(Topic ID: 200569)

SOLVED: Genesis sys80b display issue

By jmountjoy111

6 years ago


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#1 6 years ago

I have two Genesis machines. One that I picked up fully working and the other is project. I am planning on making one super nice machine out of the two and making the other into a solid player.

I was replacing the u4 riot chip today. I added a socket for the future and put in the new chip. It solved the switch issues I was having before so I feel good about that. Here is where my stupidity took over....

I plugged the a24-p2 plug (pictured) from the reset board in the machine backward. I looked at the notches when I put it back in the board but didn’t realize there were two notches. Game wouldn’t boot. So I turned it back around and now the game boots and plays. However, the display won’t come on. The display is good I checked it in the other genesis but it won’t come on in this one. I’m assuming I’ve burnt a chip on the board. I just need some help in finding out where to go from here

Any help will be appreciated

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#2 6 years ago

It will occasionally show this on the display

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#3 6 years ago

bump for the morning crowd.

Someone has to be a system 80B expert that is willing to guide me in the right direction.

#4 6 years ago

Anyone... I don’t have a manual.

#5 6 years ago

Hello,

About the manual : read your PM
It's weird, but I do not see how the connection to the back of the A24-P2 (TC1) connector could burn something related to the SYS80B display.

In general, it is not necessary to reconnect the small RESET card because it generate more problems than it solves. The pinball can work very well without, and for a particular, this card has no use. It was only used in operation, to force a RESET of the pinball when the CPU card was hanged. Disconnecting this card while solving the problem, it is absolutely not necessary.

In SYSTEM 80B, the display works with commands and datas sent by the CPU (8-bit commands, read directly by the circuits 10939/10941). As a result, compared to a SYSTEM 80 / 80A the display circuit is much simpler.

2017-10-19_07h20_00 (resized).png2017-10-19_07h20_00 (resized).png

The data / commands are sent by the RIOT 6532 (U5 - not shown, on left of the picture). They pass through a buffer 7404 (Z16) and then into two latches 74175 (Z18 and Z20). The reset signal also passes through buffer 7404 (Z17) and this IC serves only for this purpose (not shared with matrix or decoding).

Maybe one of these circuits is roasted, but diagnosing which is difficult without a test bench or logic analyzer.
As you can see, there is no connection with the display and the TC1 connector, so I do not understand what could have been broken. If the decoding of circuit U5 (6532) is burned out, the CPU board will not work at all.

A trick also to try: invert the EPROM with the other Genesis that works. To be sure that the problem does not come from a partially erased EPROM.

#6 6 years ago

Awesome. Thank you so much. I will pull the ROMs today and see if they will work properly in my other genesis. I appreciate it. At least now I know the direction to head in

#7 6 years ago

So I didn’t have a ton of time this weekend but tonight I tried the ROMs from the other Genesis. There was no difference.

I then went ahead and replaced Z16, 18, and 20 also no difference. Could the 6532 be bad and the game still boot? I know the display is good. I hooked it up in the other Genesis.

#8 6 years ago
Quoted from jmountjoy111:

...
I then went ahead and replaced Z16, 18, and 20 also no difference. Could the 6532 be bad and the game still boot?

Yes it could. To boot, the CPU need that full amount of RAM (384 bytes) be present (the RAM is located 1/3 in each 6532 - 128 bytes).
The RAM can be good, and also the registers/timers, but one or more output-buffer can be damaged. In this case, the chip works but not the I/O.

I've seen all types of failure on the 6532: sometimes, a divider/timer doesn't work but others are fine. In this case, the chip work fine at U4 location and not if placed at U5 (because the CPU doesn't use the same divider for U4 and U5). Sometimes, the RAM is defective but I/O works. This a complex component and not all elements fail at the same time.

Note that the issue with the display comes mandatory from this area - there no other parts involved in the display system. As you replace Z16, 18 and 20, nothing remains but U5 (6532) or Z17 (7404). Another possibility is that there is a cut track somewhere (you have to search with an ohmmeter).

#9 6 years ago
Quoted from noflip95:

Yes it could. To boot, the CPU need that full amount of RAM (384 bytes) be present (the RAM is located 1/3 in each 6532 - 128 bytes).
The RAM can be good, and also the registers/timers, but one or more output-buffer can be damaged. In this case, the chip works but not the I/O.
I've seen all types of failure on the 6532: sometimes, a divider/timer doesn't work but others are fine. In this case, the chip work fine at U4 location and not if placed at U5 (because the CPU doesn't use the same divider for U4 and U5). Sometimes, the RAM is defective but I/O works. This a complex component and not all elements fail at the same time.
Note that the issue with the display comes mandatory from this area - there no other parts involved in the display system. As you replace Z16, 18 and 20, nothing remains but U5 (6532) or Z17 (7404). Another possibility is that there is a cut track somewhere (you have to search with an ohmmeter).

I will start working on that today. I have the 6532 riot chips. I am out of 7404 so I will have to order some. I’ll check continuity of all the board traces while I’m at it. Thanks

#10 6 years ago

It was not the 6532. I added a socket and put in a new one. No difference. I have the 7404 on order. I may try one out of a parts board just to see. Also I plan on checking for broken traces. I supposed I could have flexed the board at some point taking it in and out of the game.

#11 6 years ago

And the problem has been solved.... I had repinned the connectors on the power supply and one of the wires has come loose.

I appreciate he help so much!!

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