It's starting to look like a design flaw. The 68B21's RESET line is simply tied to Vcc through a pull-up resistor. I've now jumpered that to the board RESET line and I get a clean start up almost every time (I have substituted a good 68B21 to no avail previously). Sooo, if this works in the game, another Tech Tip will appear on RGP I guess. John :-#)# At 11:51 AM -0600 2/23/05, Robert C. Bullock wrote:
John:
----- However when I replaced the suspect 175, no sounds, and when I put the original 175 back in the (now socketed) PCB, I don't get the sounds even with the Bugtrap the same way. -----
Hmm, the only similar thing I've seen is a bad -5V reg (7905) on Atari Roadblasters in the sound section. Putting my logic comparator on one of the chips fixed a problem with the scrolling playfield, but killed the sound. Take it off, it scrolls again, sound works.
Replacing the voltage regulator fixed it. The voltage on the pulse to sync that section of the board was low.
You'll have to Google this or give me a deal on that pinout cart :) and I'll dig the particulars back out. This sounds somewhat similar.
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