HP-10 disassembly/reassembly

[[kby]]

To answer my own question, it appears that the proper assembly procedure is to attach the CPU board to the power supply and then carefully fit the cover. If aligned up properly at the bottom, you can usually (but not always) detect when things are lined up for the keyboard to mate to the CPU board without bending pins. But it's still possible, where as the other way (affixing the CPU to the keyboard and then trying to mate that to the power supply) seems nigh impossible. Note that when you take the case apart, the CPU board will generally be attached to the keyboard.

So, many fixed traces and a few parts later, I do have a mostly working "reserve power pack" for my HP-19C, aka the HP-10, but it has one odd problem I'm hoping I can get some hints on: the "5" and "2" keys don't work. To answer the obvious questions:

1. the "8" key does work. Inspection of the keyboard shows that this does make sense; the "8" key is on a different trace.

2. The keyboard does not appear to be the problem. I have figured out with a meter which contacts appear to be closed when the buttons are pushed, and they work. They also work if the CPU board is connected and a meter is used to check the connections that should close. The "5" and "2" keys go into a single pin on the CPU chip. All traces and pins look reasonably clean and firm, but this is not possible to verify with the case completely together. Shifting the case slightly does not help. No corrosion or weak traces on the keyboard.

3. The problem is solid; not intermittent.

4. I have re-soldered the pin of the CPU that the trace connects in to, although it did not look like it was the issue.

The only other obvious answer to me is that that particular pin on the CPU is bad (bummer). That does seem rather unlikely with all other functions working to me, but am I missing something? Can someone (Tony? Katie?) suggest another thing to check?

The machine is a bear to service. Although it superficially looks like at least the printer is the same as the 19C or 97, the logic is somewhat different so that other than the actually printer, parts are not that interchangable. In particular, the quad switching transistor array is PNP whereas the other two machines use NPN. I post this as a warning that you can't assume you can substitute dead chips on a 10 with some from a 19C, let alone a 97.

Thanks in advance for any suggestions.