Tech: System 1 Board repair question...

vbtalent

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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,
 

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the VSS and VAB are merely shortcuts in the circuit diagram, so that connecting lines didn't have to be drawn which might confuse the diagram.

so, all places that are labelled VSS are connected together, and similarly with VAB.

at least that's the normal convention in circuit diagrams.

there are also some agreed-upon definitions for various voltages in circuits as well - see, e.g., http://encyclobeamia.solarbotics.net/articles/vxx.html where you find Vss usually means ground. that doesn't *necessarily* mean it means that in this diagram, but it might. Vab doesn't have a usual meaning that i know of, but i've been out of my circuit diagram reading days for a looong time :).

EDIT: oh, and the -12V and +5V arrows that go to "nowhere" just meant that they connect to the power supply voltages of those values. that is common convention, as is not connecting together all points that are at ground, but rather just showing them connecting to the "ground symbol". it's just shorthand to save drawing lines around on the diagram.
 
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Slam switch has been bypassed on the cpu.

CPU board is out and on the bench with an led on z16 to see when/if it boots as described in pinrepair.

The thing that is strange is that at tc1 pin 14 never goes negative which is why I think it isn't booting...

I didn't have a way to check the crystal so I just replaced it, no impact.
 
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the VSS and VAB are merely shortcuts in the circuit diagram, so that connecting lines didn't have to be drawn which might confuse the diagram.

so, all places that are labelled VSS are connected together, and similarly with VAB.

at least that's the normal convention in circuit diagrams.

there are also some agreed-upon definitions for various voltages in circuits as well - see, e.g., http://encyclobeamia.solarbotics.net/articles/vxx.html where you find Vss usually means ground. that doesn't *necessarily* mean it means that in this diagram, but it might. Vab doesn't have a usual meaning that i know of, but i've been out of my circuit diagram reading days for a looong time :).

EDIT: oh, and the -12V and +5V arrows that go to "nowhere" just meant that they connect to the power supply voltages of those values. that is common convention, as is not connecting together all points that are at ground, but rather just showing them connecting to the "ground symbol". it's just shorthand to save drawing lines around on the diagram.

OK, thanks for the link and the information, very helpful and informative.

I do have -12v at R162 and +5v at Q6 but still TC1 Pin14 never goes negative..????

Looking at the schematic is it reasonable to assume that the reset circuit is nearly self contained except for the voltages and that if I'm not seeing the -12v on TC1 Pin14 that it has to be one of the parts, not a spider chip, etc.?

Looking at the schematic I don't see how the crystal even ties into the Reset circuit unless its related to voltage somehow.
 
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Been through the "Reset circuit" component by component and just don't see it... arrrgh?

Everything seems to be correct, all the voltages, etc. Anyone have any other ideas on how to fix or where to look?

Thanks,

EDIT:

Question about the transistor image. Is this the transistor that holds the voltage at -12v and then goes to +5v? I was thinking about applying -12v to the +5v leg for a moment while booting the board to see if it would boot, thoughts?
 

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Well I finally got the board to boot, ultimately it was a bad Z22 and a cut trace, the "Reset Circuit" was the culprit...

Now that I have the board booting I put it back in a working game and I am not able to get it into test/reset mode, anyone have any ideas on that?

It also wont take credits but I was assuming they are related somehow maybe the book keeping needs to be cleared?

I was focusing my attention on SN7416n the buffer right before U3... thoughts?

VB,
 
For anyone following this thread I was able to get into the test/reset mode after fixing the slam switch bypass, again. I originally had jumpered C2 but after a few conversations with a seasoned System 1 technician he suggested bypassing the slam switch earlier on the CPU board near the J6 connector by grounding Pin2 to Ground.

The only issue that I am experiencing at the moment with this board is that player 1 and 2 display is missing a segment as well as the status display which I think is Z16...

Hopefully my experiences will help someone in the future.

VB,
 

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