Radio Shack TRS-80 Service Manual page 49

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Service Manual
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Troubleshooting
the Power Supply (Secondary Drive Only)
Section
1
Equipment
for Test Set-Up
1.
Isolation Transformer (minimum
of 500
VA rating)-
***CAUTION***
Dangerously high voltages are present
in
this power supply*
For the safety of the
individual doing the testing, please
use an isolation transformer. The 500 VA rating
is
needed
to
keep the AC waveform from being clipped
off at the peaks
.
These power supplies have peak charging capacitors and draw
full power at the peak of the AC waveform.
2.
0-140 Variable Transformer (Variac)-
Used to vary input voltage
.
Recommend
10
Amp, 1.4
KVA
rating minimum.
3.
Voltmeter-
Needed
to
measure
DC voltages
to 50 VDC and AC voltages
to 200
VAC. Recommend
two digital multimeters.
4.
Oscilloscope-
Need
X10 and X100 probes.
5.
Load Board with Connectors-
See
Table
1
for values of loads required. The
entry
on
the table for
Safe Load Power
is
the
minimum power
ratings for the load resistors used.
6.
Ohmmeter
Set-Up Procedure
Set-up
as
shown
in
Figure
8.
You will want
to
monitor the
input voltage and the output voltage of the regulated bus,
which
is
the +5 output, with
DVM's. Also monitor
the +5
output with
the
oscilloscope using 50mv/div sensitivity. The
DVM monitoring
the +5 output
can also be used to check the
other outputs. See text of Section
III for test points
within power supply.
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