To view the HP-150 Diagnostic LEDs remove the back of the Control Console Enclosure. The LEDs can be seen through a horizontal slot in the back of the HP-150 to the right of the Keyboard connector. There are 6 red LEDs in the array. The first two (from the left side) indicate the sequence digit and the remaining four, the failure code segment.
For example, a display pattern of 00aaaa, 01bbbb, 10cccc and 11dddd would generate a failure code aaaabbbbccccdddd. This is read as a 4 digit hex number. The a,b,c,d in the above failure code represent any combination of 1s and 0s.
When the HP-150 is powered up, the reset signal from the power supply turns the LEDs on. This confirms that the LEDs have power to them and that all the LEDs work. The first instructions executed from ROM turns off the LEDs and then turns them on one at a time (scans the LEDs). The scan (also called the walking pattern) verifies that the core components are at least partially functional and capable of being used to test the remaining hardware.
If during power up, this sequence does not take place (LEDs come on but don't go off, two or more LEDs come on at the same time during the scan etc.), then the LED codes throughout the remainder of the test are unreliable and the HP-150 Processor CPU board would appear to be the problem.
As the test proceeds, failure codes are displayed in a four segment code on the LEDs each time faulty hardware is found. Each segment will flash on for about four seconds. In addition, all LEDs will be off for about one second between segments.
The first LED code displayed following the LED scan is usually correct. Use it first when troubleshooting power-on test failures. Any codes displayed after this may be invalid.
Replacement of a component will not always solve the problem; the LED code is only a good estimation of what is wrong. It merely gives you the best opportunity to diagnose a problem without sophisticated test equipment.
Refer to the listing below for interpretation of the LED codes.