Ah, but I do have 5 litres of wd40
If wd40 then please use the switch cleaner. Originals main purpose is eliminate water which in your case is unlikely. Also it can become sticky and be a "magnet" for dust.
The Comp mod pcb stopped working as well, so I'm back to rf output. To be honest the new trimmers seems to have made the display a bit more stable.
Someone on Facebook suggested that the booting issue might be down to c27 (the one that the plus reset switch is soldered to). I've ordered a recap kit plus a simple capacitor for another go at the comp mod. Also a simple digital oscilloscope from China. Think this is going to be a long term project .......
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Holding the reset button for at least two seconds, prevents the Z80 from entering the special reset mode if C27 has died.
The most likely cause of intermittent operation is either a poor connection somewhere (poor chip/socket connection, dry solder joint, cracked PCB track, damaged plated through PCB via) or a DRAM chip fault (either ‘lower’ or ‘upper’).
I’ll also scope the supply rails to see how much noise is getting through to the memory and video chip. Seen some nasty traces on a couple of YouTube vids which recapping the regulator/inverter circuits made a great difference.