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REPAIR HP 8412B
PHASE - MAGNITUDE DISPLAY
January 2011

The HP 8412B Phase - Magnitude Display (S/N 2246A) is not displaying a trace and possibly has multiple faults. It was bought as a maintenance spare but I will try and repair it. Removing all the covers for access and inspecting the pcb's and wiring did not reveal any obvious faults.
Top view of A1 Phase Detector and A2 Phase Offset
LHS view of A3 Multiplexer Horizontal & Vertical Deflection Amplifier
RHS view of A5 High Voltage Power Supply &
A6 High Voltage Transformer & Rectifier
DANGER 3kV exposed!
Bottom view of A7 Amplitude Channel Log Converter
A8 Low Voltage Power Supply
A9 Test Channel Phase Amplifier
A10 Test Channel Amplitude Synchronous Detector
Following the 'Troubleshooting Tree' in the Service Manual and having a tested unit to substitute pcb's with certainly makes fault finding easier. The first fault identified is A9 U3 having no output. This is a RCA CA3146E Transistor array in a 14 pin DIL package. Need to find a replacement IC. Second fault was no trace, just one spot on left hand side of screen. Removed A3 pcb to check sweep input via horizontal gain control and ok. Plugged A3 back in and now have amplitude and phase traces! Maybe intermittant connection but will have to watch this. Completed troubleshooting tree ok but some adjustments will be needed.

There was a mark on the screen, stripping the front assembly down and cleaning the mesh and perspex cover removed the mark and all the dust! Tricky to reassemble though. Following the adjustments procedure and several areas are outside specification. The trace is distorted at the right hand side of the screen with +ve or -ve gain. The phase calibration is outside specification. The high voltage is out of specification, probably leaking capacitor(s).

As I already have a working unit, I will keep this one as a maintenance spare.
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