Radio Shack TRS-80 III User Manual

TABLE 1. LOAD BOARD VALUES
SAFE
MAX
SAFE
OUTPUT
MIN LOAD LOAD R LOAD POWER
LOAD
LOAD R LOAD POWER
+5
0.45A
11.11 ohm 5W
2.5A
2 ohm
25W
+12 0.3A
40 ohm 5W
2.02A 24.6 ohm 50W
-12 NONE OPEN
0
O.lA
120 ohm
2W
START-UP
Load power supply with
minimum load as specified in
Table 1. Bring power up slowly with Variable Transformer
while monitoring +5 output with scope and OVM. Supply
should start
with approximately 40 - 60 VAC applied and
should regulate' when
95 V AC is reached. If output has
reached +5 volts,
do a performance test as shown in Section
IV .If there is no output refer to Section III.
Section III
NO OUTPUT
E. Check Q2 Waveforms
Using
X100 probe on heatsink of Q2, check collector
waveform. The collector of 02 is the pin closest to the
large capacitors C5 and C6. Transistor should be switch-
ing,
correct waveform is shown in Figure 2. If this is not
present, check for shorted junction on Q2. If 0 K check
the base waveform. Base of Q2 is pin of transistor nearest
the edge of PCB. The correct waveform is shown in
Figure 3. If this waveform is not present, check L2, Q1,
and 01, 'and secondary components Q3, 08, 09,010
and L3. If any of the semiconductors are found shorted
or inductors open, replace them.
FIGURE 2. 02 COLLECTOR WAVEFORM
A. Check Fusable Resistor
If fusable resistor is blown replace but do not apply
power until cause
of failure is found.
B. Preliminary Check on Major Primary Components
Check diode bridge (B R1), power transistor (T2), and
catch diode (03) for shorted junctions. If any com-
ponent is found shorted, replace.
C. Primary Check on Major Secondary Components.
Using ohmmeter from output common to each out-
put, .with output loads disconnected, check for shorted
rectifiers or capacitors. If +5 output is shorted also
check
crowbar SCR (SCR1) and zener (Z1).
O. Check for B+
Set
power supply and attach X100 scope probe ground
to end of R10 nearest OB1. Slowly turn up power and
check for B+ on end of R9 nearest mounting hole.
With
input at 95 VAC, this point should be between
120 - 140 VOC. If this is not correct, check BR1 and
if necessary,
check R2, 02, and finally input capacitors
C5 and C6.
~,
'I"''''
I\.
~Ib..
l..;
1.0 V/DIV
5/JSec/DIV
Input and Loads
same as above.
50 V/DIV
5 /JSec/D IV
Input - 120VAC
Loads - +5 @ 2A
+12 @ 1A
-12@0.lA
"'~
.. ~
,-
_ J
68
FIGURE 3. 02 BASE WAVEFORM

Radio Shack TRS-80 III Specifications

General

Brand	Radio Shack
Model	TRS-80 III
Category	Desktop
Language	English