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HP 8702A Source Problem Fixed (partly)

Question asked by Francis on Jan 23, 2012
I had a problem with intermittent loss of lock on an HP 8702A.  The YIG oscillator output would fall to zero, and  I traced to problem to an intermittently open circuit potentiometer on the YIG oscillator bias board. See schematic at:  After repair I continued to observe loss of lock -- though much less often.  The second problem was traced to the 1 MHz PL reference, as measured on the A12 Reference assembly, TP10.  It was also noted that the 1 MHz signal was not present on pin 17 of U 36 (the output at U36, pin 13 also failed).  The 4 MHz clock was always present.  I replaced U 36, but the problem persisted.  The problem only occurs on switch on; pressing the reset button always corrected the loss of lock.  I was thinking it could be a firmware glitch.  My A9 CPU ROM chips: U4, U5, U24, and U25, part numbers 08702-80008, 9, 10, and 11, Rev. 2.01.  I wonder if anybody else has observed a similar problem.  One of the U 36 "write", address, or data lines must not be sent from the CPU board, so I guess I need a logic analyzer to verify this.  Interested in any comments.