Tac/Scan wont boot

hatster

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Hi people,

Wonder if someone wouldnt mind helping me, spent hours on this now..

no LED on cpu board at all, Reset on Z80 is high (low, then high on powerup), clock is running (from xy timing board), if i press the diagnostic switch, i can see pin17 of the Z80 go low, then back to high when i release the button, but still no LED. HOWEVER: if i remove the XY timing board and powerup, the led comes on, if i remove any of the 2114 ram on the cpu or the security chip then powerup, the led comes on. If I remove the eprom card and powerup, NO LED even when diag pressed, same goes for XY control pcb.
Does this mean the cpu is running properly ?
(I replaced the Z80 and the 4x 2114 on the cpu board), and all 2114 on the xy control board.

Please Help before i go insane !
 
When you say that it won't boot, do you mean that you don't get video and/or sound?

Is it possible that the game is running with no output?

There are several pieces to a G80 puzzle.
 
thanks, erm well ok,

if i put a scope / dmm accross either X or Y ouput i get 3.9v steady so no picture. R G and B are all showing absolutely nothing.
I think the sound board has a fault as if i connect this and the sound cable i get loud white noise..
Its possible that the game is running but with no video or sound, how do i tell ;)
even if it is running, by pressing the diag buton shouldnt i get some sort of output on the LED ?

damn what a nightmare this is :|

Thanks,
 
can anyone with a g80 game please press their diag switch and see if anything happens to the LED ? also does the LED do anything on powerup ?

thanks.
 
Welcome to the world of Sega G80 vector games!

Pull the sound board, as it may cause an issue. Just keep in EPROM, the two XY boards, and the CPU board for now.

Pressing the plunger on a good working system usually gives a brief LED flash; also at startup there might be a brief flash, but then it should stay off.

If you get a steady halt light, processing is failing; either the Z80 or 2114 RAMs or something else.

If you get a constant flashing led, most likely an EPROM is bad.

If you get a morse code-like output from the LED...like 1 long...2 short...7 short...repeat...that's the CPU telling you what RAM on the XY pair has a bad 2114 RAM (in the above example, the 2114 RAM at U27 is bad).

If you get nothing, could be just about anything. <- Sounds like you are here.

These boards are very difficult to work on without a known working set, since you can't usually reach any of the boards to see what the problem might be.
 
thanks for the feedback..

what i dont get is, if the clock is running, the reset pin on the cpu is high (low then high on powerup), there is life on all data pins, then surely the program in eprom1 ie the diags, must be running ? or does this not work like other boards with a watchdog that will constantly reset the cpu when faulty.
btw: The LED definately works as if i take the XY timing board out, the LED comes on as soon as I powerup the cab. This makes me think even more that the program is running. I really dont get it.
It would be nice to know if this problem can be caused by any of the boards, or the cpu board in particular.
I already removed the sound board as you suggest.

Thanks again !
 
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