bad Hsync at top of screen

 Improved Video Sync Generation on a Rev 0 Motherboard

A few monitors have trouble maintaining horizontal sync lock on the video signal generated by a stock rev 0 motherboard.  The loss of lock usually occurs during vertical retrace.  This results in horizontally distorted video at the top of the video display.  I only heard of this problem recently, but Apple must have known about it early on, as they made improvements in this area when they came out with the rev 1 motherboard.

The change detailed in the unshaded area of the top schematic will bring a rev 0 board up to later rev sync generation standards.  This should solve this problem.  The change takes just one 74LS02 chip (placed into a socket soldered into rework area B-14) and 13 wires.  This change is totally reversible and does not require modifying your motherboard.

Make sure that ground is connected to pin 7 of the 74LS02, as the rework area is designed to allow a 16 pin chip in this spot.  

Note that C11-3 is connected to two locations.  The wire that is connected to B14-11 is connected to the pad on the back of the board.  The wire that is connected to B14-10 is connected to the lifted pin.  All other wires that go to a lifted pin location should be soldered to the lifted pin, not the pad.

This change took a little over an hour for me to implement and test.

section layout

The bottom schematic shows the original, much simpler rev 0 sync generation.

The pictures below show the top and bottom of a  rev 0 board with this change applied.

transistor packages
Four wires are connected between lifted legs on parts C-11 & C-13 to pins on a 74LS02 placed into a socket at B-14
transistor packages
Nine wires on the bottom.  Two are the short jumpers between B-14, pins 3 to 4 and from pin 7 to ground on B-14.

A Different Fix

I found that there is a very simple hack that can also be helpful if you experience this problem.  The video signal seems to have a lot of noise in it and I believe the distortion is due to the horizontal sync lock of your monitor attempting to lock onto this noise.  Solder one side of a 10K pot to each side of the 2K resistor in the video driver circuit.  Try adjusting the pot, which reduces resistance, to see if you can make the distortion disappear.  Reducing this resistance raises the sync signal higher above the noise floor.  Also note that it also causes your composite video output signal to move further from NTSC standard signal levels.  This may degrade the video signal, if carried too far.

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