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25. RA-6 High Voltage Circuit Troubleshooting
Troubleshooting
High voltage circuit failures generally fall into four categories: No HV, low HV, excessive HV and excessive
current. The latter two will cause the HV circuit to cease operating and a high will be sent to IC704 on the A board
to place the unit into protect-mode. The timer LED will flash in sequences of eight.
Dead HV Circuit: The initial concern with this symptom is whether the HV output Q8038 has shorted. This will be
evident by the fuse link being open on the 135V line. If Q8038 is not shorted, it is important to be certain the high
voltage did not come on and shut off suddenly by observing the timer LED for a diagnostic indication. Once it is
confirmed the HV is not starting, a dead HV circuit will be caused by a shorted HV output transistor Q8038 or a
malfunctioning circuit. The approach for either reason will be different.
No HV – Q8038 Shorted: Verify Q8035 is also not shorted. It may have failed after the fact or could be
the reason for Q8038 shorting. The extremely functional soft start circuit in the power supply will provide
enough current limitation to prevent a repeat failure of Q8038 in most cases. If the HV regulator is
malfunctioning or there is excessive current draw, the HV or 135V protect circuits will activate during the
soft start period. If the unit shuts down after replacing shorted components, the HV circuit will need to be
monitored to locate the cause. 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. Connect a DVM to CN8016 and monitor the DC voltage. It must not rise above 7.5VDC, which
would signal a HV regulator problem. Apply AC power with the variac and watch for the pulse ( if it
appears). If the flyback is defective, one of two waveforms is likely to appear and you are going to have
to attempt to observe them during the startup and shutdown duration. 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, which also indicates a
defective flyback. If the retrace pulse looks great but starts to jitter and dance on the scope screen, this
usually indicates excessive loading of secondary lines. In this unit, it can be loads on T8004. Measure
for shorts on the filament and 200V line. The flyback transformer T8003 only has two load circuits and
they are the HV and focus blocks.
Do not worry about excessive high voltage problems. If the high voltage rises too high, the horizontal
drive pulse will be cut off, the circuit will stop functioning and the unit will shut down. If the pulse still looks
good by the time the circuit activates, you are good to go. If it goes into HV protect, proceed to HV
shutdown section to follow.
No HV – Q8038 Not Shorted: Check for 135VDC at Q8038-C. This will verify proper operation of the HV
regulator stage. Since the unit is not shutting down and generating an H Stop event in the diagnostics,
horizontal drive is being output by IC309. Work your way back from Q8038 to locate where the drive
signal is being lost.
If the voltage at Q8038-C is extremely low or missing, the HV regulator is at fault. Check pin 3 of IC8008.
There should be 12VPP pulses with a long duration positive pulse. This indicates the PWM driver is
attempting to get proper HV and Q8035 is unable to amplify. If pulses are not present, IC8008 is not
working properly. Verify 12VDC at pin 1 and H pulses at pin 14 before replacing IC8008.
High Voltage Too Low: This is not a common occurrence in this circuit design. PWM regulators will generate low
voltages if the reference sawtooth frequency or level changes, or if the error loop voltage rises due to pull-down
resistors increasing in value. The saw reference is locked to the incoming H pulse and the level is controlled
inside IC8008. A quick method to isolate the cause is to measure the voltage at pin 12 of IC8008. If it is higher
than 7.5V, the HV block is defective. If it is significantly lower, check the PWM signal at pin 3. If the positive duty
cycle exceeds the negative, IC8008 is sensing the low HV and trying to compensate. Check for clean pulses at
Q8038-C. If they are OK, the HV block is defective.
If the negative duty cycle exceeds the positive, IC8008 may be defective. If the duty cycle is near 50%, someone
has managed to turn VR8001 out of adjustment.
