Fortunately, Williams included some nice built-in diagnostic tools in Star Rider. To access it, press the ADVANCE button during the red 'moire' test (shown below), or any point thereafter during the attract mode. Make sure the AUTO-UP/MANUAL-DOWN switch is in the DOWN position when doing this. Both of these controls are supposedly inside of the coin door on a real cabinet but I've never had access to a real cabinet, so I am only going off what I've heard.
Press ADVANCE once you see this screen to get into the diagnostics screens.
The first test that comes up is the ROM test. The board on the left is the ROM board that sits on top of the VGG board. If all of the ROMs are failing, check the ribbon cable going from the ROM board to the VGG board. Also check the two PROMs on the VGG board as they need to be good in order for these ROMs to be correctly read by the CPU.
The board on the right is the CPU board. If any of these 5 tests fail, do not even bother continuing as nothing else is likely to work if you have CPU ROM read problems.
There are several more tests before the disc test: RAM test (uses rug pattern, failures here may be non-fatal or fatal depending on whether it's bad video RAM or CPU RAM), CMOS test (tests battery backed up RAM, failures here may be non-fatal), sound test (a missing or defective sound board will cause this test to not allow you to advance to the next test without spamming the ADVANCE button for about 30 seconds), and input test (steering mechanism, brakes, turbo, throttle, etc).
None of these aforementioned tests will tell you anything about Dexter so examine them at your leisure.
After the input test, hold down advance to get to the DISC TEST. Make sure the AUTO-UP/MANUAL-DOWN switch is in MANUAL mode.
The four tests on the DISC TEST screen are controlled using the AUTO-UP/MANUAL-DOWN switch. When in manual mode, the same test will execute over and over again. When in auto mode, each test will execute in sequence one time. I prefer a combination of the two: going into auto mode just long enough to advance to the next test, then switching back to manual mode.
Reset response test: This tests the communication from the VGG board to the PIF board. It has nothing to d with whether Dexter is operational. If this test fails, your game will not work period because it will not be able to talk to Dexter. There are many points of failure that could cause this test to fail, but in my experience, a bad or unreliable PIF ROM is the #1 culprit. The next thing I would check is the cable going from the PIF board to the VGG board. I have yet to see a bad PIA6821. Again, if this test fails, it's I/O from the VGG board to the PIF board, and so Dexter is not the cause of this problem.
The walking bit test is similar to the Reset Response test. It tests communication from the VGG board to the PIF board. It tests each of the 8 data bits one by one to make sure each bit is working properly. If this test fails, it's a problem somewhere from the CPU/VGG to the PIF board, not Dexter.
The third test is the SEARCH TO test. This test does test Dexter's ability to receive and execute search commands from the PIF board. This test _does_ test whether Dexter is working properly. If this test occasionally fails, it may be because you have not performed my PIF ROM modification yet. Other reasons for this test failing are the VGG board not being able to correctly decode picture numbers from the video stream that Dexter generates, but this is just theoretical as I haven't actually seen a VGG board with this defect (yet).
Last but not least is the STEP DISC test. This test checks to see whether the PIF board can run the laserdisc player in a variable speed manner, which is what the game needs to do in order to make your 'motorcycle' accelerate. I've never seen this test fail if the SEARCH TO test is passing. If it is failing but SEARCH TO is passing, then I would suspect that it's not Dexter's fault but instead a hardware defect on the VGG board or PIF board somewhere. Like I said, there are many things that can fail to cause the whole system to break.
Whether your motorcycle gives hope to the helpless is in your hands... good luck. Oh wait, wrong game!