Pixel garbage with floppy activity

[pixelpusher]

Hi folks,

one riddle for the hw experts in the forum: I've a machine which shows some pixel garbage in the color modes moving over the screen if the floppy is active (e.g. loading Gembench like in the screenshot) and the machine was already warm - it's fine if the machine is cold and I can't see the issue in monochrome mode either. The floppy is a Sony MPF920 (running as DD drive in this machine), the machine is attached via exxos's UBE video adapter to a LCD screen:

PixelGarbage.jpg (97.94 KiB) Viewed 3719 times

- I swapped in different power supplies (all re-capped) like ASP34-2 or DVE DSP508A (that work properly in other machines), but the problem remains
- Actually running Gembench in a loop for an hour shows no issues (stability is fine and while running and not accessing the disc, there's no pixel garbage)
- testing the memory with YAART021 for an hour showed no problems either

Any "usual supects" that come up to your mind? (my next test would be replacing the 16V 4700 uF capacitor on the mainboard)

[exxos]

What motherboard is it exactly ? Some STF machines (without modulator) were terrible for that kinda thing. There is no fix for that, other than get a STFM machine which has a better board revision normally.

[pixelpusher]

What motherboard is it exactly ? Some STF machines (without modulator) were terrible for that kinda thing. There is no fix for that, other than get a STFM machine which has a better board revision normally.

It's "the" other Mega ST 2 I have here (which has no solder joints for ram extension and which also had a quite beaten ASP34 psu I had to repair - there was no output).

MEGA ST CA 200092, C100501-001 Rev 1.0

[exxos]

Is it missing the filter caps on the RGB port ?

[pixelpusher]

Is it missing the filter caps on the RGB port ?

I've no schematics for this board (revision) - any specific location where they should be? This is how it looks:

MegaST_MB.jpg (618.27 KiB) Viewed 3675 times

(mainboard is in stock condition, as far as I can say from looking underneath at the soldering - and I haven't removed any caps):

[exxos]

Add some 47pf caps on the rgb lines to gnd. Also slap a 1,000uf cap or higher on the 5v to the floppy.

[pixelpusher]

Add some 47pf caps on the rgb lines to gnd. Also slap a 1,000uf cap or higher on the 5v to the floppy.

Thx - will give it a try once my toolbox is re-stocked next week (couldn't find anything in it below 2.2uF to put on the rgb lines...)

[pixelpusher]

Add some 47pf caps on the rgb lines to gnd. Also slap a 1,000uf cap or higher on the 5v to the floppy.

I added a 2200uF on the 5V line (and also replaced the large 16V 4700uF on the mainboard) - no change. I'll check the 47pF once I get hold of them ...