Testing And Troubleshooting - StewMac ’64 REVERB UNIT Assembly Instructions Manual

Testing and troubleshooting

Any multimeter will work fine for
the two types of tests we're about to
do: checking continuity and reading
voltages . The instructions that came
with your meter will be helpful .
Continuity testing is simply making
sure current flows between two
points successfully . Touch the meter's
red lead to one end of the section
being tested, and the black lead to the
other end . If the continuity is good,
your meter will beep or register this
on the display .
Voltage testing is where you need
to be careful . Some steps require the
unit to be plugged in and turned on .
This becomes dangerous if you're not
cautious . Respect the voltages and
follow the directions, and these tests
are safe and easy .
STEP 84
Perform a safe power-up
At this point, there should be no tubes
installed .
Before plugging the unit in, turn the
power switch to ON . Switching the
unit on before the first power-up
protects you from shock if a mistake
in your wiring has created a short
to the chassis . If this short exists, an
indication would be that the pilot
light will not turn on, since the AC
current is going directly to ground .
Plug the power cord in . The pilot lamp
should light .
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At this point, dangerous voltage
forming in the filter caps. Always
discharge them before working
on the circuit, even if the unit is
unplugged. See how to use a snuffer
stick on page 6 .
For a few minutes, watch for smoke
or unusual smells . If anything seems
unusual, disconnect the power
immediately and carefully review all
your connections .
Set your multimeter to 20V AC . Check
the heater voltage across pin 9 and
pins 4+5 on socket V1 . This should
read between 5-7V AC . If this reading
is drastically different, disconnect
power and check your connections .
If the unit seems normal, unplug the
power cord while still leaving the
power switch ON .
32
STEP 85
Test the dangerous DC voltage
The dangerously high DC voltage that
passes through the three diodes and
the filter caps is referred to as "B+" .
The next step is to test this B+ voltage .
Plug the power cord back in .
Again, spend a few minutes watching
for smoke or smells .
For safety, use only one hand to
probe the unit during DC tests.
is
Keep your other hand behind your
back when you need to probe a
component . This way, you can't be
a path between B+ voltage and
ground—a mistake that would send
a dangerous charge through your
heart .
Seriously, keep one hand
behind your back!
Set your multimeter to 500V DC and
connect the negative lead to ground .
Once the negative lead is secured
to ground, measure the DC voltage
at eyelet 2 for your B+ voltage . This
should be roughly 420V DC .
Unplug the unit .
Danger: Remember to discharge
the capacitors before working on
the circuit. See how to use a snuffer
stick on page 6 .
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