Space Invaders - Need Inspiration / Thoughts

sometimes you never figure it out. was working on a taito 3 board version with the power supply that shall remain nameless since no one has ever seen a schematic for it. I screwed up and accidentally bridged 12 volts to 5 volts. took about 2 days to run down the faults I put into the boards. one was truly bizarre that allowed the test to work and give me bad ram for the taito self test but the braze test never worked. It had a shorted address decoding line. The power supply took me 3 weeks to fix and I never figured it out. regulators and power transistor were shot but I still got double voltage when they were replaced. Got desparate and pulled the large filtering cap off the board to test it, it was good and I put it back and the power supply functioned properly. 2 years later and its still good and I still have no clue why.
 
Before I put it into the cab, I wanted to figure out the difference in Freq measurements I was getting.

Since I had previously socketed A7 (don't remember why), I started trying different chips 7404s. My working board would be @19.95Mhz at C7-14 and end up at 59.97hz at end of counter chain E7-11. One 7404 put me at like 19.89Mhz and @58hz, a 74ls04 was closer at 19.92Mhz and 58.9hz, and a different 74ls04 was a bit worse. I went and grabbed a donor board 74s04 (which is called out in the schematic) and sure enough, I was right at 19.95Mhz and 59.97hz.
**Use 74s04 at A7**

I put the board in the cabinet and it boots up with no problem. Woohoo.
Hoping I could reproduce the original behavior and reason for this thread, I swapped in a 74ls04 at A7, (thinking the slower clock was leading to slower sample, vra, etc) but the damn thing still booted up in the cabinet. Maybe it isn't the same 74ls04 I had in there originally or maybe that was not the problem.

So unless it flakes out again, I can only assume that the crack was at fault, it is old and random stuff is failing. On to fixing the saucer hit where I had previously replaced both 3900s and I am sure something else will fail after that.
How were you measuring frequency? Counter or scope? The S part is significantly faster than an LS part and at 20Mhz the LS part is likely not swinging its normal range. I don't think it would change the frequency but much more likely randomly miss clock pulses. Missing random clock pulses will cause bad read/write behaviour for sure. It would also show up on a counter as a slightly lower frequency.
 
How were you measuring frequency? Counter or scope? The S part is significantly faster than an LS part and at 20Mhz the LS part is likely not swinging its normal range. I don't think it would change the frequency but much more likely randomly miss clock pulses. Missing random clock pulses will cause bad read/write behaviour for sure. It would also show up on a counter as a slightly lower frequency.
I was using the freq counter on my tek 2236a scope.
 
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