Atari DigDug sync help

bunnygrunt

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Donor 2023, 2025
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Hi all, I would like to humbly ask for help with, what I believe to be, a sync issue with my Dig Dug PCB.

Much of this hobby is still voodoo magic to me but I am learning and appreciate the deep knowledge on this forum.

Background on this game/issue - I purchased the game non working. It came with all the original parts with the exception that the monitor chassis was missing. Luckily I had a spare working GO7-CBO chassis. I put this on the tube and was able to get the tube to sync and display patterns with TPG. Overall, as far as I can tell, there have been no hacks, mods, or repairs to anything. I have worked through some edge connector and AR II issues, however I believe these are resolved now. The PCB boots( PCB red light on), game will coin up, there is audio, game plays "blind" ( there is audio, I can hear movement sounds from game when I press buttons and move stick, etc)

The monitor looks like this with the sync connector connected to the G07 chassis. It looks the same without the sync connector connected to the chassis.
2025-02-01-152311_002.jpeg



According to my research:
Sync_Generator.png


Unfortunately I don't have an oscilloscope. I do have a logic probe, however I don't have a lot of experience using it. Just the same I have attempted to research sync issues with Dig Dug and located the following http://www.arcaderestoration.com/RepairLog/15/Scrambled+video+ROM+issues.aspx This was inspiring so I ordered some LS369s, LS00s, LS74s, etc

Things I have tried to check:
  • 4E(LS368) pin 3 is the output composite sync signal (CYSNC) from the sync generator circuit.
    • I have continuity from 4E pin 3 to R87
    • I also have continuity from the other side of R87/CSync test lug to the CSYCN pin on the edge connector (position "U" ; purple wire on harness that runs to sync pins on monitor chassis).
  • My CSYNC line does not appear to be stuck low or high. Instead it pulses low and high at same time faster than the probe can probably keep with.
  • As far as I can tell, none of the LS369 inputs or outputs of the sync circuitry are stuck low or high (I can provide more trouble shooting notes but this is getting long)
  • I do notice an audible difference in the logic probes pulsing depending on the side of R87 I test from.
    • 1) very fast pulse, with high and low lights on on but no yellow light - between edge connector "U" and the CSYNC Lug
    • 2) less fast pulse audio, with high and low lights on, and yellow light blinking - when testing 4E pin 3 or from the other side of R87
    • I realize "the sound" probably doesn't help and that a scope would be more ideal here.
    • I also realize this probably has something to do with the resistor pulling the line

One thing that has me concerned is that my DMM shows continuity between edge connector "U" and the 10.6v line on the PCB when the wiring harness is attached to the board. I noticed this when testing continuity of the harness CSYNC wire from cync connector for chassis to harness edge connector pin "U". When the harness is not connected to the PCB I don't get continuity between 10v and the CSYCN circuit or between the harness CYSNC wire and harness 10,6v wire ("E")

My back can't take leaning into the cab to test anymore so I have a jamma adapater ordered and on the way so I can setup on a bench.

Thanks for reading this far :) and thanks in advance for any tips or advice.
 
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Namco customs can be problematic. Did you CAREFULLY remove 4C (07) and inspect its pins, clean its pins and inspect the socket?
 
Namco customs can be problematic. Did you CAREFULLY remove 4C (07) and inspect its pins, clean its pins and inspect the socket?
Thanks VectorCollector. I did, just for 4C. Its pins were very tarnished. I carefully removed, cleaned pins with a fiberglass, sprayed pins with Deoxit, sprayed socket with deoxit, and carefully reinserted. Have not yet done that with any of the other customs.
 
Sync is probably find on the board...
Just adjust the setting on the monitor.

If the 07xx was bad enough for hsync to be bad, vsync would be bad too, as it's divided down from H.
 
True. Just because the monitor sync'd with the Crafty TPG, does not mean it is exactly set for use with a game logic board.
 
Thanks for the recommendations. I revisited the monitor and tried readjusting the pots, but still not getting sync. I also hooked the board up to a jamma rig and LCD but can't get picture. Work is about to get busy so ill have to put this down for a bit and revisit in a few weeks. Will try a second monitor at that point and maybe invest in an oscilloscope
 
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Thanks for the recommendations. I revisited the monitor and tried readjusting the pots, but still not getting sync. I also hooked the board up to a jamma rig and LCD but can't get picture. Work is about to get busy so ill have to put this down for a bit and revisit in a few weeks. Will try a second monitor at that point and maybe invest in an oscilloscope
Hopefully, this log helps someone else facing a similar issue in the future.

I finally found some time to resume working on my Dig Dug sync issue and am happy to report that it's resolved! I replaced IC 6C (74LS74) after a lot of poking around with a logic probe and a cheap Amazon oscilloscope. ChatGPT was surprisingly helpful in walking through the logic and helping interpret what I was seeing. I also used AI to help summarize all the steps here.

  • PCB Version: "Type B"with original harness
  • Symptom: Rolling or unstable video on G07 CRT (see photo in first post) and no signal at all on an LCD/Supergun setup
  • Fix: Replaced IC 6C (74LS74 flip-flop)


Troubleshooting Summary

The game booted, had sound, and clearly ran — but the image wouldn't lock on my G07 or LCD. The G07 showed vertical striping and rolling, while the LCD just stayed blank. Since the harness was original and other test boards displayed fine on the same monitor, I suspected a problem with the composite sync (CSYNC) signal. The G07 expects CSYNC to be looped into both H and V sync pins via a single wire.

Using the scope, I confirmed that the Namco 07 custom chip (IC 4C) was generating:
  • A stable HSync waveform at ~15.7kHz (pin 12)
  • A clean VSync pulse at 60Hz (pin 17)
That confirmed the core timing circuitry was alive — which was a relief, since I had started looking at reproduction 07s (which aren't cheap).

Next, I focused on the CSYNC generation circuit. Based on the schematic:
  • HSync and a gated VSync are combined by an XOR gate inside IC 4A (74LS86)
  • The output from pin 8 goes to IC 4E (74LS368) and out to the edge connector on pin 3 as CSYNC
When I scoped pin 8 of IC 4A, I saw a nice clean square wave — but I wasn't sure VSync was being included. It looked like only HSync was passing through, and I wasn't confident I was using the scope optimally to tell for sure.

To dig deeper, I probed pin 9 of 4A, which gets input from pin 9 of IC 6C. That signal was flat — no toggling. A logic probe confirmed it was stuck HIGH. I checked the flip-flop's CLR pin (12), which was LOW (meaning Q should be LOW too), but pin 9 remained HIGH. That strongly suggested 6C was not behaving correctly.

Even more telling, pin 10 (clock input to the second flip-flop in 6C) was also held HIGH. It's driven by the /Q output of the first flip-flop (pin 6), which was likewise stuck. So neither flip-flop inside IC 6C was toggling — despite getting valid signals. I went in circles for a while here (with ChatGPT's help) because I wanted to be absolutely sure before desoldering.

Eventually, everything continued pointing to IC 6C as the failure.

I desoldered and replaced 6C with a new 74LS74. I later tested the original chip with a cheap IC tester I found on eBay — and it confirmed the chip was bad.

After installing the replacement and powering up the board, the difference was immediate. Dig Dug booted and sync locked properly. I had to tweak the G07 slightly to center and size the picture, but the image was now stable — no rolling, no striping.

signal-2025-06-28-100808.jpeg
signal-2025-06-28-100654.jpegIMG_3005.jpg
 
Hopefully, this log helps someone else facing a similar issue in the future.

I finally found some time to resume working on my Dig Dug sync issue and am happy to report that it's resolved! I replaced IC 6C (74LS74) after a lot of poking around with a logic probe and a cheap Amazon oscilloscope. ChatGPT was surprisingly helpful in walking through the logic and helping interpret what I was seeing. I also used AI to help summarize all the steps here.

  • PCB Version: "Type B"with original harness
  • Symptom: Rolling or unstable video on G07 CRT (see photo in first post) and no signal at all on an LCD/Supergun setup
  • Fix: Replaced IC 6C (74LS74 flip-flop)


Troubleshooting Summary

The game booted, had sound, and clearly ran — but the image wouldn't lock on my G07 or LCD. The G07 showed vertical striping and rolling, while the LCD just stayed blank. Since the harness was original and other test boards displayed fine on the same monitor, I suspected a problem with the composite sync (CSYNC) signal. The G07 expects CSYNC to be looped into both H and V sync pins via a single wire.

Using the scope, I confirmed that the Namco 07 custom chip (IC 4C) was generating:
  • A stable HSync waveform at ~15.7kHz (pin 12)
  • A clean VSync pulse at 60Hz (pin 17)
That confirmed the core timing circuitry was alive — which was a relief, since I had started looking at reproduction 07s (which aren't cheap).

Next, I focused on the CSYNC generation circuit. Based on the schematic:
  • HSync and a gated VSync are combined by an XOR gate inside IC 4A (74LS86)
  • The output from pin 8 goes to IC 4E (74LS368) and out to the edge connector on pin 3 as CSYNC
When I scoped pin 8 of IC 4A, I saw a nice clean square wave — but I wasn't sure VSync was being included. It looked like only HSync was passing through, and I wasn't confident I was using the scope optimally to tell for sure.

To dig deeper, I probed pin 9 of 4A, which gets input from pin 9 of IC 6C. That signal was flat — no toggling. A logic probe confirmed it was stuck HIGH. I checked the flip-flop's CLR pin (12), which was LOW (meaning Q should be LOW too), but pin 9 remained HIGH. That strongly suggested 6C was not behaving correctly.

Even more telling, pin 10 (clock input to the second flip-flop in 6C) was also held HIGH. It's driven by the /Q output of the first flip-flop (pin 6), which was likewise stuck. So neither flip-flop inside IC 6C was toggling — despite getting valid signals. I went in circles for a while here (with ChatGPT's help) because I wanted to be absolutely sure before desoldering.

Eventually, everything continued pointing to IC 6C as the failure.

I desoldered and replaced 6C with a new 74LS74. I later tested the original chip with a cheap IC tester I found on eBay — and it confirmed the chip was bad.

After installing the replacement and powering up the board, the difference was immediate. Dig Dug booted and sync locked properly. I had to tweak the G07 slightly to center and size the picture, but the image was now stable — no rolling, no striping.

View attachment 829646
View attachment 829647View attachment 829648
you're a good citizen. I don't know you, but this was a pretty significant win. pop some Pookas now
 
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