A500 issue - floppy drive dims monitor

[pyrre]

Hi, I have a very nice condition PAL A500, with Rev6a, all stock, with original chips and no mods done to it. The only expansion it has is a 512K trapdoor RAM expansion. It works fine, but has an issue in that every time the floppy drive clicks, or other wise reads, the monitor dims. It dims rhythmically with the click of the drive when there is no disk inserted. I have tried several PSUs, including ones of new construction, but, the 500 does the same with all of them. My other A500s also do not have this issue with the same power supplies, so, I think we can rule out there being an issue with the PSUs themselves. Where should I begin to look to try to fix the issue?

Many thanks,

Kirk

could be failing caps on the mainboard......