Metal detector keep alive circuit not working

[jgroleau]

I am new to this forum so please forgive me if this is too technical. I have an old Whites Eagle metal detector that won't turn on. There is a main processor and I/O processor in the metal detector. There is a keep alive circuit that keeps the main processor in standby mode until the on/off switch is selected at which point the metal detector goes through a boot sequence and starts the metal detector. When I press the on button the metal detector does not seem to have the correct voltages to the all the ICs. The main power supply produces 13V, 8V, and 5V to supply voltage to all the IC's. The voltages on the main power supply are wrong also but not sure if that is because the keep alive is not working or something else. I am trying to eliminate the keep alive circuit as the problem. I decoded the Keep alive portion of the power supply which is shown in the following diagram:



The voltages on top are what I measure in standby mode. The bottom voltages are what I measure after I press the on/off switch. Q1 gets an input from the microprocessor I/O line D7 during startup. It seems to switch Q1 on., but the voltages on Q2 appear to be low. Can anyone tell me if the circuit is working as expected or if Q2 is bad or if Capacitor is bad? Thanks.

[vtech]

At first glance, based on your numbers. the circuit appears to be working. There is a possibility of a leaky Q2 but you should be able to check it in the circuit OR better yet swap it with Q1.
Not sure of your 2.66v reading at emitter of Q2 though. Usually, the power on/off control line should swing from high to low. Also, you can always isolate Q2's emitter and use a resistor to pull it down to verify that your supply does come up.