HP 25 not working



#4

Hi everyone! Maybe someone could help me. My HP25 was left unused for a long time -- battery pack removed. I've recently tried to put it back to use, unsuccessfully. The original batteries are now worthless and a regular alkalines were installed. I switch on the calculator and it seems to be ok, numbers can be entered and are shown in the display, etc. However, if any other key is pressed, the display sort of turns off for one second and, then, it's filled up with just zeros. What could this be? Any possibility of fixing this? Many thanks for your help, whoever you are!


#5

Hi;

I had one with this very problem, but a few functions operate as well: [STO], [RCL] and two others I can't remember.

I concluded that the only possible "bad guy" should be a faulty ROM. I removed the two of them, one by one, and tested in other working calculator. Bingo: one of them was faulty. I did not even checked for possible broken copper tralis because I used the ROM from the other calculator and the one that was faulty worked fine.

But let me tell you that in your case you may have problems with broken trails, why not? I remember that under the LED assembly you have one LED driver (the biggest chip) and one ROM (the other one). In the lower part of the mainboard you see (components side) a big chip (the Arith/Control/Timer, or ACT), the RAM at its right (or upper) side and the other ROM ath the left side. I don't remember one thing: the RAM position is related to the caculator type: HP25 or HP25C. I know it is in the upper side for one type and at the right side for the other. In both cases, the second ROM is in the left side.

Searching for broken trails (and eventual cold soldering) is a time consumming task, and you'll need a good magnifying lens. If you intend using a measuring instrument, use a voltmeter and carefully check for zero volts at both ends of each copper trail, calculator ON. I'd not use an ohmmeter to check copper trails unless all IC's are removed (voltage across measuring terminals when measuring resistence may exceed the IC's tolerance).

Success. And let us know if you need more advice. There are some gurus here that will guide you much better and with measurement details.

Cheers.

Luiz (Brazil)


#6

Luiz,

I have exchanged some emails with Marcos about his 25. Unfortunately, before I could warn him, he connected the AC adapter without batteries in the calculator. I suggested him to contact you, and told he would probably need spare parts for his calc. I guess he has a bad rom, due to using the AC adapter.

Renato


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