48
11. RA-4/4A High Voltage Circuit Troubleshooting
D BOARD
+135V
6
7
4
3
2
1
1
2
3
4
6
7
8
9
18
17
16
15
13
12
11
10
1
HV
2
3
4
5
6
7
8
9
10
12
11
HV
T8001
HDT
FROM
HV
DRIVE
Q8007
D8011
D8007
R8019
C8018
D8010
C8014
C8021
C8012
C8009
FB8001
Q8008
L8003
D8009
D8008
R8023
T8002
LOT
PS8001
R8199
L8004
D8003
FB8002
R8012
HEATERS
D8013
C8020
C8023
R8039
R8038
D8014
R8016
C8025
R8200
R8020
R8183
H+
H-
210V
TO
HV
BLOCK
T8003
FBT
TO
FOCUS
BLOCK
ABL
(HV
STOP1)
NL8001
C8032
TO HV
STOP2
PWM
FROM
HV REG
FIGURE 11-2 - HV OUT
11.2TVP14
5/30/02
Troubleshooting
Shorted HV Output Transistor: As with all horizontal output transistors, care must be taken to avoid failure of the
replacement part. Always check Q8008 since it can be easily damaged by the large current surge if Q8007
shorts. There is even the possibility that Q8008 was the cause of the failure. Disconnect or short the B-E
junction of Q8002. This will run the driver stage at low level. The output regulator will now try to compensate so
it must also be disabled. This can be done by unsoldering the gate of Q8008. The main relay can now be
jumped and power can be applied with the variac while monitoring the pulse at Q8007 collector. This is a little
extra work but it is better than losing the replacement part(s) and it is part of a critical safety circuit.
Place your scope probe somewhere near the HV output transistor or flyback. Most scope probes are not able to
handle the direct spike level from the flyback so this is a safe method. Just turn up the gain a little on the scope
and you will easily see the retrace pulse. Apply full AC power with the variac and watch for the pulse (assuming
it appears) before the circuit shuts down. If the flyback is defective, one of two waveforms is likely to appear.
The first may look like a sinusoidal waveform with a distinct ringing appearance. This definitely indicates flyback
problems. The second phenomenon will be a retrace pulse with a second, lower level, “phantom” pulse. It is
sometimes normal for this “phantom” pulse to appear at low B+ input levels but it should disappear quickly as you
increase AC voltage. Monitor the AC current level closely while doing this. If the phantom pulse remains and
current starts to rise, it is time to replace the flyback. One last situation that might occur is that the retrace pulse
looks great but as you approach higher AC input, it begins to jitter and dance on the scope screen. The current
will also be higher than normal. Back off the AC power quickly as this usually indicates excessive loading of
secondary lines. In this unit, it can be loads on T8002. Measure for shorts on the filament and 200V line. The
flyback T8003 only has two load circuits and they are the HV and focus blocks
