Radio Shack TRS-80 Service Manual page 75

TABLE 1. LOAD BOARD VALUES
SAFE
MAX
SAFE
OtJTPUT MIN LOAD LOAOR LOAD POWER LOAD LOADR LOAD POWER
"
O.45A 11.l1ohm 5W 2.5A
20hm 25W
. " O.3A
"'.hm
5W
2.02A
24.6ohm "'"
-12 NONE OPEN 0 O.lA
120 ohm
' W
START·UP
Load power wpply with minimum load as specified in
Table 1. Briny power up slowly with Variable Transformer
while monitoring +5 output with scope and DVM. Supply
should nart with approximately 40 . 60 VAC applied and
should regulate' when 95 VAC is readled. If output has
reached +5 volts, do a performance test as shown in Section
IV. If there is
00
output refer to section III.
S.ction III
NO OUTPUT
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
(BRl),
power transistor (T21, 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
SCA
(SC Rll and zener
(Z 1).
O. Check
for
B+
Set
power
supply and attach X100
scope
probe ground
to end of RIO nearest OBl. Slowly tum 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 SRI and
if necessary, check R2, 02, and finally input capacitors
C5 and C6.
68
E. Check 02 Waveforms
Using X100 probe on heatsink of 02, check collector
waveform. The collector of 02 is the pin closest to the
large capacitors C5 and CG. Transistor should be $Witdl-
ing, correct waveform is shown in Figure 2. If this is not
prescnt, check for shorted junction on 02. If 0 K check
the base waveform. Base of 02 is pin of transistor nearen
the
edge
of PCB. The correct waYeform is shown in
Figure 3. If this waveform is not pr~t, check l2.01,
and 01, 'and secondary components 03, 08, 09, 010
and L3. If llIny of the semiconductors are found shorted
or inductors open. replaa! them.
50 V-,>tv
5 JbeC/D1V
I"put _ 120VAC
Load' -
+5.
2A
+12@lA
·12@O.lA
FIGURE 2.
Q2
COLLECTOR WAVEFORM
1.0 VIDIV
5
Jbec{OIV
Input
and Load.
same a
above.
FIGURE 3. 02 BASE WAVEFORM