I have a system 9 board I am trying to repair for a friend. He said the person he bought it from said it worked. It did not when he put it into his Space Shuttle. U41 got very hot so he did not try anymore and brought the board to me. U41 was indeed burned up, case cracked. I replaced that. U11 was also missing but from what I can tell that is the sound cpu.

When powering up on the bench it comes up with 7 on the led display. I have replaced the cpu with 6802s, replaced all the PIAs plus another few chips plus installed a 6116 NVRAM. Still get a 7. Programed a 27128 eprom with the Leon test program, still get a 7. Went through everything on Leon’s write up and everything checks okay but still get a 7.

I have been testing address, data and signal lines with my logic probe. I checked a couple with my oscope and they looked okay.

Anyone have some pointers on how to trouble shoot this?

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I have replaced C9. Reset works fine and cpu appears to be running, all address and data lines toggling and looks okay with a scope as far as I can tell. Will need to get a working board to compare with to be sure. It seems like it never gets to read the eprom but all the signals look okay to me with my limited knowledge of these boards. I will continue to replace chips and see if I come up with anything.

Finally got this going thanks to MarAlb sending me the file for his SYSTEM 9 TEST EPROM.

Game eprom was bad as well as a couple logic chips. With the test rom was also able to find three bad transistors in the switch matrix.

Thanks Marco