Rob Robson wrote: >The op amp non-inverting input is coupled to the output of an LM1972 digital >attenuator via a 1uf electrolytic. This part of the circuit is identical to >one that has been in use flawlessly for years. The inverting input is tied >directly to the output, which feeds a 5532 summing amp via a 0.1uF ceramic >cap. > >The time it takes for the fault to appear (approx 22 hours) is >slowing down the troubleshooting process, but I have several units to play >with. > > > Hmmm, 22 hrs for the fault to appear? Methinks this is a big, BIG clue.... I'm suspicious of something wrong externally, like a piece of equipment that kicks on when the office gets cold, or an arc in a phase-correction box down the street, or maybe cellphone interference...? "Curiouser and curiouser", said Alice... --Bob -- Note: To protect our network, attachments must be sent to attach@engineer.cotse.net . 1-520-850-1673 USA/Canada http://beam.to/azengineer -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist