...nothing happened when 5v was tapped against the wire going into the 10k resistor.
I have not disconnected the 555 from the 2N3704 because one of the goals of this project is to have the chart recorder work with the Arduino, but also to preserve its original functionality. Do you foresee any problems with leaving the 555 connected to the 2N3704? (There will be no output from the 555 at that time.)
What value for the resistor between Vcc and pin 3 do you recommend? The one the chart recorder uses for pin 15 is 2k2, the closest I have are 1k or 3k3. Would any of them be suitable? (Vcc is around 15v, the SAA1027 registers HIGH at >7.5v, LOW at <4.5v)