Annoying A1200 screen flicker / wavy on disk access

[mykrowyre]

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

[mykrowyre]

@mykrowyre, I've seen this to varying degrees and it has varied for me by screen and connection. Sorry if I've missed it on previous posts but how do you connect e.g. RF modulator, composite, RGB, and to what type of screen?

RGB.  I see the disk drive pulse on composite signal also but it doesnt effect the composite display only RGB because I think the issue is the sync signals.  On RGB it causes the sync signals to dip and the screen jumps.

[mykrowyre]

oops replied to myself

[mykrowyre]

From memory D215A. Is there a component there?
Fix is normally, ceramic cap across collector and emitter pads.

Yes a cap is there. The 1.2V reference is perfectly stable.  Anything pulled up to 5V pulses with the floppy drive access.

I measured about a 150mv dip each time the floppy ticks.

[mykrowyre]

Hi,

Simple answer is “Yes”, have had this even on a Rev 1D1 mainboard. The answer maybe that the top RF shielding is needed. Or the drive motor has become too noisy on the drive.
Is the drive by chance a Panasonic one?

Actually yes it is a newer panasonic drive.  This is an Escom Amiga made in France.  I've suspected it could actually be the floppy drive drawing too much current on motor start.  Its not a shielding problem, its the 5V rail dipping and the monitor loses sync, I can see it on my scope.  It happens even with all shielding installed.  I've tried different power supplies so its not the supply.  I've tried a different video cable.  This is a bare bones mainboard no accelerator.