{"id":64,"date":"2016-07-27T23:56:17","date_gmt":"2016-07-28T07:56:17","guid":{"rendered":"http:\/\/wonghoi.humgar.com\/blog\/?p=64"},"modified":"2025-10-18T14:01:43","modified_gmt":"2025-10-18T22:01:43","slug":"front-end-attenuator-autopsy-of-hp-54616b-digital-oscilloscope","status":"publish","type":"post","link":"https:\/\/wonghoi.humgar.com\/blog\/2016\/07\/27\/front-end-attenuator-autopsy-of-hp-54616b-digital-oscilloscope\/","title":{"rendered":"(Front-end) Attenuator Autopsy of HP 54616B Digital Oscilloscope"},"content":{"rendered":"

One day I got a call from a friend\u00a0saying that he accidentally\u00a0fed high voltage signal (line voltage) and broke one of the channels.<\/p>\n

My hunch is that since he didn’t feed kilo-volts into it, the damage is limited to the\u00a0(front-end) attenuator (an expensive part).\u00a0I had\u00a0an extra unit lying around so I just\u00a0swapped it for him\u00a0so he can get it up and running.<\/p>\n

Once I got the time, I opened it up the broken unit to see the damage.\u00a0My hunch was correct:<\/p>\n

\"Fried<\/a> \"54616B<\/a><\/p>\n

The 500Mhz attenuator used in 5461XB, 54505B<\/span>\/54506B<\/span>\/54510B<\/span>\/54512B (has to be version ‘B’)<\/span><\/span>, 54520\/54540 series, and early Infiniiums 54810 series, E1428A (VXI board), 166XCS\/166XES (MSO), 167XG-003, etc. are all the same:\u00a01NB7-8303 (or 1NB7-8458). For 5461XB, it’s tucked inside 54616-68401*, which is a big metal case that covers 2 attenuators and an external trigger PCB (see the picture on the right hand side). Everything else is tucked individually inside 54512-63402 metal shield.<\/p>\n

In this autopsy, the resistive trace of 1NB7-8303 was blown open-circuit (charred), so bad that it spot-welded the metal casing.<\/p>\n

I\u00a0took a good attenuator and measured\u00a0resistance that the blown open ‘trace’ should have been. Turns out it’s a little less than 1\u03a9 so I used 4-lead Kelvin sensing\u00a0from 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.<\/p>\n

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.<\/p>\n

Now you know what happens when you feed line voltage to a scope.\u00a0The damage is usually limited, but the front-end (attenuator) is the most expensive RF part\u00a0that makes a scope\u00a0truly a scope (see Dave’s rant<\/a> on EEVblog). The ADC hybrid can get\u00a0just as expensive, but usually the attenuator takes the hit\u00a0before your input gets to fry the ADC.<\/p>\n

\n

* Be very careful about a similar looking block with similar number 54503<\/strong><\/span>-63401 or 54503<\/strong><\/span>-63402. It has the same metal housing, but inside the attenuator chip 1NB7-8116 or 1NB7-8288, which is NOT compatible with first generation Infiniiums, later 54500 series or 5461XB series. They are used in 54502A, 54503A, 54504A, 54510A<\/span>.<\/span><\/p>\n

[EDIT 2025-04-25]: There’s a newer revision of 54054A that has the power switch in the front (the serial numbers do not start with a country code). They use 54512-63401 (the newer attenuator design) instead of 54503-63401 and they are not compatible forward or backward.<\/span><\/p>\n

54501A never showed up in the discussion because 100Mhz scopes do not need the individually shielded metal block. The attenuator was directly soldered to the main assembly.<\/p>\n

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One day I got a call from a friend\u00a0saying that he accidentally\u00a0fed 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\u00a0(front-end) … Continue reading →<\/span><\/a><\/p>\n

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