46
Epiphone Valve Jr.
•
Mercury Upgrade Kit
This project and its documentation is the result of technical investigations made by the engineering staff of
Mercury Magnetics
. The disclosure of the information herein may pertain to proprietary rights and
the furnishing of these documents does not constitute an expresed or implied license to use such materials.
2. Digital multi-meter.
Capable of reading 500
volts DC.
3. Capacitor discharge tool.
A jumper wire
consisting of alligator clips on the ends, with a
resistor in series in the middle of the jumper.
4. Needle nose pliers.
With very good insulation
on the handles for holding the alligator clips of
the capacitor discharge tool.
Always make sure your tools are in good condition.
If there’s any question, replace them. Cracked
insulation is one of the first things to check, and
check often.
Turn the power switch OFF, and place the standby
switch in the ON position. This isn’t the same as
having your amp “on standby.”
Always unplug the amp from the wall. Unplugging
from the wall will not drain the filter capacitors
completely, but again this is for our own safety.
Unplugging is the equivalent to “locking out” before
working on a downed machine.
Because filter caps are almost always of the
electrolytic type, they’re “polarized” and therefore
have a positive and negative end. We can identify
the positive or negative ends by looking for the
“polarity indicator” printed on the cap’s side.
Our first objective is to check for residual voltage
with a multi-meter. In some cases most of the
voltage may have already been drained. Many
amplifiers will incorporate “bleeder” resistors, which
will drain the capacitor charge automatically in
a few minutes after the amp is turned off. Don’t
depend on this resistor to do the job. Never assume
the caps have been drained to a safe level without
checking with a reliable multi-meter.
Here’s a couple of ways to measure the residual
voltage contained in the filter capacitors. Note that
these are also equally valid for draining the caps:
1. Across the capacitor
.
2. From the positive side of the cap to the
chassis
. This is essentially the same as
draining “across” the caps since the negative
side is at the same electrical potential as the
chassis.
3. From the chassis to a tube pin
. This can be
done at pin 3 of any power tube, or pins 1 or
6 of any preamp tube. By shorting the chassis
to the correct pin the caps will be forced to
drain through the plate load of that tube. This
prevents sparking from high current.
IMPORTANT NOTE, HOW NOT TO DO IT:
In old
electronics books they tell you to use a screw driver
with an insulated handle to short out the caps.
“Shorting” with a screwdriver will almost instantly
drain the caps to zero volts, causing sparks from
high current. This is not acceptable. If there are
several hundred volts stored in the caps, be
prepared for a nice sized spark. It will most likely
leave a black mark on the chassis, and in extreme
cases, partially weld the screwdriver to the chassis.
Now we’re ready to start testing and
discharging
Set your multi-meter to read DC voltage. Adjust
the meter’s range so that it’s above 500 volts. Clip
the black lead of your multi-meter to the chassis.
Using only one hand, with the other hand far from
the chassis (preferably in your pocket or behind
your back), measure the DC voltage across one
of the filter capacitors. You can adjust the meter’s
range down if you can’t get a clear reading. Any DC
voltage readings less than 5 volts are harmless, so
the amp can be safely worked on, but do a double
check of all the caps just to make sure. If not we
must drain off the residual voltage with a resistor
until it is safe.
Use a resistor value somewhere between 10K and
100K, with a power rating of 5W or so (a high power
rating is not really necessary for heat dissipation
reasons, but mainly because the higher power
resistors are physically more robust and won’t tend
to break). The larger the resistor value, the longer it
will take to discharge. If your capacitors do not have
a dangerously high voltage, say around 30 volts,
feel free to use a smaller 1K 1 Watt resistor.
Now take your capacitor discharge tool and clip
one side to the chassis. Always clip to the chassis
ground first before connecting to the circuit. If
you clip to the circuit first you will have a hot wire
hanging out of your amp, which is very dangerous.
Once you have one side clipped to the chassis,
take your needle nose pliers with good insulation on
the handles and hold the alligator clip that will go to
the circuit with the needle nose pliers.
Clip this to pin 1 or 6 of the first preamp tube. This
seems to be the safest way of discharging the
capacitors. If you’re working on the first preamp
tube socket, then clip your discharge tool to pin 1 or
6 of the second preamp tube. Wait a few minutes
(about 5 minutes, if the amp has bleeder resistors
this time will be shorter) then, while the discharge
tool is still in place recheck all the capacitors with
your multi-meter and keep that other hand behind
you. Keep checking until the voltage has reached
a safe level. Note: This procedure must be done
every time you turn the amp on then off.
Remember to remove the discharge tool before
powering on the amp. Make it a bright color like
Содержание Epiphone Valve Junior
Страница 1: ...VERSION 012011 ...
