Many many moons ago I did OCR and to amplify the signals from the sensors in the 'retina' we modulated a (I do not remember exactly) 600kHz AC with the DC from the sensres (this was before intefrated chopper stabilized OP-amps). We installed a unit and just could not get it to read. I happened, accidentially, to run a blank piece of paper through and the unit 'read' a lot of charcters from it. Finally I got it, on top of the building was the antenna for an AM radio station transmitting at the exact same frequency. We replaced a lot of capacitors and tuned the amplifiers to a new frequency - problem solved for a while. Then I learned a lesson: CHANGE THE PARTNUMBER WHEN YOU MODIFY A BOARD. At a later call I found that two of the amplifiers had been replaced with the standard unmodified board. The unit then, when you opened the box had a LARGE sign "the amplifiers are non-standard, do not use the one in your bag without modifying it"

i garee with thensentiment of some of the other posts in this thread "do not make fun of the mistakes made by the less experinced".

Erik