Agilent / HP 54820A Oscilloscope Battery / Real time clock replacement

My old HP/Agilent 54820A Oscilloscope produces an error during every single boot. After it has been switched off it always looses date and time and all CMOS settings. It does continue to boot up after asking for user input. This is because the CMOS battery that keeps the CMOS settings is flat. Most 54810A, 54815A, 54820A have in common that the battery is not a simply CR2032 coin cell that is easy to replace like it is in more modern versions of these oscilloscopes like the 54830A or the DSO8000 and DSO9000 series. Instead this series of oscilloscope have a Dallas Real-Time Clock chip with embedded battery. Even worse, the chip is soldered directly to the motherboard.

The after the oscilloscope range of HP became Agilent and more recently Keysight, the Service Manual can be found here:

They’ll want you to provide name and email address, but that’s a rather small sacrifice to make for a service guide. The service guide is helpful, since it does walk you through some diagnostics and will inevitably tell you to replace something expensive, since the diagnostics do not go down to component level. – Don’t get me wrong, I am not complaining. I am very thankful for the fact that there is actually a service manual.

Pages 5-30 ff show diagrams of the PC motherboards used in this range of oscilloscope and pages 7-17 tell you what motherboard is in your oscilloscope based on your serial number. This is however the hands off way of finding the board suitable for anyone who does not want to open the oscilloscope (which is inevitably required later on, when the battery will be replaced). Anyways, in short:

  • serial prefix range <US37349999: ”PC Motherboard AMI Series 727 (Atlas PCI-II)”. Dallas RTC. Likely a DS12887A.
  • serial prefix range US3805 through US3838 with rear-panel label VIN # 020: ”PC Motherboard AMI Series 757 REV C (Atlas PCI-III)”. Dallas RTC. Likely DS12887A
  • serial prefix range US3845 through US3909 with rear-panel label VIN # 025: ”PC Motherboard AMI Series 757 REV D (Atlas PCI-III)”. Dallas RTC DS12887A.
  • serial prefix range US3919 through US3935 with rear-panel label VIN # 025: ” PC Motherboard AMI Series 757 REV D (Atlas PCI-III)”. Dallas RTC DS12C887A. RoHS compatible replacement DS12C887A+, which is difficult to google because of the ‘+’.
  • serial prefix range US3949 and above with rear-panel label VIN # 03X: ”PC Motherboard FIC VA-503A”. Lithium Coin Cell 3V (likely CR2032). If you have one of these, you’re lucky! Replacement is about as easy as replacing any other coin cell.

In short, you will likely need either of these two chips:

Their RoSH compliant counterparts are:

Here is the datasheet for the Dallas RTCs mentioned above.

Furthermore the DS12887A can be replaced with Tech Houston chips that use the same nomenclature as Dallas.
The Benchmarq BQ3287 is also compatible with the Dallas DS12887 and so is the Odin OEC12C887A and the ST M48T86. I will stick to the original Maxxim Dallas parts as they are readily available (see links above).

Maxxim were nice enough to produce an application note on the replacement of these chips, which is still floating around on the net. I took the freedom to provide it here for reference: Maxxim Application Note: Replacing Dallas RTC.

In my particular case I had a PC Motherboard AMI Series 757 Rev D (Atlas PCI-III) with DS12887A  real time clock with embedded battery soldered directly to the motherboard. Argh#%@±!

AMI Series 757 Rev D, Atlas PCI-III


Dallas DS12887A, Maxxim

This is now the point where a decision has to be made:

  1. With a bit of courage the existing Dallas DS12887A can be (literally) hacked open and get a piggy back CR2032 Li Coin cell for easy replacement. There is a great how-to for that over here: Call me paranoid, but I don’t like the idea of an open chip that allows the battery to leak onto the motherboard. I read that these RTC chips don’t leak, but I am not sure that this is true after filing them open.
  2. With a bit less courage the Dallas DS12887A can be socketed and replaced . I like this idea better.

This is basically down to your personal preference. Soldering to the motherboard will be involved anyway.

After carefully unsoldering the chip, I soldered a 24 pin socket with 2.54mm pitch to the board and inserted the brand new DS12887A+. Job done!

Posted in Misc Electronics and tagged , , , , , , , , , , , , .

One Comment

  1. Seems fun, I had the same issue wich I fixed easily, and some others (+2), My bios flash would not work, dead chip and the GAL16v8D, with the sticker Pxxx is also fried. I was able to get a new flash and program the bios onto it. Now, next item on my list is the GAL. Would you be able to read the latter chip with an flash programmer and make the code available to me. I have a blank GAL , a programmer, so the job can be done. Thanks for your input. Cheers,

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