HP 25 Display


I have an HP 25 which works well but the display is a bit dim certainly when compared with my HP 35 and HP 33E. Is there anything I can do about this or could it be terminal ?


If you feel like going into your calculator and doing a small modification, you can make your display brighter.

Open your machine. At the top of the main circuit board there are two IC's, one with 18 pins, one with 20. Pin 6 on the larger IC is connected to ground through a 3K resistor. This resistor sets the display current and hence the brightness. Replacing it with a somewhat smaller value will brighten the display.

I tried this on an HP-21. I didn't remove the existing resistor but temporarily put a 4.7K resistor in parallel with it, giving an equivalent resistance of about 1.8K. The increase in brightness was modest but noticable.

Warnings: Don't do this unless you are very comfortable about disassembling your calculator and replacing components. Don't overdo it -- too much current could damage the display or its IC's. Finally, more current will reduce battery life and increase recharge time while the calculator is on.

Have Fun!


Hi Michael;

you mentioned both IC's under the LED assembly (viewing from the top). Which one is the HP25 ROM? I always forget that, and it's been mentioned so many times... q:o)

Not a "senior moment", instead a lazy guy. :(

Thanks... :)

Luiz C. Vieira - Brazil


I don't have my 25 open in front of me, but an HP memory chip should have a part number beginning with 1818-. Other digital logic IC's like the ACT will start with 1820-. Of course, there are the parts with alphanumeric part numbers (like the 1LB3-0003 HPIL chip for example, also most of the 41C chips including the memory chips) which seem to be HP manufactured chips. The "other" chip (not the ROM) under the Woodstock display might be such a chip but I think the ROM will have an 1818- part number. So will the other ROM and the RAM(s) on a 25, while the ACT will have an 1820- part number


Hi, Ellis;

thanks for your help. I have mine in hnads right now and it's convinientely without the two small screws.

On the top of the smallest IC, 18-pins, I read

MK60030N (MOSTeK)

The other one, 20-pins, allows the reading of a faded "20" and nothing else. This "20" comes from 1820, I guess.

May I consider this 1818-0163 the ROM chip, with almost all HP25 internal resources? I want to xchange the 25 ROM with the 25C's to verify if the 25C's can be repaired. AFAIK, they are interchangeable, right? If so, I prefer having an HP25C in hands and save the HP25 as for spare parts. It also has a damaged LED display.

Thanks again, Ellis.

Luiz C. Vieira - Brazil


The only thing I have an opinion on is that 1818- means a memory chip! I don't know what's interchangeable except what David Smith has said - including that the "only" ROM in a 21 (under the display) isn't the same part used in the 25. Another thing, the 25 should have two ROMs and one or two RAMs. The RAM or RAMs are nearest to the ACT, I believe, and the second ROM is at the bottom of the PCB but further from the ACT.


The HP25 has two ROMs. The first is included in the 18 pin display driver chip under the display module. The other is the vertical chip on the left side of the big 22 pin ACT chip. This one seems to be the one most likely to die.



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