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Defective HP 54504A --- NVRAM substitute?

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Wim_L:
I recently acquired an old HP scope that was going to be thrown away. Poor sampling rate (200MS/s), good bandwidth (400MHz). It's clearly defective, but may be fixable. It gives plenty of errors when booting, and fails several of the self-test procedures. However, using the appropriate magical incantations from the service manual (power cycling with a button pressed, then going through the full self-calibration procedure) its full functionality can be restored. Until the next time it's turned off, when it fails again. So it's fairly clear the NVRAM battery has failed. Now, for the fans of instrument pictures...

First a front view. Showing its age with some visible screen burn-in, and a missing knob. Standard shaft size, so it should be no problem to find a replacement for that, I'll see if I can find something with a large diameter and not too high. The metal box to the left is the power supply which I had removed there. All seems fine there, the voltages where it matters are still spot on. Electrolytic and tantalum caps all over the place in this scope, and yet no sign of obvious failures during a first inspection. I did clean some dust out, the fan pulls a lot of dirt in.

Next, after all calibration procedures have been performed. It can be used in this state, but the rather lengthy procedure makes it impractical to do that every time. It wouldn't be as much of a pain if it were used as part of an installation where it's always on, but for home use, its fan is a bit too noisy.

The insides then! After removing the back panel, the main board slides out through a slot in the rear part of the screen. It's a heavy construction, full of thick metal plates, though the scope isn't all that hard to lift. Fairly light compared to some older portable analog scopes even. An inside view of the scope front panel shows they made room for more connectors than are actually present on the front.

And the main board, top and bottom. Yep, that's gold plating all over the tracks. The whole thing is shiny yellow metal.

Final picture, the likely problem case! An NVRAM chip that, as far as I know, isn't being made anymore. DS1235YWL-120. Maxim still makes a similar chip, the DS1230 (in various versions, Digikey has a whole page full of variants). Looking at the datasheets, they look quite similar indeed, though which of the many variants would be the best replacement will require a closer look. Anyone who has done this before and knows which one will or won't work? The replacement procedure itself may also turn out to be tricky. It's not socketed (they probably figured the scope would be discarded before the battery ever ran out) and the pins are under the package so cutting them and desoldering one by one isn't going to work.

Finding probes may also be tricky, considering it only has 7pf input capacitance at 1MOhm, which is outside the compensation range of many cheap and some not so cheap passive probes. Of course, I could make some passive resistive divider probes to use on the 50 Ohm inputs, which probably is the best way to measure at the high end of the frequency range anyway. But conventional passive probes can be useful too at lower frequencies.

So, any suggestions on where to go next with this one? I do already have an 150MHz Hameg combiscope 1508-2.

Rerouter:
top right, is that a speaker or a battery?

Wim_L:
Top right on the mainboard? A speaker.

Having the battery separate from the RAM would be very convenient, but I'm not so lucky here... That kind of NVRAM module has the battery built into the IC package.

Rerouter:
well then as an alternative if it really is shy of impossible to replace it, how many mA does it chew under normal operation, perhaps you could fit some external battery to maintain it, its dip so you could lift the power leg and run it that way as a worst case option,

Wim_L:
That's possible in theory... With the disadvantage being the place to do that is buried inside the IC. It would involve drilling into the package and connecting wires to that. I'd take my chances with desoldering and trying the Maxim IC as a substitute before considering that ;-) (especially as misjudging the location of components inside the package means a risk of drilling straight into a lithium battery)

Oh yes, you can see a little bit of slightly dodgy stuff I did too... On the EPROMs, they were all exposed because the paper stickers had fallen off. So before I made the pictures, and not having good black tape left, I used scotch tape to stick black pieces of paper over the windows before taking pictures with flash.

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