Fwapp wrote:If it still "sparkles" after the fitting the recom, try applying something suitable metallic to draw heat out of the ULA. If that cures the sparkles, it'd be worth fitting a DIP40 heatsink which might prolong the ULA's life a bit.
If that is the case, PM me your address and I'll send one over, return the favour for the sideways ROM a bit
Sorry to be the nay sayer but fitting an heat sink to a chip in an enclosed case with no air flow will do no good what so ever.
It will only serve to reflect heat back into the chip at worst or reach thermal equilibrium at best as there is no convection/conduction at all inside a closed system like the ZX81.
I base this on 45 years of building audio amplifiers and their respective cooling systems. when ever a customer said "hide the heat sink inside the case please" it always ended badly when they were insistent.
The heat has to go somewhere and inside a closed system like the zeddy there ain't nowhere for it to go.
The biggest culprit is the regulator so fit a switching jobby and remove the biggest heat source in one go or as karl says use a 9v switching power pack and at least reduce any heat the reg might give off.
Another thing to remember is that the chips inside a ZX81 do not create argon arc furnace temperatures , they are relatively warm at worst and people do tend to panic a little where the ULA is concerned.
Of all the ULA's I've knackered in the last thirty odd years it has always been down to me messing about with peripherals IE removing them when powered up etc, I've never buggered one up through heat problems and don't know of any one else who can PROVE they have either, wether or not your 81 has been used 10 times or 10,000 times in the past your ULA has survived a third of a century and is more hardy than most theorists seem to think.
As a final note, Lardo, should you or anyone on the forum be able to prove your ULA has been totalled due to excess heat I will happily dismantle one of my zeddies and send you the most up to date replacement F.O.C.