25C needs to be warm for registers to work


I am running into a strange problem with my 25C. If the machine is cold, the registers and the program memory will not store or recall data properly.

I found out that by warming up the machine, such as by plugging it into the charger. Then everything will work perfectly, including the Continuous Memory, which indicates that the RAM chips are probably OK. It continues to work even after I disconnected it from the charger, but as the machine cools, the problem returns.

I read the article on "A Common Woodstock Problem and Repair Suggestion." Does the fix for the 25 apply to the 25C? I am hesitant to try it as it says "The 25C and 29C use some CMOS technology and some NMOS, however they seem to have an entirely different set of problems. The following solutions have been found to fix this problem on the 22/25/27 so that the calculator works as it should initially"

Any guidance would be greatly appreciated.


Hi, Gordon;

what I can take from your text is that some "cold soldering" is the cause. When gets warm, all metal (exceptions?) dilates. I'd search for a Vcc (Vdd?) terminal in the RAM IC that's probably wiht a bad contact and, when warmt, extends enough to contact with the board. These are mainly the edge-positioned terminals. I had an HP25 with this problem, ie. no way to keep information in RAM. In my case it was a bad RAM because I resoldered and tested it in another unit.

The HP25C has a single RAM that's continuously fed for as long as the batteries are installed and with enough charge. I saw some units with a provision for another IC, but I never saw an HP25/25C with two RAM IC's.

What bothers me is the fact that the 1Watt resistor (current limit), that's mostly the one that heats up, is in the opposite side of the RAM IC; in fact, it is closer to the second ROM and the ACT. Maybe, just maybe, you have a broken copper trail close to the 1W resistor that connects after getting warm.

Not much of a help, I guess you have figue it all out, but I could not help posting.

Cheers. And success!

Luiz (Brazil)


I did a little more testing last night to see if the "heating" it needs is related to the 1W charging resistor or the battery, or just general heating of the calculator.

By leaving the calculator on top of my DSL modem, which gets slightly warm, the calculator is able to retain its memory and program information overnight without being plugged in.

Is the RAM chip the ones with the gold top? In my case, I have two of them. One to the right, and one above, the ACT chip. It looks identical to the 25C interior picture on the museum website.


I think the problem lies in the ACT chip. This chip is the same in the 25 and 25C. Katies fix is definitely worth a try.


Hi, David;

do you mean this one? I read it and I see what you (and Katie) mean.


Luiz (Brazil)


Yes, that is the one.

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