I bought this unit a few months back and have been looking at it between other projects but now its time to sort it out. I have spent a lot of time with the scope trying to understand the digital signals around the MPU and the video chip.
What I have found so far. No HSYNC or VSYNC timing pulses from the HD63484 chip, but I did notice that when measuring levels with the DMM the LCD did start scanning line by line as I touched the HSYNC connection on the video chip with the probe which suggests to me that the LCD is fine and just waiting for a sync pulse. All clock lines are present and healthy at around 6.4Mhz, the power on reset delay is working perfect with a 214ms delay to the processor and video chip.
My initial assumption was a failed video chip but replacement made no difference.
I have looked in more detail at the signals from D0 to D15 on the MPU and all look healthy and visually similar when viewed on the scope, the signal lines then go in two different parallel directions with one direction passing through a pair of HC245 transceivers then on to the Scan/AD board, the other direction passes through a pair of AC245 transceivers and then it gets more confusing.
The output of one the AC245 chips feeds directly to the D8 to D15 pins of the HD63484 video chip and the data viewed here is again healthy and visually the same for each data line, all good.
The output from the second AC245 connects to the D0 to D7 pins, but also connects to the M66011FP serial/parallel chip and the ADV476 Colour RAMDAC. When scoping around the D0-D7 lines at the video chip it looks as if the data is becoming corrupted/changed. I have what appears to be a good data stream on 3 of the eight pins but very unusual pulses on the other 5.
The problem I have is my experience with digital electronics is very limited, so with that in mind I may just be mis-interpreting what I am seeing. It appears to me that the video chip is not receiving the correct information for it to operate correctly. My suspicion is one of the other chips sharing that part of the data line has gone bad, but that is just a best guess based on the little I know so far.
I welcome any input from anyone that be able to put me on the right track with this one as I am getting a little lost and don't want to just start randomly replacing ICs which may or may not be at fault.
What sort of signals should I expect to see on the D0-D15 pins?
I have added a photo with the data paths marked, what isn't shown is that the two AC245 transceivers are between the MPU and the rest of the lower chips but are mounted on the underside of the board. ( As a quick check I have removed and swapped around the AC245 chips but the data issue stays the same)
Any help or suggestions as to which direction to go next are greatly appreciated.. Karl