Space Invaders Repair Help

[epicfatigue]

Hello everyone, i brought a friend a complete old table top it pretty rough condition.

He has recently given up on it as its a little to much work for him so now i plan to restore it.

The table top is running a old B&W 14inch Toei GM-140 B&W.
The monitor has screen burn but when its all lit up you cant see it, Screen functions perfectly.

Moving on the the power side of things all the correct voltages are there and minor repairs are needed, broken fuse holders cable management and the odd replacements to comply with Australian Standards.

The game board itself is not functioning, i noticed an op-amp was missing on the sound board so i replaced it and got spares to replace them all as i had to buy a tube, but for now i want to just get it going.

The PCB is a dual board setup made by Logitec titled T.T
All i get is a scrambled picture, and i thought i would start with just replacing the ram and see how i go from there.

[epicfatigue]

So i took the roms all out and fired it up just to see if i had any action.
I get as follows in the video below.

I noticed that Eprom 6 has a leg cut off but this looks intentional i will look into this further.


Seeing the board is pretty much impossible to make anything out on it i decided to give it a good wash. ( its currently drying in the oven).

I did notice something unusual when pulling it apart it appears that the ribbion conector might be in the wrong way, as it has triangles marking the correct orientation. these were not lined up. however if you were to follow these markings the cable would be twisted unless it was mounted in a L shaped fashion.

Will look into all this further once its dry.
(ALL STICKERS WERE REMOVED AND SAVED)

[epicfatigue]

So i have ordered the ram, but i hate being in the unknown about these Eproms, they are ancient and very few eproms can read them and even fewer can write to them.

So i sat down and started researching, but i came up very dry but i noticed along my reading that a drop in replacement was available.
2716 could be used in place of a 2708 if you altered the power source.

Now this got me thinking i quickly checked my programmer and the 2716 was on the list.
So i sat down and looked at the data sheet and there was only voltage differences and size differences.

So i built an adapter then set my programmer to read a 2716 and only read to the size of a 2708 and applied the voltages according to the data sheet and wella read the eproms!!!
i compared the roms with the Logitec rom set and they are not a match!!!!
Time to use clrpro and see what is on these chips!

[epicfatigue]

okay so they have been fully redumped with the correct and still now go,

So i have had alittle time today i sat down and went through and removed all the connectors and started to rewire them manually.
Just to ensure that no one had tried this themseleves and sure enough it had been wired as a tatio space invaders which is wrong because you send voltage to ground points on chips.... sigh... might be a few things dead now

So i have checked where everything should be going as best as i can by following chips etc and fired it up and got this.
Seems like its sending sounds to the board. Good news is my amp is working! bad news is it still doesnt run, i will have to start investigating this further.

[epicfatigue]

i decided to go back to basics again now that the wiring is semi complete and i noticed something!.

I got out my logic probe and the reset pin is low, then goes Hi for a split second i have never used an 8080 is this rebooting all the time or is it just me.

Second reading is a clock pin (alternating looks good)
Third is the other clock pin, (alternating looks good)

Will get a scope out and see the clock when i can find it.

Is this normal operation for an 8080 guys?

[epicfatigue]

Still waiting for the ram to come,

But i replaced all the caps and soldered the eproms to the pcb, couldnt be bothered with sockets. if i have a rom problem down the track ill just remove them all anyways.
fired it up and bam !

think its looking more like a ram issue
i am still having the cpu reset but it really looks like its trying to start now!

I somtimes get the alphabet and others i get space invaders!

[jennifer]

     " Cant be bothered with sockets" ?.... Jenn get all amused,. I feel bad that I missed out on this thread it looks like a really fun project. And apparently its about done, When you say new caps, thats caps on the board right? how many were there?

[epicfatigue]

i didnt want sockets because sockets cause more problems then good and they were the only socketed item on this pcb.

There was about 12 caps in total, i dont think the project will be finished for awhile i believe i have issues in the sound board and maybe some logic issues.
Time will tell!

any advice or words of wisdom, done this pretty much on my own its the first one i have ever done.,
i am praying the 8080 is reseting due to a bad crc from ram

[jennifer]

     Ive always been a bit suspicious of logic probes and never really believe them, thats just me however and most likely unfounded, a scope will tell the real story... But since your clocks are running and the chips not triggering my first thought would be to replace it, (And in typical Jennifer fashion)
would probibally change it out anyway cause of its age, 

[dddirtman]

looks like a ram problem.....
these older boards didn't buffer the ram so they are prone to be very static sensitive and blow...