I have a 1D4 mainboard, and I have this annoying problem that the screen flickers and become a little wavy with any disk access. The floppy drive tick will cause the screen to jump, especially when inserting or removing disks.
I'm aware of the A1200 "flicker fix" on the Video DAC reference line where a cap is installed to make sure the 1.2V is clean. That cap always exists on my board, and the 1.2V is clean and does not have any trace of noise.
The 5V rail however which runs the floppy drive is very noisy. Each time the drive ticks the voltage dips. When the floppy tick is disabled (sticking a disk in), the screen stops jumping, but is still wavy any time I'm accessing the HD so the noise on 5V is still there just not as bad.
Since the 5V rail is used to pull up the Horizontal and Vertical sync pins, I think this may be where the video is being effected as 5V rail sags.
I've tried placing various capacitors across that 5V line to ground but it seems to have no effect at all.
I thought it might be the power supply, so I removed my accelerator and it didn't seem to have any effect on voltage.
Obviously it still could be a the power supply.
I considered that I could have a bad floppy drive which is pulling excess current, but even when unplugged the screen is wavy on HD disk access.
Has anyone run into this before?
Thanks