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    So either E4 or D4 are bad, or there is something else on A8 holding it low (be that a short circuit to 0V or something else driving A8 when it shouldn't).

    Dave

    Comment


      E4 is a a hex buffer i believe, pin5 being an input to one of the buffers and 6 being the output of it, so 0 output from a pulse is wrong? correct?

      Comment


        Correct.

        Pulse going in ==> inverted pulse coming out. Simples!

        Dave

        Comment


          Parts ordered will report back in a few days.

          Comment


            Originally posted by daver2 View Post
            ...or there is something else on A8 holding it low
            I would consider replacing D4 as it is a candidate for holding A8 low... I would suspect a 74LS157 over a 7417. I think the 7417 is open collector, that's why R37 is there as a pull-up. Potentially anything on the BA8 will cause it to short low... a good time to remove any dodgy 6540s perhaps? And order a spare 74LS157 just in case.

            Comment


              It is a little disturbing that I don't see anything that is consistent with A5 or A8. A5 passes through D3 and E3. A8 passes through D4 and E4.
              There is not much in common for a single point failure. One wonders what the extra ROM/RAM board adds to the problem.
              It would be difficult to trace issues with the NOP and not having 2 probes. One probe could trigger on D2 pin1 while the other looked as the various RAM addresses from the NOP.
              I'm of course assuming it is a two channel scope.
              Dwight

              Comment


                I have ordered, twin packs of, should be here by sat 74LS157 and 7417.
                I bow down to all of your knowledge i'm going to ask a really dumb question here what are you referring to when you talk about A5 and A8 are there address lines for the video memory, and why specifically are you mentioning these, is it the way that's mapped to the screen and the type of corruption shown in my video points to them as mentioned by Dwight Elvey btw yes its a 2 channel scope i just only have one probe at present.
                Also so i can get things clear in my head Nivag Swerdna you say D4 is most likely the cause and im sure your right, just in my head it is giving pulses at the outputs, maybe not correct ones granted, but he input on pin 10 is 0 which is fed from E4 pin 6 which is also 0, but the input to that buffer on pin 5 is 1, is that not very suspect?
                Once again many thanks for all the advice from all

                Comment


                  You would naturally expect the driver of a signal to be suspect if you find a signal that is wrong however it is possible that a receiver of the signal is responsible by loading (or pulling) the signal to the wrong value. If you follow the BA8 signal you will find it goes to quite a few places (actually it is this fan-out that explains the presence of the 7417 driver) and any of those consumers of the signal could be responsible. It has been observed in prior repair that the chips that commonly fail tend to be the counter, flip-flop and mux types so they are normally considered the usual suspects.
                  The reason I mentioned A8 earlier is because the screen showed corruption at 256 character boundaries which is consistent with A8 appearing low to the video (it's why blocks with A8=1 are not cleared). You also discovered the low value on BA8.
                  You should try and divide and conquer to remove consumers of the BA8 signal where you can... so depopulate the 6550 memory (not video) and 6540s (ROM) so you can at least eliminate those; put them somewhere safe for laters.

                  Comment


                    Nivag Swerdna that makes total sense thanks, and i shall depopulate the rom/ram later today and keep safe

                    Comment


                      Nivag covered most of what I was about to say.

                      The CPU produces address lines A0 through A15. However, the CPU can only drive a small number of external devices directly - hence buffers are required. You can make a very small system without buffers, but any more substantial system requires buffers (address, control and data).

                      The address lines are buffered and change their name from Ax to BAx (buffered address x).

                      The buffered address lines (BAx) go all over the place (literally)... ROMs, RAMs, video circuitry and address decoding logic.

                      The video circuitry as a bit different, however. The video RAM addresses can come from two (2) locations (1) the CPU and (2) the video circuitry divider chain - hence the reason for the multiplexer integrated circuits.

                      Dave

                      Comment


                        Originally posted by Nivag Swerdna View Post
                        You would naturally expect the driver of a signal to be suspect if you find a signal that is wrong however it is possible that a receiver of the signal is responsible by loading (or pulling) the signal to the wrong value. .
                        I have worked with TTL logic gates for many decades. A failure mode on the gate inputs is invariably an open circuit problem. I don't have any single TTL IC in my possession where a gate input has failed in a manner where it can source or sink enough current to the extent that it can drag the output of a TTL gate to ground or pull it up to 5V.

                        Invariably, when a logic level is stuck high or low (excluding physical pcb shorts etc) it is the fault of the output stage in the IC driving that line that is the cause, not "loading on the line" by failed inputs of other IC's. (Though sooner or later somebody will find an exception to this "rule" and make a chump out of me).

                        So the best move, when a line is stuck low, is to just solder suck the output pin on the IC that drives that line and free it up in the hole. Then confirm that the TTL lines it was driving self pull up to around 3.5V.

                        Comment


                          One of the difficulties of looking at the video addresses from the CPU, is that to see anything happening, it has to be addressed. Normally the CPU is not doing any video access if there is nothing to display. That is why we want to use the NOP generator. It runs through all 64K of addresses.
                          The symptoms we have are that part of the screen's RAM space was not cleared. Part of the screen was incorrectly addressed, when written to. That was why I jumped on A5 but I may have not thought it out completely.
                          We've seen a lot of failures of the address muxes. That is the job of the 74LS157's. They determine which source the addresses come from, either the CPU or the video scanning counters.
                          The fact that you see a full screen, including random stuff indicates that at least the screen is accessing the full RAM address space.
                          I'd been thinking because of the switching at the 32 byte boundary that the problem was A5. Actually, after giving it clearer thought, A5 is working but part of the address higher up is failing. That would be A6 and up some place. That is consistent with D4 or E4 failing. From previous history, that would be D4 as the most likely source of failure.
                          If you map out how the video display is by address, you'll see how it works. The screen is 40 characters wide. That is not a nice binary type address( 2 4 8 16 32 64 128 256 512 1024 ).
                          So mapping binary boundaries to locations on the screen, we have some lines, expected to show at the left, ending up on the right, after 32 characters. I didn't count how many returns you entered to see what line was suppose to show up where. After giving it some thought, one can see that the are two areas not being addressed. The address 1024 does roughly 1/2 of the screen ( the last 24 bits do not show are off the end ). That would be A9. That seems to be working. Next is A8 or the 512 blocks. It is used for roughly 1/4 of the screen. Bingo! That is what we are seeing. One 1/4 on, next 1/4 off, next 1/4 on and last 1/4 on. As Dave said, that would be A8 always zero and never one. D4 or E4 but most likely D4.
                          Dwight

                          Comment


                            One of the difficulties of looking at the video addresses from the CPU, is that to see anything happening, it has to be addressed. Normally the CPU is not doing any video access if there is nothing to display. That is why we want to use the NOP generator. It runs through all 64K of addresses.
                            The symptoms we have are that part of the screen's RAM space was not cleared. Part of the screen was incorrectly addressed, when written to. That was why I jumped on A5 but I may have not thought it out completely.
                            We've seen a lot of failures of the address muxes. That is the job of the 74LS157's. They determine which source the addresses come from, either the CPU or the video scanning counters.
                            The fact that you see a full screen, including random stuff indicates that at least the screen is accessing the full RAM address space.
                            I'd been thinking because of the switching at the 32 byte boundary that the problem was A5. Actually, after giving it clearer thought, A5 is working but part of the address higher up is failing. That would be A6 and up some place. That is consistent with D4 or E4 failing. From previous history, that would be D4 as the most likely source of failure.
                            If you map out how the video display is by address, you'll see how it works. The screen is 40 characters wide. That is not a nice binary type address( 2 4 8 16 32 64 128 256 512 1024 ).
                            So mapping binary boundaries to locations on the screen, we have some lines, expected to show at the left, ending up on the right, after 32 characters. I didn't count how many returns you entered to see what line was suppose to show up where. After giving it some thought, one can see that the are two areas not being addressed. The address 1024 does roughly 1/2 of the screen ( the last 24 bits do not show are off the end ). That would be A9. That seems to be working. Next is A8 or the 512 blocks. It is used for roughly 1/4 of the screen. Bingo! That is what we are seeing. One 1/4 on, next 1/4 off, next 1/4 on and last 1/4 off. As Dave said, that would be A8 always zero and never one. D4 or E4 but most likely D4. posted a second time. to fix typo
                            Dwight

                            Comment


                              Still awaiting for parts to be delivered, but I've removed all the ram and the roms (labelled roms for reference), for info and probably expected its made no difference to the issue. Hopefully some chips will arrive tomorrow, my novice money is on E4 but I've also got replacement for D4, which will arrive first is what i will replace first.

                              Comment


                                YES Replaced E4 and its working again . Many thanks for the assistance I would been in the dark without all your assistance. Now to load that game again...

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