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SCRAMBLE

FAULT

Dead boards with random static characters filling the screen. Board has been heavily modded to run Super Cobra with additional Eproms stacked on top of other eproms. Intention is to repair boards and then convert back to Scramble as it was originally. Boards are Bootlegs.

DIAGNOSIS

Boards were powered up and initial checks showed that the screen displayed a static random coloured pattern of characters. Suggests that the boards video circuits were indeed working but it was not booting up.

Using the AR81, the Z80 was removed and the Z80 pod plugged into the socket on the PCB. Attempted to read back some data from the first Eprom at address 0000h to 000ff. The data read back was grouped in blocks of 4. IE addresses 0000 to 0003 gave data as 01h and from address 0004 to 0007 was another data pattern. This repeated though the full block read back.

Suspected the 8216 buffer chips on the databus from the processor so disconnected the Z80 pod and used the ADT on the devices. These tested OK with o stuck pins flagged up. The next line of attack was the three 74LS367 buffers on the Address lines. Again these were tested and the last device checked flagged up pins 3 and 9 as stuck HIGH. This chip refused to pass the ADT test. Pins 3 and 9 are in fact address lines A0 and A1 which would explain why blocks of 4 in the data was happening. These pins stuck are outputs so the decision was made to replace this device with a new part.

Retested the chip after changing and the stuck pins had gone. More random states were flagged up with the ADT which would indicate activity on the address lines.

Switching over to the Z80 pod again and running a memory read dump gave correct data this time showing that the eproms were reading OK now.

Connecting a monitor up to the board and replacing the Z80 into its socket showed the boards were now booting and running though the attract screen! The next stages will be to check the other functions such as sound and buttons.