HP8924C ROM Checksum Error on power on

[WPXS472]

After sitting for a couple of months, my 8924C now displays ROM Checksum Error about 10 seconds after powering up. The HP documentation says to replace the A16 Host Controller card. I have disassembled the unit sufficiently to access the cards. The A16 card has 2 socketed ICs. One is an X28C256P-20, and the other is a 08920-87168960814W6. I suspect that the latter is the one that is the problem. I have an 8924E I bought some years ago when they took a drastic price drop for a while. I got it pretty cheap, and have kept it to use as an organ donor, should the need arise. While it worked when bought, it has been in storage for quite some time, and I am hesitant to just power it up. My question is twofold. First, has anyone else had this same failure, and was the A16 card the problem? The second part is: If I put the A16 card from the 8924E into the C would it work? Assuming it is good, that is. I know the E version was lacking some of the bells and whistles of the C model. Would I end up with E model features?

[WPXS472]

Addendum
After further checking, it seems that the internal LED check lights DS3, not DS2 as first assumed. The relevant HP documentation says this indicates A17, Memory card. This card has a 3 volt lithium battery, which reads almost exactly 3 volts. I will disassemble my 8924E and replace this card with the one from that unit, if it looks to be the same. I will post results when available.

[fzabkar]

X28C256P is an EEPROM ...

[WPXS472]

I pulled the A16 card from the "E" model, and put it in the "C" model. It didn't display the ROM checksum error, but thought it was an "E" model. Then, I moved both socketed chips from the "C" model card, and placed them in the "E" model card. Now, it still thinks it is an "E"model. I have ordered a used "C" model A16 board off eBay. If I put that in, and it still thinks it is an "E" model, I am stumped. The A17 board from the "E" model has a bad battery, so I didn't try it. I ordered a new one, and will try to get the "E" model up and running. I am not sure exactly what the battery does, I know it retains the memory, but what memory is it retaining? Is it such things as last setup data, power on defaults, or cal data. If it is cal data, then I think this unit is landfill material. The assembly level manual doesn't really explain things like in much detail.

[WPXS472]

I have an update. I received the used A16 card, and installed it. Now, I get a "Calibration check sum incorrect. Writing to default values" message. Booting proceeds no further. The manual says that the cal constants ROM from the old card must be moved to the new one. I did that, and get the same results. The manual also states that it should complete booting and work, except for being uncalibrated. So, I guess this is it.