Hi!
·Unlike the remaining compact Macs (SE included), the SE/30 has separate VRAM, thus the regular DRAM SIMMs shouldn't be related in any way to your video issues. The VRAM contents could be copied from DRAM sometimes, but also from ROM, from other sections of the VRAM itself, or even directly computed.
·To be precise about sync signals: the electron beam keeps moving because of the deflection sweep signals coming from the analog board... the HSYNC signal only forces the beam to return quickly to the left after a line has been drawn; in the meanwhile, the vertical deflection coil has lowered the beam just a bit in order to draw the next line just below the previous one. Once the whole 342 lines are drawn, the VSYNC signal comes up and forces the beam to go back to the top of the screen as soon as possible -- while the much faster horizontal deflection keeps swinging the "emerging" beam between left and right in a funny, but invisible fashion. Even if the sync signals are missing, these movements will keep going, although at a somewhat erratic rate. You're right about the VIDOUT signal cutting the electron beam in order to "draw" the black pixels and keeping it on during the white ones.
·The SE/30 has 64 K of VRAM, over twice the required amount for the screen (about 22 K). This is in order to implement double buffering so the screen updates cause no flickering -- the computer draws in one buffer while the video circuits show the other one on the screen. Once finished, the hardware switches quickly to the other buffer and displays the completed image, while further updates can be made on the previous one. But I think there are two VRAM chips for bandwidth reasons: each one gets the same address and supplies 4 bits a time so both in parallel can give 8 bits within the same timeframe -- otherwise it would need VRAM chips twice the speed.
·I don't think there is an issue with address lines, since each address get 8 bits (or pixels; 4+4) a time, so the flaws should be at least 8 pixels wide... the UC6 chip supplies the leftmost four pixels of a "byte", and UC7 the rightmost four.
That said, I think we can rule out a SIMM problem or any issue with the multiplexers, which just put the PAL-generated address on the VRAM chips' address lines. Defective VRAM is a possiblilty, but it looks odd to me that both chips could have the same output bit flaky...
Thus, the culprit is likely to be the UE8 shift register -- OR the lines connecting its inputs to the VRAM output lines. I'd check these for continuity:
UC7 pin 2 = UE8 pin 2
UC7 pin 3 = UE8 pin 3
UC7 pin 22 = UE8 pin 4
UC7 pin 23 = UE8 pin 5
UC6 pin 2 = UE8 pin 10
UC6 pin 3 = UE8 pin 11
UC6 pin 22 = UE8 pin 12
UC6 pin 23 = UE8 pin 14
About the "magic" PAL chips... I can't rule them out since they may do some obscure, complex tasks; but it's worth to check the aforementioned connections first.
Hope this helps,
·Unlike the remaining compact Macs (SE included), the SE/30 has separate VRAM, thus the regular DRAM SIMMs shouldn't be related in any way to your video issues. The VRAM contents could be copied from DRAM sometimes, but also from ROM, from other sections of the VRAM itself, or even directly computed.
·To be precise about sync signals: the electron beam keeps moving because of the deflection sweep signals coming from the analog board... the HSYNC signal only forces the beam to return quickly to the left after a line has been drawn; in the meanwhile, the vertical deflection coil has lowered the beam just a bit in order to draw the next line just below the previous one. Once the whole 342 lines are drawn, the VSYNC signal comes up and forces the beam to go back to the top of the screen as soon as possible -- while the much faster horizontal deflection keeps swinging the "emerging" beam between left and right in a funny, but invisible fashion. Even if the sync signals are missing, these movements will keep going, although at a somewhat erratic rate. You're right about the VIDOUT signal cutting the electron beam in order to "draw" the black pixels and keeping it on during the white ones.
·The SE/30 has 64 K of VRAM, over twice the required amount for the screen (about 22 K). This is in order to implement double buffering so the screen updates cause no flickering -- the computer draws in one buffer while the video circuits show the other one on the screen. Once finished, the hardware switches quickly to the other buffer and displays the completed image, while further updates can be made on the previous one. But I think there are two VRAM chips for bandwidth reasons: each one gets the same address and supplies 4 bits a time so both in parallel can give 8 bits within the same timeframe -- otherwise it would need VRAM chips twice the speed.
·I don't think there is an issue with address lines, since each address get 8 bits (or pixels; 4+4) a time, so the flaws should be at least 8 pixels wide... the UC6 chip supplies the leftmost four pixels of a "byte", and UC7 the rightmost four.
That said, I think we can rule out a SIMM problem or any issue with the multiplexers, which just put the PAL-generated address on the VRAM chips' address lines. Defective VRAM is a possiblilty, but it looks odd to me that both chips could have the same output bit flaky...
Thus, the culprit is likely to be the UE8 shift register -- OR the lines connecting its inputs to the VRAM output lines. I'd check these for continuity:
UC7 pin 2 = UE8 pin 2
UC7 pin 3 = UE8 pin 3
UC7 pin 22 = UE8 pin 4
UC7 pin 23 = UE8 pin 5
UC6 pin 2 = UE8 pin 10
UC6 pin 3 = UE8 pin 11
UC6 pin 22 = UE8 pin 12
UC6 pin 23 = UE8 pin 14
About the "magic" PAL chips... I can't rule them out since they may do some obscure, complex tasks; but it's worth to check the aforementioned connections first.
Hope this helps,