Possibly unrelated, but I have an Agilent PNA? from about 2006.? It had a number of issues including a "non-functional" CPU board (according to Keysight).? Symptoms were similar to yours - 3 or 4 leds would light, but it would not boot or do anything. I tried a number of things (including removing, reflashing, and resoldering the EPROM), but in the end I discovered that when you replace the coin cell back-up battery, you have to short the "reset" pins that are positioned about 4" from that battery.? They are labeled on the board, but a possibly a bit tricky to find.? After I shorted these (with a pair of tweezers), it booted fine.? It is kind of insane to me that Agilent didn't catch this in when they quoted the job.? Note the battery in the upper right and reset pins in the lower left.
