Game Plan MPU-2 help needed

jeff412

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I've got a Sharpshooter II and I am having difficulty with the MPU. I get no flashes from the LED. It comes on and is very dim. All of the voltages are good. The reset circuit is working. I have 4.7 V on pin 26 of the Z80 processor.

Jeff
 
what are your voltages at the test points? what plugs are connected when your trying this, have you repaired any acid damage? what all have you done to try and get this booting.
 
The Sharpshooter II MPU had some serious battery damage. All of the roms, the z80 and the z80 CTC experienced some pin damage. I got an MPU-2 from a Coney Island when I bought the game. There was some battery damage on it. I replaced the enitire reset circuit. I also jumpered it to accept 2716's and burned some new roms based on the files on www.pinrepair.com. Here are the voltages as I remember them. I got the same results with another power supply.

J1 pin 3: 27 V
TP1: 4.7 V
TP5: 1.8 V
TP2: 12.5 V
U11 pin 25: 4.7V
U11 pin 26: 4.7V
TP7 to cabinet ground: 0V
 
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have you jumped u11 pin 25 to u11 pin 26 with power to the board?

when you said the reset section has been rebuilt, did you replace everything? or do the mentioned dallas mod?
 
Yes. I still don't get a flash. I just have a dim LED. According to pinrepair I need 4.9 V at TP 1 and 2.1 V at TP5. Do you think 4.7 and 1.8 are close enough?



Jeff
 
me personally i say close enough, but im not positive as to what it MUST be. how much resistance from the ground TP i believe its TP7 to ground of the machine do you have? might possibly not have good ground due to acid damage?
 
I'll check that, but I'm assuming it will be pretty low since I had nearly 0V between TP7 and the backbox ground.

Jeff
 
4.7 on the +5v bus is too low.

The spec for TTL logic is 5v +/- 5% or 4.75 to 5.25v

Things start to get weird as you approach either limit... so check your power supply.

Next, if a CPU isn't working you must start with the basics - Power, Ground, Clock, Reset, Control Lines, and Address/Data busses.

Read the schematics and check for continuity on those lines from the CPU to the other chips on the MPU board. Patch any bad traces and test.

If all those traces are good then watch the reset line with a logic probe. When the board is powered up it should be momentarily low then flip to high and stay high. After that, look at the clock pin and see if you have a good clean clock signal. Then check the read, write, IRQ, and other control lines for proper signals.

If all that is working then check the ROMs for activity on the control lines (/CE and /OE) - keeping in mind that some systems ground one of those so check your schematic. No activity on the control line (that isn't grounded) means you need to check the address decoding circuitry. Those control lines need to be LOW for the chip to come active. If they are working then check the RAMs for address/data bus activity and for the control lines on them.

It's not hard to troubleshoot the older systems... but wtih all the acid damage you MUST double and triple check that the traces are good.

CONTINUITY TESTS DO NOT WORK IN ALL CASES. Use your ohmmeter and make sure you have close to zero ohms. My multimeter reads zero on continuity if the traces are less than 25 ohms in resistance. This can lead to problems. I had a Pac Man board on the bench a couple of months ago that refused to boot. There were a few traces reading from 9 to 20 ohms in resistance when they should've been almost zero. A few patches later and the board was working fine.

RJ
 
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