Purcased a untested mr do! booty some weeks ago from a member,the pcb arrived and in good condition,so here it is on my bench.
So it looks complete well almost as the Z80a cpu is missing,right next i put in a tested z80 and fired it up.......
Ok the good news is we have sync,rgb and some audio.Scoped the z80 cpu with clock good,reset high but all the address bits were in high impeadance i.e floating,the data bus bits were low.
briefly when powering up i can see on my scope the data bits signal but rapidly falls to ov.
This sugests theres a fault somewere on the databus,before fault finding i did a romident on all the eproms and they idetified as ok,roms 9+10 (tiles gfx) were MR DU?
Next i pulled the work rams out which are 2 of them and sit below the program roms,verified them and all was good.
Next i looked under the pcb to see if there was any trace damage and no traces damaged,while inspecting componets on the pcb i discovered something very odd.
As you can see on the picture there is a 74ls139 decoder chip fitted in replace of a 76489 melody chip?so it looks as though this may be the culprit on corrupting the data bus,this melody chip has its outputs tied to the databus.
the other melody chip which sits above had a broken leg and all legs were rusted.
ok so i fitted 2 melody chips and fired it up.....
cool the program is running but the tile gfx are corrupted,sprites are ok.looked at the tile gfx circuit in the schematics and scoped the tile rams which there are 2 (6116),the one at loction W8 had poor outputs on bits D0,D1.
Put in a new sram and............
oh yes,mr do! lives.
So it looks complete well almost as the Z80a cpu is missing,right next i put in a tested z80 and fired it up.......
Ok the good news is we have sync,rgb and some audio.Scoped the z80 cpu with clock good,reset high but all the address bits were in high impeadance i.e floating,the data bus bits were low.
briefly when powering up i can see on my scope the data bits signal but rapidly falls to ov.
This sugests theres a fault somewere on the databus,before fault finding i did a romident on all the eproms and they idetified as ok,roms 9+10 (tiles gfx) were MR DU?
Next i pulled the work rams out which are 2 of them and sit below the program roms,verified them and all was good.
Next i looked under the pcb to see if there was any trace damage and no traces damaged,while inspecting componets on the pcb i discovered something very odd.
As you can see on the picture there is a 74ls139 decoder chip fitted in replace of a 76489 melody chip?so it looks as though this may be the culprit on corrupting the data bus,this melody chip has its outputs tied to the databus.
the other melody chip which sits above had a broken leg and all legs were rusted.
ok so i fitted 2 melody chips and fired it up.....
cool the program is running but the tile gfx are corrupted,sprites are ok.looked at the tile gfx circuit in the schematics and scoped the tile rams which there are 2 (6116),the one at loction W8 had poor outputs on bits D0,D1.Put in a new sram and............
oh yes
,mr do! lives.