Quoted from woodmedic:... I installed a socket and a new 7400 at Z13. I also replaced the 7404 at Z11 which was already socketed. The game boots fine and sounds are working. Top half of the switches are still not registering. Looking at the diagram, other areas that could be bad are Z33, Z15, and U4. I should also note that now the display only shows all zeros while in attract mode. ...
Hi, your analysis is fine.
Indeed, when there was a short circuit between the 24 volts of the coils and the switch of the bumper, it injected high voltage into the matrix of contacts.
On Genesis, the bumpers are the SW71. So there was 24 volts on Strobe 7, and/or , Return 1.
Since Z13 burned, we can assume that it was on Return 1 that there was the short circuit.
This is a case that we often meet on cards that are repaired.
Thus, Z13 burns immediately and sometimes, the 6532 (U4) which is downstream too. But in this case, usually, it is all the port A that toasted, and thus, all the matrix is dead. It is very rare that Z15 (or even Z33 ???) are affective.
Since it is possible to start a game (button REPLAY = SW47), or the test mode (button TEST = SW07), it means that the Return 7 signal works (so Z14 - 7400 is good), and also the strobes (Strobe 0 and Strobe 4) so, we can assume that Z11 - 7404 and Z12 - 7404 are also OK.
You should be able to check, using the test mode, that some DIPs (eg DIP 1) are working. So, this means that your Z15 - 7432 is also OK.
But, as Z13 and (may be) U4 are dead, you notice that the half part of the matrix is out.
And as the half part of DIPs is driven by the same circuit, this mean that some DIPs should be permanently read to "OFF".
When Z13 is burned, this affect S5..S8, S13..S16, S21..S24 and S29..S32.
And indeed, when S23 and S24 are read as "OFF", the high-score is no more displayed in the attract mode.
This explain why you see "0000000" (scores) but no high-score. This is a coherent behaviour.
To resume, the issue is simple : either Z13 is dead and may be U4.
For sure, as you change Z13, it should be U4.
But it's strange because, usually, U4 completly burned (and system don't start), or sometimes partially (but often all port A burned).
Before to replace U4, check your replaced Z13.
Be sure that the new component (7400) is in good condition (if you own an EPROM programmer, they often include a component tester). Maybe the short circuit damaged tracks under Z13. Check that it is well fed.
Hereford, TX
