It reminds me, in my day when diagnosing microwave oven problems. I would regularly tap into the magnatrons HV circuit to test current draw whilst running the oven and also HV output. AlI done using an analogue AVO meter. Fluke digital meters back then weren't able to measure the peak HV
-too high.
Some years later we had a warranty claim for our home microwave. The technician would replace a suspect part, without any circuit testing, put the case back on and test.
I asked him why he didn't circuit test to diagnose the problem instead of parts swapping. He said they weren't allowed to test live equipment.