Mem Expander 1 (bell games) or repairing a locked bally mpu with some great italian stuff...
While I was repairing a bell super bowl (a x and o conversion), I found a little pcb mounted over the mpu. There where no game roms in U2 and U6. The little pcb (labeled bell memory expander) was connected to J5.
There are two 2716 eproms and some adressdecoding stuff (done very complicated with a 74ls00 and a 74ls00 , I think one 74ls138 would also do the job) on this pcb.
Here you see the schema of this pcb:
How does this memory expander works:
All adresslines and datalines are availiable at the MPU connector J5. Due to the fact that no games roms are present on the bally mpu and the adressdecoding is done on the memory expander board, the 6800 (u9) reads the game roms completely out of the memory adapter after power on. !!
I took the game roms from Sterns "pinball", this game fits on two 2716 eproms, and inserted them into the memory expander. (I think you could use every other bally / stern game that fit on two 2716 eproms, like mata hari)
I found out that this pcb with these eproms is quite handy for reparing mpus where the LED stays on continually after power on.
Proceed in this way with your locked mpu:
1) Remove game roms (U2 and U6) from the mpu.
2) Connected the memory expander to J5.
3) Power on.
Now, If you get the first flash from your mpu, you know that the following components are good on the mpu board:
U9 and U11 and their sockets
no shorts (adress and datalines)
And you also know that your problem may be related to:
bad sockets (U2, U6)
bad roms / eproms
wrong jumper settings
bad U14, U17 or U18 (but if your LED flashes seven time with the memory expander, you can be quite sure that these chips are working)
If your mpu is still locked on with the memory expander mounted you should check:
the reset circuit
the U9 and U11 and their sockets
the clock circuit
for a short (data or adresslines)
With this little board you can narrow the search direction for the culprit !