Annoying A1200 screen flicker / wavy on disk access

[screwtop]

In my experience it's quite common to get one or more colour channels going intermittent on the 1084 monitors. The input connector at the back gets stressed (especially when cables are squished against a wall or left dangling) and the solder joints get cracked. Touching up the solder joints with a soldering iron to reflow them is often enough.

In older Panasonic floppy drives the leadscrew grease can harden and cause a lot of mechanical resistance, which could cause excessive current draw, though I think the leadscrew stepper is normally driven by +12 V rather than +5 V (though I've seen some drives that are the reverse, and of course external drives only use +5V). In addition to recapping the drive, you could try degreasing and regreasing the leadscrew, paying particular attention to the tricky ball bearing setup at the tip of the leadscrew (watch out for balls falling out!). I'd suggest testing without the Panasonic drive connected at all.

[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.

[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?