I'm troubleshooting my Roland TR-909 drum machine, which doesn't power on anymore. All of the voltages come up fine, but the reset line stays low. My suspicion is one or more of the components of the reset circuit have failed. Edit: the trace connecting D702 and D701 is highZ and C704 in the same area looks like it's leaked some electrolyte, which probably caused an open on the trace.
My understanding after staring at it for a little while is as follows:
At first I thought D701's purpose in life is to bring the drive voltage of Q701 down below 5V so as not to forward bias it's Vbc, but the excess voltage would still be dropped across R702.
Question 1: What is D701's purpose? (Note the schem was revised from a 6.8V to a 5.6V zener at some point)
Question 2: Why not just drive Q701 from the 7805 directly by removing D701, and connecting the output of the 7805 to where the anode of D701 used to be?
Q701/Q702 and surrounding resistors function as an inverter use output is high when power is applied. When power is not applied, the transistors are off, the 7805 is off, and it's output is low, pulling reset low via R705.
R705 and C701 set an RC time constant such that RESET goes high roughly 30ms (6 time constants) after 5V is up.
Question 2: Why is an inverter needed here? Wouldn't the circuit work easily as well with just R705 and C701?