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Hi all,
I am trying to repair my HP29C I've had since new. I'm told the ACT and ROM chips are likely to be unservicable due to overvoltage when the NiCads died many years ago.
Can anyone confirm if these parts can be sourced from other Woodstock models please?

Hi Russell,
the ROM could be sourced only from another 29C with other issue.
The ACT from another Woodstock (except the 27) and from the 67 and 97A/S.



In addition to the 27, the ACT from a 21 is not compatible as it does not support the I/O bus.

FWIW, 29C troubleshooting is straight-forward. Google "HP Calculator Chips" and use Eric's great list to identify the five rom and ram devices. Remove them. Leave the one 8 pin next to the ACT, it is power up reset device. Power it up. If you get a 0.00 display, the ACT is okay, basic math should work. If not, check for oscillations at the tank circuit, pins 13/14. If it is dead there, the ACT is dead.

If the ACT is okay, re-install the roms and ram one at a time and test after each addition. Start with the 8 pin devices, finish with the 14 pin rams.

The purpose of the procedure is to isolate the data bus as any defective device will load the bus and cause the ACT to hang.

While the ACT is a common failure, so are the roms in the 29C. The combo rom/ram is especially fragile.

Edited: 27 Mar 2012, 8:38 a.m.

Thanks for that post. That made me have a look in my box of dead Woodstocks. And with the newly accquired knowledge that all the Woodstocks (except the 27) have the same ACT, I brought a 21 with a bad ACT back to life by stealing the ACT of a 25 with a bad ROM or RAM (don't know which) chip.


I cannot say for sure that all the ACT's can go up in functionality but I think it is a safe bet to assume they could go down. By that I mean a 25 ACT may not function 100% in a 29C but the 29C would be okay in the 25. Perhaps they were the same die but sorted for speed and or current consumption, I really have no idea but there must be some reason for the different part numbers.

There were posts from David Smith many years ago that they were all compatible with the exception of the 27. I have not verified this beyond learning the hard way that the 21 had limited functionality.

At this point, I don't think anyone is willing to invest the time to figure out the differences...

HI, all;

I think this thread and particularly the IC list pointed by the link might be of help.


Luiz (Brazil)

Thanks Luiz, I found the names of many contributors I don't see here anymore...
What a pity!


Felt the same thing...