prehistoric isle fault

[grobda]

Ive got a SNK prehistoric isle jamma board that used to work, but was in storage for a while and now doesn't.

It does one of the following things;

Displays 'ALL MEMORY OK!' and crashes.

Displays 'ALL MEMORY OK!' and resets continually.

Displays 'WORK ERROR 70003000' or similar (70004000,70001F03 etc.)

Displays garbage and crashes.

I've checked the basics; rom sockets, ribbon cables, +5v across the board. The only further investigation is to check the 68k halt and reset pins, which are both high if i get a message, and both low if i just get garbage.

these are the boards;





anywhere i should be focusing? im suspecting the RAM labelled '6116' below the eproms on the CPU board to be the work ram that the messages refer to, any hints would be appreciated; im borrowing a scope to have a look at the outputs of the ram chips any giveaways i should be looking for?

cheers

Rick

grobda2010-08-27 11:54:38

 

[trm]

Hi Grobda,

What kind of skill level you have at this stuff? I don't want to write a load of replies which are just teaching you to suck eggs And do you have a logic probe - I'm assuming so from the halt/reset test but wanted to check.

Often /HALT and /RESET are tied together and your description of their state vs screen display makes sense. High when code is running, low when board is resetting. Are you seeing the game watchdog or do you just see active low signals on /RESET when the game is first powered up?

As for the RAM locations I've looked through MAWS and there is no indication there which RAM occupies that address space, nor is considered work ram so I think you're either going to have to follow the 68k address lines and figure out the address decoder for anything in the 0x70000000 range or just brute force replace/test those RAMs.

At least we know it's not the Z80 sound co-pro as the address space is a bit optimistic for that CPU

.

Sorry I can't come up with anything more helpful and please let me know if you need help with the address decode path. I've got no idea of your skill level (other than you clearly have some familiarity) so I don't want to baffle or offend

cheers
tim

 

[grobda]

My levels of skill is fairly basic. I've fixed obvious stuff in the past (ie visibly broken/missing/oxidized) but when identifying logic faults i get out of my depth pretty fast. Just wondered if there was anything obvious I should look for or what a bad ram chip would look like on a scope.

I guess I should start with the basics, download the galaxian troubleshooting manuals and work on those boards, then work my way up to late 80's stuff.

Annoying thing was this worked before I put in in the loft for a while, and died while it was up there, in an antistatic bag, inside a cardboard box. As it was reporting a fault I thought it may be an easy fix.

Anyway cheers for your input I hadn't thought of using mame drivers as a reference. I'll keep plugging away and maybe get somewhere, maybe not. Learning all the time anyway And I'll play it in mame in the meantime.
grobda2010-08-27 22:46:31

 

[trm]

I'll be back in the morning with some pointers on what to look out for.

 

[grobda]

well tim never got back to me and i fixed the original faults years ago (work ram at 1K and 3K on top board). then some corrupt graphics due to a dead mask rom at 3H on the bottom board. also looks like i replaced some ram chips but dont remember details.

took it out of the box this week and it displayed 'front ram 000A00000' error.

as my google search popped this thread up I thought id update it

after looking at the mame driver it showed sprite ram at 000A00000. located these on top board at 6H and 7H. replacing 6H fixed the fault.

self test error codes;

work ram (1k,3k) top pcb error 'work error 7000XXXX'
sprite ram (6h,7h) top pcb error 'memory error front 000A0000'
tile ram (12k,13k) bottom pcb error 'memory error back 000B0000'
palette ram (6h,8h) bottom pcb error 'memory error color 000D0000'

the schematics available on klov do not match my pcb, 'prehistoric isle' is hand drawn on the 1st page but the labels seem to point to gang wars or sky adventure, which have a the same mame driver, while prehistoric isle is unique.

 

[PfefferT]

Ive got a SNK prehistoric isle jamma board that used to work, but was in storage for a while and now doesn't.

It does one of the following things;

Displays 'ALL MEMORY OK!' and crashes.

Displays 'ALL MEMORY OK!' and resets continually.

Displays 'WORK ERROR 70003000' or similar (70004000,70001F03 etc.)

Displays garbage and crashes.

I've checked the basics; rom sockets, ribbon cables, +5v across the board. The only further investigation is to check the 68k halt and reset pins, which are both high if i get a message, and both low if i just get garbage.

these are the boards;





anywhere i should be focusing? im suspecting the RAM labelled '6116' below the eproms on the CPU board to be the work ram that the messages refer to, any hints would be appreciated; im borrowing a scope to have a look at the outputs of the ram chips any giveaways i should be looking for?

cheers

Rick

grobda2010-08-27 11:54:38

By the way, I found a lot of interesting things here.

If you have a scope or a logic analyzer, check for activity on the 68000 CPU and RAM address and data lines. If one of those lines is stuck high or low, that would definitely explain the random error codes and the garbage on the screen.