Power Supply; Functional Specifications; Troubleshooting; Set-Up Procedures - Radio Shack TRS-80 Service Manual

FUNCTIONAL SPECIFICATIONS
The power supply for the TRS-80 Model 111 is a 40 watt,
switching power supply. The printed circuit board is mounted
directly to the case. line input to the power supply is made
through a locking 2-pin PCB socket header (SK-1).
Pin 1 Line - Neutral
Pin 2 line - Live
Outputs are taken from locking 4-pin PCB socket headers
(SK-2, SK-3, and SK-4).
Pin 1 -12V
Pin 2 +12V
Pin 3 Common
Pin 4 +5V
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 overcurrent protection.
The power supply module can withstand the following
maximum ratings:
Vin (AC continuous) - 95 to 135V AC@ 60 Hz
Short Circuit, any output - indefinite
TROUBLESHOOTING
Section 1
EOU IPMENT FOR TEST SET-UP
1. Isolation Transformer (minimum of 500VA rating) -
Dangerously high voltages are present in this
power supply. For the safety of the individual
doing the testing, please use an isolation
transformer. The 500V A 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 - 140V 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 volt-
ages to 200 V AC. Recomme.id 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 1. You will want to monitor the
input voltage and ·the output voltage of the regulated bus,
which is the +5 output, with DVMs. 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 111 for test
points within power supply
POWER SUPPLY
AC
INPUT
FIGURE 1. TEST SET-UP
Section II
VISUAL INSPECTION
+12V
·12V
oc
OUTPUT
Check power supply for any broken, burned, or obviou?IY
damaged components. Visually check fuse resistor, if any
question check with ohmmeter.
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