(Front-end) Attenuator Autopsy of HP 54616B Digital Oscilloscope

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:

Fried 1NB7-8303 54616B Attenuator Block

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.

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