Dig Dug PCB Troubleshooting

TheDrewster

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Alright, so I got myself into yet another project with a bakers dozen Dig Dug PCBs that I am going to repair and hopefully teach myself more about board repair along the way. I have a couple boards I am having issues with and would like a push in the right direction. My tools are limited to a soldering iron, a logic probe, GQ-4X Programmer, and a multimeter. I am testing all these boards in my Dig Dug cabinet which is putting out all the correct voltages. I'll put the DOA and likely time consuming boards aside and work on these four partially working ones first.

Board 1 passes ram and rom check but resets right after the crosshatch and never loads the game. Grounding the watchdog will keep the crosshatch on the screen and will allow me to coin up, but game does not play. Self test mode seems fully functional. FIXED: Bad 3rd Priority CPU

Board 2 resets after ram check and never checks the ROM. Grounding the watchdog will give a ROM B error, which does not make sense. Self test does not work.

Board 3 boots and gives ROM 1 error without the need to ground the watchdog. Tried known good ROM in the corresponding socket and the issue is not resolved. Self test does not work. Update: Several traces damaged while attempting to remove the socket. Ill put this one on the back burner for now. :(

Board 4 boots to squared colors (distorted sprites) and crosshatch screen is completely white, which is where it freezes. Cannot coin up or make out any graphics/text. Possible video processing issue?


Can you give me some pointers? :)
 
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First things first, you need to get yourself an eprom programmer. It is absolutely pointless trying to repair a board when you don't even know if it contains the correct code.
 
You could order a whole set of ROMs from steph. Then you have a known good set.

But I would follow Hewitson's advice and then post back. Why? Because the EPROM programmer will help you repair any board.
 
You could order a whole set of ROMs from steph. Then you have a known good set.

But I would follow Hewitson's advice and then post back. Why? Because the EPROM programmer will help you repair any board.

Agreed. Heck, you can grab a GQ-4X for close to the same price as a complete rom set. Then you can verify those ROMs and burn any replacements you need yourself.

The ROM sockets on those boards are notoriously bad. I had a lot of intermittent boot failures on my Dig Dug until I replaced those sockets.
 
Sorry, forgot to mention that I do indeed have a GQ-4X programmer and I have verified the ROMS to be good, at least on the boards throwing up ROM errors.
 
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SWEET! Board 1 has been fixed. I took the long route though. I replaced all 3 CPU sockets (what a pain in the ass...) and put the CPUs back in and my heart sank as it didnt fix the issue. So I swap 3 different processors and I get absolute garbage. So now I thought I was going backwards. So I replace each CPU one at a time and I eventually found the culprit, being a bad Z80 in the 3rd priority CPU socket. Suh weet!!!
 
That's great news, having a working board will come in very handy for troubleshooting the others.

Especially since you can verify the ROMs are good and the CPUs are good.

I've run into situations where a ROM checks good in an EPROM programmer but would not run at speed in the game.

Also, anything that causes problems reading the ROMs and calculating the checksum can cause problems with the ROM tests. This includes address decoding issues for the ROMs or RAMs, bad sockets on the ROM, CPU, or RAM, and simply bad components such as ROMs, RAMs, or CPUs.

Now why the RAMs? Easy. When computing things the system has to have a place to temporarily store the data. Wait? Isn't this done in the CPU? No. Every so many milliseconds the CPU is interrupted so it can do housekeeping tasks such as check for control panel inputs, service the video subroutines, etc... and if the RAM has issues that can affect things in a bad way.

Bad sockets - that's a no brainer. Atari used some shitty sockets! If they aren't already double wipe sockets, replace them. If they are double wipe check them carefully for corrosion or simply replace them.

RJ
 
Especially since you can verify the ROMs are good and the CPUs are good.

I've run into situations where a ROM checks good in an EPROM programmer but would not run at speed in the game.

Also, anything that causes problems reading the ROMs and calculating the checksum can cause problems with the ROM tests. This includes address decoding issues for the ROMs or RAMs, bad sockets on the ROM, CPU, or RAM, and simply bad components such as ROMs, RAMs, or CPUs.

Now why the RAMs? Easy. When computing things the system has to have a place to temporarily store the data. Wait? Isn't this done in the CPU? No. Every so many milliseconds the CPU is interrupted so it can do housekeeping tasks such as check for control panel inputs, service the video subroutines, etc... and if the RAM has issues that can affect things in a bad way.

Bad sockets - that's a no brainer. Atari used some shitty sockets! If they aren't already double wipe sockets, replace them. If they are double wipe check them carefully for corrosion or simply replace them.

RJ

So in the case of the board (actually, two boards) that reset after the RAM check, we know at least the RAM is good, right? So since I have verified the ROMs to be good with the reader, should I start replacing the ROM sockets first or the CPU sockets? Eventually I will do them all, but I want to see the board working first, then do the bulletproofing later.
 
So in the case of the board (actually, two boards) that reset after the RAM check, we know at least the RAM is good, right? So since I have verified the ROMs to be good with the reader, should I start replacing the ROM sockets first or the CPU sockets? Eventually I will do them all, but I want to see the board working first, then do the bulletproofing later.

If the design is similar to Galaga which uses 3 Z80 CPUs and those pesky 8xx chips to control memory access then I'd say that the 2nd or 3rd CPUs aren't starting properly.

On Galaga they stay in reset until the main CPU finishes the self tests then they come out of reset and start running. If they don't run then the watchdog timer kicks the main CPU in the head and the process starts over.
 
Alright, Ill replace the CPU sockets for starters and report back.

Also, quick question about the slew of custom Namco ICs on these boards... Are they interchangeable if the first two numbers printed on the chip match? For example, can you replace a chip that says 0472 with a chip that says 0439?
 
Alright, Ill replace the CPU sockets for starters and report back.

Also, quick question about the slew of custom Namco ICs on these boards... Are they interchangeable if the first two numbers printed on the chip match? For example, can you replace a chip that says 0472 with a chip that says 0439?

Them are the ones I would start with if you just want to get working, if cpu is processing code.

Replace the sockets with good quality dual wipe sockets.

Jork has redone some of the customs and are available for about $40 ea.
 
Them are the ones I would start with if you just want to get working, if cpu is processing code.

Replace the sockets with good quality dual wipe sockets.

Jork has redone some of the customs and are available for about $40 ea.

Jork? ;)

Jrok
 
Alright, so I worked on board 4 tonight and another board from the pile that I havent mentioned yet, we will call him Board 5.

Anyways, with Board 4, I was able to fix the sprite issues by reseating a ROM with a leg sticking out. Then it gave me a RAM error and I replaced the affected RAM and I am now stuck here. "QAM OK" it says. I have reseated all socketed chips and tried known good CPUs It is stuck in resetting itself every second. It never gives the preemptive jumble of graphics that good PCBs typically do, it just boots straight to what you see in picture 1. Also, there is no audio. No distorted tones or squeals.

And on to Board 5, I was originally given a RAM 2L error, so I replaced the corresponding RAM and the issue persists. Board continually resets. Except now what I see is after about 30 seconds, it changes to 2H. So I replaced the IC socket that 2H refers to (according to the manual), and I replaced the 1st Priority CPU socket and I have no change. I tried a known good IC as well, no luck. There also seems to be another issue with the color palate possibly? The board also gives out an annoying high pitched tone, that has since given me a headache... Pictures 2-5 show what is displayed at boot, in this order.
 

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1) Extract all the Namco customs, Z80s & EPROMs.
2) Sell them.
3) ???
4) Profit!
 
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