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19. RA-5A 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, depending on which protect IC detected
the problem, may shut the entire set down. Since the HV protect circuits are not monitored by the self-diagnostics
feature, conventional procedures are needed to perform the initial diagnosis.
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. In extreme situations, HV shutdown can occur so fast that it
appears to have never started. A quick check of pin 7 of IC8007 for a high will confirm a shutdown since D8016
latches the IC. IC8006 pin 7 also latches but, unfortunately, outputs to the main relay latch circuit and will shut
the unit down. A peak-hold DVM or oscilloscope will be needed to catch the momentary high output. Once it is
confirmed the HV is not starting, a dead HV circuit will be caused by a shorted HV output 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. Just turn up the gain a little
on the scope and you will easily see the retrace pulse. 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 full AC power
with the variac and watch for the pulses (assuming it appears). You are going to have to attempt to
observe them during the startup and shutdown duration. 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, which also indicates a defective flyback. If the retrace pulse looks great but
it begins 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 may go into shutdown. 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 IC3006. 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
