shuyge
Member
I recently picked up a non-working Stargate. This is my first Williams game so I've had to come up to speed on the basic hardware.
IANAEE. I am a S/W Eng. by trade which means that I can understand the basics of an arcade schematic and I understand address and data buss's, address decode logic, etc.
In other words, I know enough to be dangerous..
The CPU board flashes a 1-3-1 but that doesn't matter since I don't have good voltages coming from the power supply. The 4116 RAM can't work without all 3 voltages.
I have two leds lit. #1 and #2. #3 won't light.
I should mention that this is the later power supply board with the power transistors located on another board with a large heat-sink. I'm using the schematics from Robotron to troubleshoot the problem since they seem to match my board.
So the first thing that I did was to purchase a cap kit and installed it. Same problem.
So I'm starting to get a bit aggravated with the power supply. So I start looking harder at the schematics and the -5V supply looks like a standard power supply circuit that I've seen many times.
There's the normal A/C input, a bridge rectifier, filter cap, voltage regulator. Normal stuff. The only part that is odd is that the outputs of the bridge rectifier outputs are hooked up backwards to provide negative voltage. But since this is a -5v circuit, it makes sense.
So I grabbed my multi-meter and started probing the board. I'm actually getting -1.2v out of the circuit. I checked the legs of the 7905 to see what I can find..
From pin 1 to 2, I have -10.8v
From pin 1 to 3, I have -5v.
WTH?
So now I really start looking at the board and I pull it out of the machine again and I start doing some continuity checking and actually looking at the traces on the board. What I see confuses me. The ground that is labelled as 0v that comes from the bridge rectifier isn't connected to the other grounds in the circuit. If you look at the schematic, there are two grounds that are shown in the circuit, the output of the bridge rectifier, and the one that the #3 led is connected to. Those two grounds are not connected in the circuit. The ground from the led is connected to the other grounds on the board and also the ground that goes to the CPU board. The ground used with the 7905 isn't connected to the rest fo the board at all...
So I put the board back in the game and I locate and measure all of the test points on the board:
TP1 - Supposed to be 5.1V, measured 5.2V
TP2 - Ground
TP3 - Supposed to be 12V reg, measured 12.46V
TP4 - Supposed to be -4.9V measured -1.2V
TP5 - Supposed to be 12.8V unreg, measured 14.21V
TP6 - Supposed to be -14.8V unreg, measured -14.62
So then I also measured from the 7905 pin 1 (ground) to TP2 and I find my missing -3.8V.
So it appears that my grounds are not the same and the -5V circuit ground is floating at -3.8V.
Have any of you guys seen this problem before? Is it the bridge rectifier? Something wrong with the input A/C voltage?
I do have a scope, but I'm a noob with it and I don't know where I should hook the ground clip to start probing this circuit...
TIA, Stan
IANAEE. I am a S/W Eng. by trade which means that I can understand the basics of an arcade schematic and I understand address and data buss's, address decode logic, etc.
In other words, I know enough to be dangerous..
The CPU board flashes a 1-3-1 but that doesn't matter since I don't have good voltages coming from the power supply. The 4116 RAM can't work without all 3 voltages.
I have two leds lit. #1 and #2. #3 won't light.
I should mention that this is the later power supply board with the power transistors located on another board with a large heat-sink. I'm using the schematics from Robotron to troubleshoot the problem since they seem to match my board.
So the first thing that I did was to purchase a cap kit and installed it. Same problem.
So I'm starting to get a bit aggravated with the power supply. So I start looking harder at the schematics and the -5V supply looks like a standard power supply circuit that I've seen many times.
There's the normal A/C input, a bridge rectifier, filter cap, voltage regulator. Normal stuff. The only part that is odd is that the outputs of the bridge rectifier outputs are hooked up backwards to provide negative voltage. But since this is a -5v circuit, it makes sense.
So I grabbed my multi-meter and started probing the board. I'm actually getting -1.2v out of the circuit. I checked the legs of the 7905 to see what I can find..
From pin 1 to 2, I have -10.8v
From pin 1 to 3, I have -5v.
WTH?
So now I really start looking at the board and I pull it out of the machine again and I start doing some continuity checking and actually looking at the traces on the board. What I see confuses me. The ground that is labelled as 0v that comes from the bridge rectifier isn't connected to the other grounds in the circuit. If you look at the schematic, there are two grounds that are shown in the circuit, the output of the bridge rectifier, and the one that the #3 led is connected to. Those two grounds are not connected in the circuit. The ground from the led is connected to the other grounds on the board and also the ground that goes to the CPU board. The ground used with the 7905 isn't connected to the rest fo the board at all...
So I put the board back in the game and I locate and measure all of the test points on the board:
TP1 - Supposed to be 5.1V, measured 5.2V
TP2 - Ground
TP3 - Supposed to be 12V reg, measured 12.46V
TP4 - Supposed to be -4.9V measured -1.2V
TP5 - Supposed to be 12.8V unreg, measured 14.21V
TP6 - Supposed to be -14.8V unreg, measured -14.62
So then I also measured from the 7905 pin 1 (ground) to TP2 and I find my missing -3.8V.
So it appears that my grounds are not the same and the -5V circuit ground is floating at -3.8V.
Have any of you guys seen this problem before? Is it the bridge rectifier? Something wrong with the input A/C voltage?
I do have a scope, but I'm a noob with it and I don't know where I should hook the ground clip to start probing this circuit...
TIA, Stan
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