I've read the other responses to your question to date so I won't repeat those. I know that there's likely a bad logic IC somewhere that isn't gating or counting or shifting like it should, but that's hard to find without special equipment so I'll give you a few simple hints that often go a long way.--------------------------------------------------------------------------- ** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org ** Please direct other questions, comments, or problems to chris@westnet.com Received on Mon Jan 26 19:54:51 2009
1/ I know that you said the clock is running but the main crystal or oscillator circuit could still cause an issue if the levels aren't correct or if the crystal broke and is running the wrong speed. Sounds weird, but I've seen it happen. Also, check for the clock at other chips such as counters, shift registers, latches, etc., not just at the CPU.
2/ Check the halt or reset pin of the 6502. See if it is being told to stop and if so, backtrack that signal to be sure that the chips generating it are getting what they need. If yes, then they are your problem.
3/ Replace the 2116's with 5116's or 6116's. Bad sram can cause weird behavior and the game certainly won't run.
4/ Inspect those 377's again. Sometimes when people replace them, they rip out vias and lose chunks of the traces on the component side under the new socket. Beep them through from the socket contact through to their signal destination/origin.
5/ Beep check the DIP sockets by measuring continuity from the chip pin (on the shoulder/knee) to the pad underneath. Those old leaf contacts sometimes snap/crack under the socket body near the PCB. If any doubts exist as to the reliability of the socket, replace it... carefully and patiently. If you have little soldering experience or don't have the proper tools for socket removal, then don't attempt it. It's super easy to do more harm than good when trying to remove multi-pin components. If you are proficient, don't hesitate.
6/ If the board is really dirty, clean it thoroughly and don't be afraid to get it completely wet. I wash my big boards and wiring harnesses in the dishwasher with no ill effects, although certain residual soldering fluxes (specifically no-clean type 245) can turn an ugly white. It can be removed with simple rubbing alcohol and a toothbrush and/or q-tips. The point here is that contaminates can prevent proper board operation depending on where it is located. When I wash boards in the sink, I use mild hand soap and a toothbrush or paintbrush to clean everything. I usually hold the PCB on edge and drip dry for a minute, then either blow-dry with compressed air or I lay it on a rack inside the clothes dryer and dry at lowest heat setting for 45 minutes. I've never had a problem come up from doing this, and a clean PCB is a happy PCB. Besides, I hate getting my fingers dirty while working on boards anyway. If there's a ton of old rosin flux, it can be removed with rubbing alcohol. The 70% works fine. The 99% works faster and is stronger but also can cause some old solder masks to discolor but most will be okay. The nice thing about rubbing alcohol is that it works, it's cheap, and you can buy it almost anywhere. For really flux laden boards, I fill a glass tray and set the board right in and let it soak for a while. I gently brush it (with paint brush) while submerged until it looks clean and then I drip dry for a minute and then blow dry it. I usually do this outside in the open garage in the summer when it's warm. I would not recommend exposing yourself to that much alcohol vapor indoors.
7/ Look at the entire board on both sides under magnification and the brightest light that you have. Look for any sort of trace scratches that has cut through and caused an open. Look for a broken trimpot leads, bent or broken leads on resistors or diodes or anything. When a large part is bent over (say a big film cap), it can hit and even break an adjacent resistor. Check for broken SIP resistor networks. Check for DIP pins on the bottom that are bent right over that may be touching adjacent pins or traces. Oftentimes the problem is visually detectable if you have the patience to really look over everything carefully. A lot of small damage can occur when people carelessly stack their PCB's. I find this a very common cause of failure on almost every sort of boards. People stack boards and they either just don't care what happens to them or they are blissfully unaware of what can happen. Either way, things get bent, broken, or scratched. Also, check for any IC that is cracked, has a small hole or crater or swelling right in the center of the body where the chip is inside. When chips blow or fail, they can show such a physical defect and they can leak black stuff out of the body along the lead frame and out onto a pin where you can see it.
Dunno if that'll help, but good luck.
William Boucher
----- Original Message ----- From: "Chris Brooks" <armis@pcdochouston.com>
To: <vectorlist@vectorlist.org>
Sent: Monday, January 26, 2009 7:30 AM
Subject: VECTOR: Space Duel PCB weirdness
Hello all - new to the list, but not arcade stuff :) Howdy from Texas!
Anyway, I have a Space Duel board that is behaving badly and I am stumped
as to what is going on.
The watchdog barks, so I disabled it, then looked at the CPU bus. It
appears to address memory for a few milliseconds, then completely stop - no
activity, address bus locked, data bus locked. Clock looks clean on the
scope and is operating at the correct frequency.
I would completely understand this happening if it was a processor that
supported halting or DMA, but the 6502 has no support for such things.
Anyone have a clue on what to look at next? I'm thinking its time to invest
in a logic analyzer.
Thanks,
Chris Brooks
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