How do you track down a RAM error

The easiest way would be with an in-circuit emulator; also the most expensive. Is it possible to put a logic analyzer on the bus(es) and single-step it? And, of course, the cheapest is to just replace them all.

Back when I was doing those kinds of repairs, everything was DIP, and since we got $30.00/hr for repair, and about 9 cents for a chip, we'd "shotgun" it - it was cheaper to just replace every chip on the board than to spend the time troubleshooting it.

Maybe a good hot air station would help?

Good Luck! Rich

uriah a écrit :

If you can change the prog easily (eprom on socket) then write a small prog that tests all the ram locations and report it for you.

Also keep in mind that it might not be the RAM but address decoding, bus tranceivers...

If you have the schematics that'll help a lot.

Freeze the chip and see if the problem goes away.

Note that the actual error might be most anything-- the RAM test is probably the first test done, so if there's anything wrong with the power supply, address lines, data lines, bus drivers, or corrosion, it may show up as a RAM error.

Good suggestions all. My first impression was similar to yours, from which I concluded: pitch the whole thing into a dumpster. But writing a little test program is a good idea - just make sure to keep the initialization code intact.

-- Joe Legris

On a sunny day (Fri, 15 Dec 2006 09:22:03 +0100) it happened Fred Bartoli wrote in :

Yes, absolutely, scope the RAM address pins, and also the data pins, and CE and WR lines.

Sometimes there is a short or overload, could even be a processor ouput pin.

Original poster here: Thanks so far to all. A few notes to add:

No schematics on this. Coin op game board manufactures don't like to share these things because of the fear of copies being made. As a matter of fact that is one of the main reasons the company who made the board I am working on is no longer in business. (SNK-Neo-Geo) They would make a game (Cartridge) and within a few weeks it would be hacked, copied and sold for half the price.

I do suspect that it might not be the actual RAM being the problem. That is why I don't want to shotgun the thing and find out I didn't solve the problem. The freeze thing didn't work. Writing a progam I think is beyond my abilities. Don't I need to have some clues from SNK Neo-Geo on how to do this? They have a number of Custom chips and protection of their hardware and software is important to these companies so I would think they would have something to prevent easy access to the brain of their system. But I don't know enough to answer that question. I know you can make a NOOP adaptor but the M68000 is soldered in and once again, from past experience I end up damaging things and chasing false leads. And even if I put this in I the self test on this thing is already pointing out a RAM error. Isn't that better then NOOP. That I think leaves - besides the dumpster idea- scoping all of the lines. I have looked at all of the lines and all I get is a blank look on my face. Most everything either looks like one of the other lines, some kind of square wave or little pyramids. They all compare just fine to a working board. What am I looking for when scoping the lines. I assume we are talking about address, data, CS, lines? Could someone provide details on how to spot errors with a scope on these buss lines? And how to check a RAM in-circuit, if there is a way. Thanks again, I wish me luck.

A 68k game could be 20 years old now... first, locate any connectors (like socketed EPROM) and remove/reseat them (or better, remove, clean with alcohol and treat with DeOxit or similar contact enhancer).

Then, check the power voltages (if your 5.0V supply has readjusted to

Third, a lot of these problems are solder joints that have cracked; test by pressing the chips with a stick (bamboo chopsticks are perfect) while starting up. If a pressure-stress test results in normal function, resolder that chip (flux the joints and hit them one at a time with a fine soldering iron tip), clean off the flux, and retest.

If these don't fix it... where are you going to find repair parts, anyhow? If it were a bad RAM, can you still get that part number?

That changes everything!

-- Joe Legris

So tell us more about this working board.

Is it something just sitting on the shelf collecting dust somewhere (and would make a great replacement for the faulty board)?

Was it taken from another cash-generating game machine (and therefore isn't available)?

Or was it graciously lent to you by a co-worker?

-MD

I actually have 5 or 10 different boards like these. They sold numerous versions. one has a work Ram error, another a video ram error, some just don't boot, some work fine. I will probably pull custom chips off of one of the these. The ram chips I can get. I paid someone for some and of course the guy ripped me off {Hello Alan Free ( snipped-for-privacy@comcast.net ) when am I going to get my chips??} But I can find more of them or remove them from the parts board. I have been over this board with one of those microscopes for SMD work and have checked and rechecked everything. I have re-flowed the solder on numerous chips and checked all socketed chips. I have a few Huntron style machines with up to 64 pin clips and compared almost all of the chips against the known working board and can't find anything wrong. I have changed the main ROM with a known working one and probed the board with a logic probe and pulsar where I knew what to expect. I have read a lot of books but I can't seem to find a method to track this problem down. I am not just wanting to get this one working, and the others but to actually learn something I can take with me to other boards. Most of these coin-op companies are out of business or no longer support these boards. We just lost another different game today because they no longer have some of the custom chips for the particular board and the board is rare, didn't come in kit form and had only one game on this particular hardware. Atari Rush the Rock. I constantly accumulate working and non working boards so that I won't have to junk a game because the board is fried. So that leaves me back at square one. This board with the damm RAM error. Is it hopeless?