Annoying A1200 screen flicker / wavy on disk access

[screwtop]

Sorry, I should have watched your video and read your last post properly! Glad you got the colour fixed on the monitor - it's so often something simple like a connector or cabling.

Has your A1200 been recapped? Leaked electrolyte can weave all kinds of evil magic on the motherboard, including making large areas become conductive when they shouldn't be. If it has been recapped (or had other work done), check carefully for solder bridges; I had an A600 with ticking noises in one audio channel after recapping, which was caused by a tiny solder bridge where the solder mask was deteriorating, which was shorting one of the interrupt lines into the audio signal.

[mykrowyre]

Yeah... I recapped it long ago... It only happens on disk access so I'm certain its not the monitor.  I noticed its much worse with DF1 disk access than DF0.  It's clearly something to do with noise on the power rail but I still haven't found the source.  It's possible both drives need to be recapped. I may try recapping DF1 to see if it has any effect.

[F0LLETT]

Yeah... I recapped it long ago... It only happens on disk access so I'm certain its not the monitor.  I noticed its much worse with DF1 disk access than DF0.  It's clearly something to do with noise on the power rail but I still haven't found the source.  It's possible both drives need to be recapped. I may try recapping DF1 to see if it has any effect.

Is D215 populated?
If not, add a ceramic cap or ferrite across the bottom pads.

[mykrowyre]

Is D215 populated?
If not, add a ceramic cap or ferrite across the bottom pads.

I previously installed a new 1uF cap across 215A which is unpopulated.  No improvement.

[screwtop]

Now that I think about it, the later Escom floppy drives might use only the +5 V line, which would be consistent with the voltage dropping on the +5 V line. I was thinking that +12 V runs very close to the video signals, both at the output connector and on the motherboard. Still, I wonder if it's worth scoping the video output signals while the floppy drive is active but without a monitor cable connected. And of course lubing the drive. Any chance of testing the floppy drive from a bench power supply that would show current draw?

Just to confirm, since you mentioned checking resistors and diodes at the video output: R250-R253 read the expected 4.7k?

[F0LLETT]

I previously installed a new 1uF cap across 215A which is unpopulated.  No improvement.

What about C215 and R215. If you have them both populated, try removing R215. If its still same, try putting it back and removing C215. If not, then put it back on.
Just when had this issue in past, either D215A, R215 or 215 solved it.