Sinclair ZX80 / ZX81 / Z88 Forums

..that could've been why it didnt work..???

supposedly a -6 'shouldnt' work in an issue 6 , only -7, due to timing change, but might have been a 'selected' one that did !! -7 's are supposed to be used in issue 4 and above, except apparently samsung issue 4s which can handle -6 's .. ?!

a 6mhz cpu was probably used as it may have been more easily available than a 4mhz one.....

if you do that, you wont be able to get a display as the video timings are derived from the clock, and yes, the cpu clock is half that of the resonator/crystal...you must keep it at 6.5mhz...although it IS possible to rig up a 'turbo' circuit, which runs the cpu at 6.5 mhz when running program code,...

..thin the varnish down a bit to help it seep in all the spaces..... you can get proper 'transformer varnish', but i've used normal wood varnish before, maybe not advisable really as i think the proper stuff is less flammable in case of gross overheating....but the thermal fuse should pop long befor...

many 50/60hz transformer based psus 'buzz' , especially if the laminations are a bit loose, normal,. but, if it gets warm/hot even with no load, could have shorted turns ... loud buzz on load could indicate problem with one or more of the diodes gone open circuit, making it half wave instead of full...

and..beware,..i have a UK1200 psu with the same smaller transformer as the UK700..and it appears to have got VERY hot as the pcb is blackened, although still works, gets fairly warm even with no load,..so...there may be others out there....

as for replacing the 7805 with a switchmode thing, this doesnt help with the rampack voltage problem, but will certainly reduce the heat in the computer...or you could try fitting a full width heatsink? sprayed with a thin layer of matt black barbecue paint...??

my zx81 and spectrum psu's always seemed to me as if you could fry eggs on the things...😉 but...using a modern regulated 9v psu on a zx81 could be a problems as some rampacks used the 'higher than 9v' line as a 'near to 12v' for the '12v' for a bank of 4116s, so 9v wont work, but if 11 ish, probably...

even if you want to use your machine 'every day' (through composite) , still no need at all for modulator murder, just tack the ccb board onto the top of the modulator case(it wont be seen), wire it in as usual (disconnect the modulator 5v and input if you wish, but not necessary)... this way the mo...