Tandy 6000 HD, Service Manual

Page: 190 / 416

Service Manual
TANDY 6000/6000-HD
TANDY COMPUTER PRODUCTS ------------------------------------------------------------------------
4. Check for B+
Set up power supply and attach the X10 scope probe
ground to the cathode side of D 5 . Slowly turn up the
power and check for B+ on the cathode side of D9. With
an input of 95 VAC, this point should be betweeen 130
and 170 VDC. If this is not correct, check DBl, the
fuse (FI), and if necessary, R 5 , D 1 2 , and D13. Also
check input capacitors C5, C 6 , C7 and C8.
5. Check Q1 Waveforms
Using X10 probe on the case of TO-3 package of Q 7 , check
the collector waveform. The transistor should be
switching. The correct waveform is shown in Figure
7-16. If switching is not present, check for shorted
junctions on Q7. If Q7 is not shorted, check the base
waveform.
The base of Q7 (looking under the PCB) is the pin from
the center of Q7 closest to the PCB edge. The correct
waveform is shown in Figure 7-17. If the waveform is
there, the problem is on the secondary side of the
supply. If the wave form is not correct, the problem is
in the control section of the supply and the supply
should be returned to the repair depot.
CAUTION
Your scope should be powered by
an isolation transformer.
Figure 7-16. Q7 Collector
Waveform
Figure 7-17. Q7 Base
Waveform
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