I realize that fixing a System 1 board is a thing of the past since the Ni-Wumpf was created but I'm doing a little dabbling and was hoping to get some direction or some input from the group.
The problem with this board I think is that it never goes low before going high in the "Reset Circuit".
What I've done so far: Replaced Q5/Q6, C31/C32, Z2. I have checked the voltages at C16 and C17 and I have +5v and -12v respectively. I tested TC1 pin 14 at power on and the voltage is always postive no negative voltage.
The attached image is from the CPU schematic. Can someone tell me what is meant by VSS and VAB as well as why the arrows that point to -12v and +5v that go nowhere?
From Pin Repair:
Dead CPU: Next Steps.
The CPU board is completely dead, with no score display activity after 5 seconds of power-on. Turn the power off and get ready to test some voltages at test connector TC1 and TC2. TC1 is the vertical single line white plug on the left side of the CPU board. TC2 is the vertical white plug in the dead center of the CPU board. On both TC1 and TC2 pin1 is the top most pin.
Reset Circuit: The reset circuit holds the CPU chip low for a set period of time until the +5 volts can stablize. If the CPU chip is never told to go "high" from the reset circuit, the board will never start to boot.
Measure TC1 pin 14 and power the CPU on. It should show immediately at power on -12 volts. This will rapidly changie into +5 volts after about half a second. This is the RESET signal. Another place to check the Reset is at chip Z2 pins 7,9. (both should go high to 5 volts after about one second of power-on). If the reset is not working and does not change to +5 volts, it is best to replace the Q5 and Q6 (MPS-A70) in the reset circuitry. If the reset is still not going from -12 to +5 volts, change chip Z2 (4528 CMOS). Still not working, check or replace caps C31 and C32 (.1 mfd, and these do sometimes fail). Note that the "Reset" button on the CPU board has nothing to do with this Reset signal (it is only used to reset bookkeeping values).
Any help would greatly be appreciated.
VB,
The problem with this board I think is that it never goes low before going high in the "Reset Circuit".
What I've done so far: Replaced Q5/Q6, C31/C32, Z2. I have checked the voltages at C16 and C17 and I have +5v and -12v respectively. I tested TC1 pin 14 at power on and the voltage is always postive no negative voltage.
The attached image is from the CPU schematic. Can someone tell me what is meant by VSS and VAB as well as why the arrows that point to -12v and +5v that go nowhere?
From Pin Repair:
Dead CPU: Next Steps.
The CPU board is completely dead, with no score display activity after 5 seconds of power-on. Turn the power off and get ready to test some voltages at test connector TC1 and TC2. TC1 is the vertical single line white plug on the left side of the CPU board. TC2 is the vertical white plug in the dead center of the CPU board. On both TC1 and TC2 pin1 is the top most pin.
Reset Circuit: The reset circuit holds the CPU chip low for a set period of time until the +5 volts can stablize. If the CPU chip is never told to go "high" from the reset circuit, the board will never start to boot.
Measure TC1 pin 14 and power the CPU on. It should show immediately at power on -12 volts. This will rapidly changie into +5 volts after about half a second. This is the RESET signal. Another place to check the Reset is at chip Z2 pins 7,9. (both should go high to 5 volts after about one second of power-on). If the reset is not working and does not change to +5 volts, it is best to replace the Q5 and Q6 (MPS-A70) in the reset circuitry. If the reset is still not going from -12 to +5 volts, change chip Z2 (4528 CMOS). Still not working, check or replace caps C31 and C32 (.1 mfd, and these do sometimes fail). Note that the "Reset" button on the CPU board has nothing to do with this Reset signal (it is only used to reset bookkeeping values).
Any help would greatly be appreciated.
VB,