HP-25 Repair?



#2

I recently purchased a really clean HP-25 at a church rummage sale. It came with an 82041A charger, a carrying case, a Quick Reference Guide, an Owner's Handbook and an Applications Programs book, but no battery pack - all for under a dollar. I charged a battery pack in my inventory and demonstratd that it worked by inserting it into an HP-27. It wouldn't work in my HP-25 until I cleaned the battery pack contacts in the machine. Once I got the HP-25 to turn on with the battery pack in place I connected the charger and proceeded to check out the machine.

The HP-25 comes on OK. I can change display modes, write programs, and enter numbers from the keyboard. I can STO and RCL. But doing any mathematical functions from the keyboard or in a program, even the decimal point, end up with 0.000000000 in the display and a changed display mode. I have tried twisting the case but that didn't help. Since the battery pack contacts were bad I suppose that it is possible that the previous owner had connected the charger and fried something. Are the characteristics I describe consistent with something being fried?, Is there a chance that I can so some good by opening the unit? I am not an expert solderer. Any ideas?


#3

Chances are if you have seen corroded terminals on a 25 it is the tip of the iceberg: every one I have seen with visible corrosion were also corroded along significant parts of the internal boards. The 25 is easy to disassemble if you'd like to look and see.


#4

Post photos of the disassembled 25 :-)., please. I think they have self-test for the rom, so if it turns on it should mean that the roms are ok.


#5

Quote:
I think they have self-test for the rom

The 20 series units have no self-tests of any kind.

Quote:
if it turns on it should mean that the roms are ok

Not correct, it is possible to have a display. It might be a bad rom but the problem as described is most likely a fried ACT chip.

The layout of the 25 circuit board is such that corrosion isn't a factor with most problem units, it is usually damaged components due to overvoltage as a result of the corrosion.

With more densely packed units like the 29C, corrosion can be a factor but ultimately the number of Woodstocks that can be repaired without replacement parts is very small indeed. Just my two cents worth...


#6

Randy wrote:

Quote:
The 20 series units have no self-tests of any kind.

And in fact it was not even possible for a calculator based on the ACT processor to test the ROM(s), as the ACT has no instruction to read an arbitrary word of the ROM as data. None of HP's calculator processors had an instruction like that until CXISA instruction of the the Nut processor used in the HP-41C.

"But wait!" you're probably thinking now. "The 30 series had a self-test!"

Yes, they did, but the way they did it was by adding an instruction that computes the CRC-10 of a 1K block of ROM. The instruction takes 1025 full cycles (57400 clocks) to execute. That was useful for self-test, but had no other benefit.

The 41C doesn't have a built-in self-test, but there were test ROMs for it, which were not offered for sale. The Voyager series (10C/11C/12C/15C/16C) do have a self-test, and use the CXISA instruction in a loop to compute a ROM checksum of the 6Kwords of ROM. (The 15C does that twice for the two ROMs.) The use of the software loop for the ROM test is the main reason that the Voyager self-test takes longer than the Spice self-test.


#7

I was thinking of this rom checksuming process as I said self-test I knew they did not have a self-test a la HP-28S, I thought the series 20 had it also.


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