Tandy 2000 Service Manual page 155

Tandy@
Model
2000
Service
Manual
7.2.L.2
Troubleshooting
The Power Supply
Equipment Required
1.
Isolation transformer,
250 VA minimum
ra'Ling.
Dangerously
high voltaqes are present in this
power
supply.
For the safety
of the person doing the
testing, use an isolation transformer.
'lhe
25O
VA
rating is necessary to keep the
AC
wavef')rm from being
clipped
off at
the
peaks.
These
power srrpplies
have
peak
charging capacitors
and draw
maximum
power
at
the
peak
of
the
AC waveform.
2.
Variable
Transformer
(Variac).
Use
to
v,ery
the
input
voltage.
A 10
drp, I.4
KVA
rating is
re,lommended.
3. Voltmeter for
measuring
DC
voltages to
4,10
Vdc and
AC
voltages to 150 Vac. Two digital voltmebers are
recommended.
4.
Oscilloscope
with
XIO and X100 probes.
5.
Ohmmeter
6.
Load board
with connector.
See
Figure 1 for
a
schematic
of the load
board.
7.
35 Vdc power supply
Min
imum
Ohms
Load
+5 VoIt
3
.5A
1.4
+I2 volt
0
.25
A
48
-L2
volt
0.05A
240
Iv1ax
i
mutn
Ohms
Load
13
.5A
0.
38
2.lA
5
.7
0.2A
60
Table
7-6.
Load Board
Values,
95W Pow,3r
Supply
Table
1 lists the resistor
values required
to simulate
the
minimum
load conditions
and
the
maximum
load conditions of
the
95W
Power
Supply.
The
ohms
values are
measur€d
at
the
connector and
include interconnecting wiring.
The
ohms
values
may
be obtained
with adjustable resistors
or
by
paralleling several resistors.
Be
sure the resistors
are
rated
f
or the current
and power
they
must
han,1le.
The
variable resistors
must be
measured and s,et
when
they
are
hot.
146