Radio Shack TRS-80 Service Manual page 74

FUNCTIONAL SPECIFICATIONS
The
power
supply tor the TRs.80 Model III is a 40 wan,
switching power supply, The printedcirwitboard is mounted
directly to the case. Une input to the power supply is made
through a locking 2.pin PCB wetet header (SK-ll.
2. 0, 140V Vilriilble Transformer (Variac) -
U5ed to vary input votUgiI. Recommend 10 Amp,
1.4 KVA rating minimum.
3. Voltmeter -
Needed to measure
ac volt..
ges to 50
vac
and AC volt-
ages to
200
VAC.
Re<:ommetld
two digital multimeters,
Pin
1
Pin 2
line - Neutral
line - Live
4. Oscilloscope-
Need X10 and Xl00 probes.
Outputs are taken from
(SK-2, SK-3,and SK-4).
Pin 1 ·12V
Pin 2 +12V
Pin 3 Common
Pin 4 +5V
locking 4-pin PCB socket headers
5.
6.
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.
Ohmmeter
In
theory,
the power supply rectifies the AC line to DC
and chops it at 20 kHz. The chopped DC voltage is then
transformed to the required output voltages and rectified to
low voltage isolated DC. Feedback loops are provided for
voltage regulation and overcUrfent protection.
The power supply module can withstand the following
maximum ratings:
Vin (AC continuousl - 95 to 135VAC@60Hz
Short Circuit, any output - indefinite
SET·UP PROCEDURE
Set-tJP as U"oown in Figure 1. You will want to monitor the
input voltage and -the output voltage of the regulated bus,
Which
is the +5 output, with DVMs. AI50 monitor tile +5
output with the oscilloscope using SOmv/div sensitivity.
The DVM monitoring the +5 output can also be used to
ctleek
the other outputs. See text of section III for ten
points within power supply
............
TROUBLESHOOTING
Section 1
EQU IPMENT FOR TEST SET-UP
1. Isolation Transformer (minimum of SOOVA rating)-
..... CAUTION ....
•
-
FIGURE 1. TEST SET·UP
:I==;l
'"r-,
.<
• ....
'''''
Dangerously high voltages are present in this
power supply. For the safety of the indivkiual
doing the testing, please use an isolation
transformer. The 500VA rating is needed to
keep the AC waveform from being clipped
otf at the peaks. These power supplies have
peak charging capacitors and draw full power
at the peak of the AC waveform.
SeetiOll II
VISUAL INSPECTION
Check power supply for any broken, burned, or
obviou~y
damaged components. Visually check fuse resistor, if any
question che<:k with ohmmeter.
67