Verifying Galaga  74LS138
On a Galaga Cpu Locations 1P and 2P
Computer Aided Testing System

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The following images shows the testing process while examining the two 74LS138 IC's located at  1P and 2P on a Galaga CPU board.  By using the external probe, or an oscilloscope, the outputs of these chips can be verified in real time. The IC's can also be verified by testing the operation / and or results of devices connected to the outputs of these chips.

View from using the  tool designed for troubleshooting Galaga PCB's

In this example,  pin 12 of 1P has been selected, causing the hardware interface device to place hex 6830 on the address bus, in the "write" mode. The result is a stream of pulses, causing the watchdog counter to reset.

This tool contains user interface screens for examining and probing different areas of the Galaga circuit. This example screen from the testing tool shows the tests performed to prove or disprove the two decoder chips 74LS138 on the Galaga CPU board. Use of this tool provides the the ability to point and click, creating  the conditions required to test and verify any selected point in this circuit.

View using direct tests from the test table

The same tests may be run using this interface screen, where groups of tests are chosen from the test table.
Either method used provides the ability view the outputs of these devices by using an oscilloscope, watching the pulses occur as the test is in process.

Contact me at   greg@gls1electronics.com  for more information regarding Galaga board repair, using the GLS1 Electronics Computer Aided Testing System.