of technical investigations made by the engineering staff of Mercury
DANGER! READ ME!
DISCHARGE THOSE FILTER CAPS!!!!!
Compiled by the staff at Mercury Magnetics
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Orange "Tiny
the furnishing of these documents does not constitute an expresed or implied license to use such materials.
– OR jumper the power amp tube plate pin to
ground for a minute or so (Class A, single power
tube only)
– OR jumper the positive (+) lead of each large
cap to ground for several seconds. A jumper with
a built-in resistor (10K or so) will help prevent
sparks here.
Some or all of these methods may result in a spark.
Obviously, your flesh can act as a jumper also.
It's NEVER a good idea to touch amp circuitry when
it's switched on. And don't work on amps in bare
feet, or on a damp basement floor.
Letting the capacitors discharge slowly is preferable
to having them do it all at once through your body
while you're poking around inside the amp. Having
the supplies you need on hand makes the job go
faster.
You'll want screwdrivers, contact cleaner (you can
use Gunk brand choke and carburetor cleaner or
WD-40), a toothbrush and – unless you own an air
compressor – an aerosol can of compressed air –
soldering supplies and a volt/ohm meter (learn how
to use them!).
Start by unplugging the amp from the wall, the
speakers from the amp, and removing the amp
chassis from the case. For Fender amps, removing
the chassis involves unscrewing the upper back
case panel, then unscrewing the chassis from the
top of the case. Put the screws, nuts, and lock
washers where they won't get lost.
With the chassis out of the case, you can locate
the filter capacitors you'll want to avoid touching as
you perform the tune-up. These will be cardboard
cylinders (usually orange or tan or vintage Fenders)
with "+" printed near one terminal. It's also a good
idea to steer clear of wiring connected to the power
transformer because it's a discharge path for the
filter capacitors.
Terror" • Mercury "Holy Terror" Upgrade Kit
Magnetics. The disclosure of the information herein may pertain to proprietary rights and
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