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4192A LF Impedance Analyzer -- Troubleshooting Problem

Question asked by larry001 on Jan 16, 2013
Latest reply on Jan 16, 2013 by drkirkby
I've got an HP 4192A with an unusual problem I haven't been able to troubleshoot.  When I power up, the unit passes all the self-tests (01-06) and says "PASS" on Display "A", shows the Hex address for the HPIB and then the Displays "A" and "B" go blank.  The manual self-check (blue-key & 6) also passes.  The auto-run doesn't work (i.e., the trigger light doesn't flash in auto and no auto triggers on any internal test point).  A manual trigger from the front keyboard will flash the trigger light, but no display of data on "A" or "B".  If the average function is selected on the front panel (blue-key & 4) the trigger light will stay on 4x longer with a manual trigger as with a normal functioning 4192A.  All display elements and LEDs light up stable and solid on the self-test.  All front panel key entries appear correct and LEDs respond as they should.  I replaced A6U33 as suggested in the Service Manual under Timing Notes on page 8-36, but no change.  I can send commands and receive data from the HPIB interface. I'm pretty sure (but not positive) that the A6 computer board and Display board are working normally.  The oscillator is stable and accurate and can be commanded from both the front panel and HPIB.  The bias also responds to commands.  I've checked the outputs from the fractional N chip with a data probe and there's data being output. The Frequency display on the upper right responds normally to all inputs (frequency, bias, oscillator levels). It appears the unit is not able to complete a measurement, get its data to the displays and initiate another trigger sequence.  Any ideas on how to progress from here would be greatly appreciated.