Hi folks, This is another Grundy Newbrain problem on a different machine that is probably easily explained by someone who understands current flow. This machine has two startup circuits that are simply a resistor and 10uF capacitor each, both feeding a Hex Schmitt Trigger (CD401068CM). When the caps have charged up sufficiently they activate both PWRUP and RESET signals one after the other thanks to the 220K and 560K resistors.
Schematic for the circuits is here - http://www.binarydinosaurs.co.uk/newbrainPowerupCircuits.png <http://www.binarydinosaurs.co.uk/newbrainPowerupCircuits.png> - top circuit is PWRUP and the bottom one is RESET that goes straight to the Z80. Only in this machine it doesn’t unless I have 3 sampling probes and a GND probe on my USB-powered logic analyser attached to pins 3-5 of the CD401068CM and the GND pin of any nearby chip. Less than 3 sampling probes and it won’t start so those probes must be facilitating current flow to GND? Cheers! PS Brent, I got the other Newbrain going by replacing the 74LS166 shift register, the one that provides the SOVSR signal. All good now! — Adrian/Witchy Binary Dinosaurs - Celebrating Computing History from 1972 onwards
