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HP4194A Analyzer Fails to POST

Question asked by kb1vyi on Jul 1, 2014
Latest reply on Jul 2, 2014 by kb1vyi
I current have an HP4194A Analyzer that functioned fine until it was turned off one day.  Upon turning it off it would not turn back on again.  The fan in the back will run and the screen will turn on but display nothing.  The two LEDS on the front remain in the lit position as well as all the POST lights on the two processor boards A6 and A8.  However, the bus halt LED is not lit on either of the boards.  Here is a list of things that I have done as a proposed solution to the problem.

The caps on all the power supply boards and processor boards have been replaced.  All resistor values have been checked for tolerances on boards A3, A6, and A8.  All the voltages/frequencies have been checked and readjusted to the correct voltages as specified per the service manual.  The service manual flow charts also lead me to dead ends.  I believe the culprit is definitely boards A6-A8.  The processor is also receiving the correct voltages after checking the halt lines from the power supply monitor.

If anyone has any significant experience with these machines and has experienced similar problems please share your experience.  Thanks!