Help fixing Vector-06c

[cyb2600]

I'll look at the sync with the oscilloscope when it happens again. 80% of the time I get a stable picture, where the screen turns black when I hit the reset buttons and the buzzer stops. 20% of the time I get the rolling picture and it doesn't respond to the resets.

This is what the composite sync looks like when I have a stable picture.


Edit: Yes, I added a 220uf electrolytic capacitor across C33. I made every modification in that circuit diagram except for wiring the resistors directly to the inverter. When I did that I only get a solid white screen.

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Perhaps I need to elaborate on the last sentence a little bit: Vector-06c uses a primitive ROM shadowing technique. On cold boot / after hard reset the motherboard gets initialized in a way that the CPU reads data from the on-board ROM but writes data to the RAM. And there's a flip-flop that can be switched either manually by simultaneously pressing the two rightmost buttons in the upper right, or (in later revisions of the motherboard and the bootloader) programmatically by the bootloader firmware). Once the switch did happen, the bootloader ROM is no longer accessible unless the next hard reset.

When I boot the computer up, all 4 chip selects on the ROM are high. When I press the reset buttons, I see continuous data on the two inverted chip select inputs. Should they go high again after the boot ROM is copied to RAM?