My Issue 2 continuing problems and Issue 6s fixed

[redgatemodels]

Better use special potentiometer cleaner like "Kontakt Chemiereiniger Spray 601"

Ah, but I do have 5 litres of wd40

[Paul]

Do you mean real wd40 or their switch cleaner?

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.

[redgatemodels]

I've ordered a couple of new trimmers, less than 3 quid

No point messing around, I've got keyboard connectors to fit anyway

[redgatemodels]

Trimmers arrived today, working from home tomorrow, let's see if it fixes things

[redgatemodels]

Which it didn't

[redgatemodels]

It’s not just the inability to sort the colour out, it has an intermittent won’t boot/boots fine. One moment it’s throwing lower ram faults on the diagnostic rom, next minute it’s fine. Problem is I just don’t have a week solid to track it down….

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 .......

[1024MAK]

C27 has a really easy life, hence it’s normally fairly reliable. If you suspect C27, or the reset button, test the voltage across C27. It should normally be approaching +5V but will not quite get to the normal supply voltage. When the reset button is held, the voltage should fall to 0V. When released, the voltage should go back to nearly +5V. A ‘scope will show the voltage ramping up from 0V rather than a sharp rise.

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’).

Mark

[redgatemodels]

Won a classic cased Spectrum 48K today on ebay, so I'll have something to compare with etc depending on if it works and what series it turns out to be. With luck will end up with a 48K and a 48K Plus lol

[redgatemodels]

My 'scope is in the country too, so should have that this week sometime

[redgatemodels]

Ok, got my little digital ‘scope and have been looking at the service manual. It’s a bit vague about where to probe, especially in the part a) where it talks about connecting an RF lead to the telly. Is there a better set of instructions out there?

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.