Advice on HP-25 repair, please.

[Michael Meyer]

I have a beautiful HP-25 (aren't they all?) with problems. It turns on and will accept numbers fine and actually programs fine. It pauses briefly and shows 0.000000000 and freezes if I try to do anything with the stack: enter, operators, etc.

I have replaced the ACT and tried Katie's 20K resistor fix with no luck.

What's next? It seems as if it would be a chip problem, and there's only two main chips left to swap...

Thank you, smart people!

[Vieira, Luiz C. (Brazil)]

Hi, doc;

I have one wit exactly the same problem.

It turns out that the basic stuff - display/keyboard scan, number and program input - seem to be native ACT resources. The heavy-duty performance seems to be related to the external ROM, and if I am not wrong, it is one of the IC´s that go under the LED display assembly.

I tried with another HP25 with a bad RAM IC and it worked fine. At least I was able to identify the problem...

Not too much of a help, I know...

Success!

Luiz (Brazil)

[Michael Meyer]

Well, I replaced the ROM(0) chip under the display. Then, the calculator would go blank with the execution of any function!

Running out of de-soldering wick and hope, I replaced the vertical ROM chip from a dead board.

To my pleasant surprise... it works!!!! Programs, runs, etc.!!!

SUCCESS!!!

Thanks, as always, for the hints!!!

[Vieira, Luiz C. (Brazil)]

Hi, Doc;

good news are always welcome!

Now that you mentioned the ROM(0) chip, together with the ACT it provides the basic operation, not the ACT alone as I mentioned... my fault! The HP21 has just this single ROM with all it does, but in the HP25 its features must be shared with both ROM's. When anything goes wrong with ROM(0), almost nothing works. And when the calls for the functions stored in the other ROM are triggered, a display full of zeros is the sign that there is no answer from the (absent) routine.

Congrats again.

Luiz (Brazil)

Edited: 9 Dec 2009, 10:34 p.m.