DEC made diagnostic programs for every hardware they build.

These were scattered over the whole net, but a approach was made to gather them in a single database.

Normally you use the diagnostics as a GO/NO-GO test to find defective components, which you replace with good ones then.

Repairing, on the other hand, means mostly to find defective ICs on the circuit boards by means of a logic analyzer.

This article show how a defective 11/34 CPU is analysed with diagnostics, down to the point were a logic analyzer is triggered on a single erratic assembler statement.