Memory Bits 7 6 5 4 3 2 1 0
Row D 1 0 0 0 0 0 0 0
Row E 0 0 0 0 0 0 0 0
Replace Chip Z506 Z505 Z507 Z509 Z508 Z510 Z511 Z512
The above example would indicate that chip Z505 is failing the RAM
Test and should be replaced. Before replacing any chips on the RAM
Memory board the following troubleshooting hints should be read.
• When a failed RAM Memory board exhibits constantly changing
failures the demultiplexing chips (Z503-Z504) are probably
defective. Both chips must be the same manufacturer and the same
date.
Figure 5-13. RAM Memory Board Locations, 16K
• When a failed RAM Memory board exhibits a 'Solid' failure (the
failure does not change when the unit is tested in the continuous
mode), probably that particular RAM chip is defective. (Second
choice, the Data Buffer chipr Z502).
• If the test fails at address hexadecimal 4000 (Rows B and C: 0100
0000 0000 0000) a nd the data read back is hexidecimal 40 (Rows D
and E: 0100 0000), the same as the high order address byte, the
board under test is probably never being enabled. The Board Select
chip, Z501, could be defective.
• If the RAM Memory board being tested crashes the system (Green
Screen) probably the Data Buffer chipr Z502, is defective. Second
choice, Board Select chip, Z501, as this also handles the refresh
signal.
• Use only known good chips and leave substituted chips in the
printed circuit board until the failing chip is identified.
5-25 System Service Manual
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