Crt Supply Dropping Out (8560E Only) - HP 8560E Service Manual

Display/Power Supply Section
A6 Power Supply Assembly
to the following settings:
Sweep time ............................................................ 10 µs/div
Vertical scale ............................................................. 10 V/div
8. A nearly-sinusoidal waveform, greater than 30 Vp-p, with an
approximately +18 Vdc offset, should be observed.
9. If the waveform is a dc voltage near 0 Vdc with narrow, positive- and
negative-going pulses, the A6A1 HV module is faulty. If the
waveform is a dc voltage near +18 Vdc with narrow, positive- and
negative-going pulses, connect the probe to TP403.
10.If the waveform at TP403 is a sawtooth waveform with a 1.8 V
amplitude, the A6A1 HV module is faulty.
If the TP403 waveform has pulses similar to those on TP402, the A6
power supply is probably faulty.
CRT Supply Dropping Out (8560E)
See function block K of A6 power supply schematic diagram in the
component-level information binder.
The CRT supply is a separate switching supply which provides the +110
Vdc for the A17 CRT driver from a winding on the A6A1 HV module.
The CRT supply operates at approximately 30 kHz. The exact
frequency is determined by the inductance of the primary winding of
A6A1T1 and A6C407. The supply will only operate if the
HV_SHUT_DOWN line is high.
If the power supply keeps dropping out, there is probably a short on the
A17 CRT driver assembly.
1. Disconnect W8 from A6J4.
2. Connect an IC clip to U401 and connect a jumper between U401 pin
10 and TP308 (+5 Vdc).
3. Connect voltmeter to TP405 and press
4. Check TP405 for a voltage of approximately +110 Vdc. It will
probably measure higher since there is no load on the supply.
5. If the voltage at TP405 is correct, suspect a short on A17. If the
voltage at TP405 is not correct, check U401 pin 8 for a sawtooth
signal. The sawtooth should be flat-topped and about 5 Vp-p at a
frequency of about 30 kHz.
6. If the sawtooth is not flat-topped, suspect U402A and its associated
circuitry.
7. If the sawtooth is correct, check the base of Q401 for 30 kHz pulses.
8. If the duty cycle is high, but there is no +110 Vdc, suspect the bridge
rectifier, CR401 through CR404.
646
to turn the analyzer on.
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Chapter 12