Recently received another bombjack board set from a member for repair, symptom was game boots and plays but no sound.Here it is on my bench, nice condition original board with jamma adapter. 
Powered on and booted fine but no sound but a slight hissing which increases when volume pot is turned up so the amp is ok.
Scoped the sound Z80cpu (top pcb) ,reset ok,clock ok,halt ok but NMI stuck low (not good)and the databus and address bus were floating. Decided to dump the audio eprom(1) and did not romident, could this be a east fix
.Burned a 2764 eprom with correct file and fitted to its socket, powered on but still no sound.went straight to the sound ram which is close to the cpu and scoped the data pins,when the game goes to its ram/rom check there is activity but signals don't look right,when the game starts the data bus pins float.Suspected this ram may be bad,took it out and tested on another game and it was bad.
Put in a new socket and new ram and powered on but still no sound. Went back to the cpu to check the NMI signal which should be toggling but stuck high.
Suspected the databus is screwed somewhere causing the cpu not to run.looking in the schematics there 3x ay-8910 melody chips and all three share the databus which goes to the cpu data pins,these were soldered to the board so I took them out and fitted new sockets.
Tested all three in my scramble board and 2 failed and 1 was ok.fitted 2 new ones plus the other working one and powered on............................
At last,it's fixed and we have music and sound effects, so 2 of these 8910 chips were corrupting the data bus.
Powered on and booted fine but no sound but a slight hissing which increases when volume pot is turned up so the amp is ok.
Scoped the sound Z80cpu (top pcb) ,reset ok,clock ok,halt ok but NMI stuck low (not good)and the databus and address bus were floating. Decided to dump the audio eprom(1) and did not romident, could this be a east fix
.Burned a 2764 eprom with correct file and fitted to its socket, powered on but still no sound.went straight to the sound ram which is close to the cpu and scoped the data pins,when the game goes to its ram/rom check there is activity but signals don't look right,when the game starts the data bus pins float.Suspected this ram may be bad,took it out and tested on another game and it was bad.Put in a new socket and new ram and powered on but still no sound
. Went back to the cpu to check the NMI signal which should be toggling but stuck high.Suspected the databus is screwed somewhere causing the cpu not to run.looking in the schematics there 3x ay-8910 melody chips and all three share the databus which goes to the cpu data pins,these were soldered to the board so I took them out and fitted new sockets.
Tested all three in my scramble board and 2 failed and 1 was ok.fitted 2 new ones plus the other working one and powered on............................
At last,it's fixed and we have music and sound effects
, so 2 of these 8910 chips were corrupting the data bus.
