One day I got a call from a friend saying that he accidentally fed high voltage signal (line voltage) and broke one of the channels.
My hunch is that since he didn’t feed kilo-volts into it, the damage is limited to the (front-end) attenuator (an expensive part). I had an extra unit lying around so I just swapped it for him so he can get it up and running.
Once I got the time, I opened it up the broken unit to see the damage. My hunch was correct:
The 500Mhz attenuator used in 54616B, 54615B, 54520A, 54540A, 54503A and early Infiniiums 54810A, etc. are all the same: 1NB7-8303. The resistive trace was blown open circuit (charred), so bad that it spot welded the metal casing.
I took a good attenuator and measured resistance that the blown open ‘trace’ should have been. Turns out it’s a little less than 1Ohm so I used 4-lead Kelvin sensing from my Agilent 34401A multimeter. I calibrated that meter using my Data Precision 8200 reference and agreed with my Fluke DMM, so hopefully I wasn’t way off.
I could have put a small SMD resistor matching the resistance of the blown trace, but since I have a few back up attenuators lying around, I’d rather not risk the signal integrity by doing so, since I cannot guarantee the temperature specification if I just replaced it with a resistor. The scope was up and running in perfect condition after replacing the attenuator.
Now you know what happens when you feed line voltage to a scope. The damage is usually limited, but the front-end (attenuator) is the most expensive RF part that makes a scope truly a scope (see Dave’s rant on EEVblog). The ADC hybrid can get just as expensive, but usually the attenuator takes the hit before your input gets to fry the ADC.
565 total views, 1 views today