USING THE DEAD TEST CART 781220


BIT  FLASHES   C64   SX   C128-B0  C128-B1

7	1      U12   UA4   U45       U53

6	2      U24   UB4   U44       U52

5	3      U11   UA5   U43       U51

4	4      U23   UB5   U42       U50

3	5      U10   UA6   U41       U49

2	6      U22   UB6   U40       U48

1	7       U9   UA7   U39       U47

0	8      U21   UB7   U38       U46


     I heard somewhere that the C128 doesn't test RAM at bootup like the 
C64 does. A RAM problem is more difficult to diagnose in the 128 because 
it has two banks of 64K RAM, 16 RAM chips! In the C64 mode, it does test 
bank 0 and that can be useful as a diagnostic. The RAM banks are accessed 
via two CAS lines at pins 15 of all RAM chips. Resistors R29 and R30 feed 
CAS signals to the two banks, so if those lines are swapped on the 128 
motherboard (lift one end of each resistor and cross connect the ends to 
the board), the computer will access bank 1 as bank 0 and vice versa. If 
the computer will not boot in one mode but works OK or marginally in the 
other, swapping the banks may help localize the fault. If the symptoms 
change, you know you have a RAM problem. 

     The Dead Test Cart was apparently designed for the C64 but its book 
states it can be used on a C128 as well but the description of how it 
works doesn't ring true. I had a C128 on the bench that would show a 
blank 128 screen at boot and a garbage screen with border in the 64 mode.
I ran the Dead Test Cart and got one blink indicating U45 was bad. OK so 
far. Why, if the book is correct, did swapping the two banks of RAM by 
lifting R29 and R30 and exchanging their CAS outputs allow the Dead Test 
Cart to ignore the bad RAM? My conclusion is that the cart can only test
one bank at a time, not both as the book states.