576/Repairs: Difference between revisions

UPDATE:07/30/2019 This issue is resolved.  Further (more rational) testing indicated that the switch was, in fact, good.  When operating the "SINGLE" push button (SW78B), the switch portion of the circuit was producing the expected momentary "Low" at the wire connected to PIN "H" and through to Pin#5 of U69B (part of 156-0030-00 or SN7400N).  I was able to clearly see this pulse on my scope when scope probe was connected at the end of  previously disconnected wire between Pin "H" of Step Family Switch and Pin "J" of the Step Generator board.  Referring to the schematic, this lead me to swap out the IC's between U69 and U20 (same part). U20 seemed to be fully functional, since the related push buttons all produced their assigned function when operated.  This part swap resulted in the single step family working, but the .5X Rate ceasing to function.  This confirmed that the issue was indeed U69(B).  I had new SN7400N IC's in my repair parts, swapped in a new one to the U20 position, the entire circuit now functions as intended.