ConMed ExcaliburPLUS PC - Page 75

Jacks.
With
10
ohms
connected,
BVARM
should
be
+0.85
to
0.98
Vdc
and
with
150
ohms,
it
should
be
+1.89
to
2.18
Vdc.
BVARM
should
also
match
VARM
at
A3TP17
to
within
10
mV
dc.
If
this
test
passes,
then
the
trouble
is
most
likely
in
the
DAC
A3U19
or
A.R.M.
comparator
circuitry
around
A3U23.
Note
that
a
faulty
DAC
is
also
likely
to
cause
Err
5.X
Alarms.
If the VARM vs. resistance test fails, check the de
current source A3VR2 and A3U21D by connect-
ing a dc milliammeter from A3TP17 to ground.
The meter should read from 0.49 to 0.51 mA.
If VARM test reads near zero and the de current
is low, repeat the current test with A3J2 discon-
nected to eliminate a possible VARM short to
ground in the A9W5 harness or on the A5 PWB
and shorted transistors A5Q2 or Q3.
With the Return Electrode Jacks open-circuited,
check the waveforms on the A.R.M. oscillator
collectors, A5Q2 and Q3. See Figure 3.7. They
should appear as half-wave rectified sine waves
with a frequency of 34.5 - 38.1 KHz. If the fre-
quency is too high, A5C22, 23, 44 or 45 may be
open. If the circuit is not oscillating and VARM
is 0.6 - 0.8 Vdc, check the feedback resistors
A5R7 and R8, transformer AST4 for opens or
shorts and transistors A5Q2 and Q3 for opens.
If the circuit is oscillating at the correct frequency
but VARM does not respond to resistance
changes, check the circuitry from the secondary to
A5T4 to the Return Electrode jacks for shorts or
open circuits.
4.7.7 RE Amplifier Troubleshooting
The first step in trouble shooting a Power
Amplifier (PA) is to remove all PA collector and
base fuses (check the fuses as you remove them
and note if any of them are blown). This will
prevent possible secondary failures in the PA due
to overload.
Use a VOM to check for shorted bipolar power
transistors (A4Q1-Q3, Q6-A8), power MOS-
FETS (Q4-Q5), and snubbing networks (A5D9-
11, D14). Check to see that diodes (D1-3, D6-8)
are functional. Check TP9 for approximately
+110 Vdc and U2-6 for +12 Vdc.
alibur
Check the Base Voltage Generator and Waveform
Generator for proper function before proceeding.
If everything checks this far, enter dIA 2 (See sec-
tion 4.7.2) and select Monopolar Pure Cut from
the front panel. Confirm that A4TP10 and TP4
are approximately 0 to 10V signal swings, and
the same shape as A3TP10.
Turn off the power to the unit and replace the
base fuses. Reenter dIA 2 and select Monopolar
Pure Cut from the front panel. Use an oscillo-
scope on the drains of A4Q4 and Q5 to confirm
that the power MOSFETs are switching properly.
Check the voltage on the cathode of D10 for
approximately 39 Vdc.
If the unit has passed all tests to this point, it will
be necessary to replace the collector fuses to trou-
bleshoot further. Proceed with caution!
4.7.8. Power Amplifier Transistor
Replacement
All bipolar power transistors A4Q1-Q3 and Q6-
Q8 may be replaced singly without replacing the
entire set. Calibration should be checked after
transistor replacement. When replacing the power
transistors:
1. Be sure to install the insulator pad. No ther-
mal compound is necessary, but the mating sur-
faces of the transistor, insulator and heatsink
should be clean. Replace any insulator which is
torn, punctured or dirty.
2. Ensure that both pins of the transistor are
properly centered in the socket before setting the
transistor.
3. Tighten the screws to 4-6 inch-pounds.
Excessive torque may cause the insulator pad to
be cut, destroying its dielectric strength and
shorting +110 Vde to ground.
4.7.9 Troubleshooting Resets
Apparently random restarts of the unit are gener-
ally due to temporary loss of power. If the unit is
the only machine in the room to restart, check for
loose connectors in power supply circuits, broken
or weakened power cords, and a disconnected or
broken +5 volt regulator on the A3 assembly.
ra 4-21