Right, I'm trying to fix a later series Defender Main board...
The screen is rugging with 1 pixel out of 8 black at the moment.
When I probe the rams, there are 4 or 5 4116s that only on pin 14 which is data out onto the Data bus on buses 2 and 3. Its not every chip, its only a few such as Ram Location 2T (D22), 2U (D23), 2Z (D27) and 3Z(D35) off the top of my head (might be 1 more).
I've looked at the video shift registers (thats as far as I can work out basically take the banked ram and puts in into some 4 bit serial flow that goes onto the colour multiplex. The relevant pins on the 74165s 9using 3Z as an example pins 3O-14 and 2P-14) also have no activity whatsoever.
Similarly the 74LS374s at 3M and 4M are also showing no activity (again using 3Z ram as an exmaple we're talking 4M-35. My had cant get round these 374s - Im thinking theyre tri-state flip-flops so presumably something else is controlling the bank select on these?) Can someone actually confirm what the 374s at 1M, 3M and 4M are doing?
But my question is where do i look next? I can see that this mP DATA BUS and index these 374s go off to the first sheet of the schems but my minds frazzled now and cant work out whats going on. As I say every connection to the 4116 shows no life on sheet 2 of the cpu board. Every other pin of every 4116 looks to be behaving normally (subject to just using a logic probe), the 7474 at 5S ahs been replaced with a tested good chip in the Video Shift circuit. Ive piggy backed the 74165s so no result and the 27257 multiplex seems to be pulsing serial 0-3 pins...
Oh Ive change 1 or 2 sockets on the 4116s but Im confident its not a chip / socket issue (this time!).
I'm open to any suggestions as Im well stuck. Thanks!
The screen is rugging with 1 pixel out of 8 black at the moment.
When I probe the rams, there are 4 or 5 4116s that only on pin 14 which is data out onto the Data bus on buses 2 and 3. Its not every chip, its only a few such as Ram Location 2T (D22), 2U (D23), 2Z (D27) and 3Z(D35) off the top of my head (might be 1 more).
I've looked at the video shift registers (thats as far as I can work out basically take the banked ram and puts in into some 4 bit serial flow that goes onto the colour multiplex. The relevant pins on the 74165s 9using 3Z as an example pins 3O-14 and 2P-14) also have no activity whatsoever.
Similarly the 74LS374s at 3M and 4M are also showing no activity (again using 3Z ram as an exmaple we're talking 4M-35. My had cant get round these 374s - Im thinking theyre tri-state flip-flops so presumably something else is controlling the bank select on these?) Can someone actually confirm what the 374s at 1M, 3M and 4M are doing?
But my question is where do i look next? I can see that this mP DATA BUS and index these 374s go off to the first sheet of the schems but my minds frazzled now and cant work out whats going on. As I say every connection to the 4116 shows no life on sheet 2 of the cpu board. Every other pin of every 4116 looks to be behaving normally (subject to just using a logic probe), the 7474 at 5S ahs been replaced with a tested good chip in the Video Shift circuit. Ive piggy backed the 74165s so no result and the 27257 multiplex seems to be pulsing serial 0-3 pins...
Oh Ive change 1 or 2 sockets on the 4116s but Im confident its not a chip / socket issue (this time!).
I'm open to any suggestions as Im well stuck. Thanks!
