Can any one offer insite into HP-25 repairs?

The STO/RCL function on a 25 does not work but all math functions are OK. Progam store is also no good. Has anyone experienced similar & identified the specific register/program store memory chip that could be replaced?

Also the No 1 key sometimes registers a 3. The KB looks clean enough - could this problem be atributed to a dodgy contact? Any tips on cleaning inside KB in that case?

Any suggestions very welcome.

thanks
George (UK)

Looks like your RAM chip is faulty. The stack registers seem to reside inside the CPU and work, but accessing storage registers or program memory fails or reads garbage. Try the following:

• Let the calc warm up for up to an hour. Some Woodstocks have problems with their internal clock signals that correct themselves after a warm up period. See http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=267 for details and possible repair suggestions.
  
• Carefully search for broken traces on the PCB or loose solderings on the pins of the chips, and repair any problems you encounter. You might also want to check the traces on the PCB with an Ohm meter.
  
• Replace the RAM chip if you have a spare one available
  
• Count on one of the more knowledgeable experts in this forum to come up with additional suggestions.
  

Success, and keep us informed!
Cheers, Victor

Hi, G. Evans;

just to helps us understanding better: what actualy happens when you punch [STO]n or [RCL]n? No [ Error ] message? The stored value is lost? Please, try [STO]8 or [STO]9 to force an error message.

RAM is shared into program steps and registers. I remember I saw images o fHP25 guts with what seems to be two RAM chips. In this case, I wonder if one of them is used to store program lines ant the other is used to store register data. If the 7 lines by register is internaly, then we can figure that one of the RAM chips contains eight registers, R0 to R7, and the other one contains the LASTx plus 7 × 7 (49) program steps. Stack registers are hold inside ACT big-chip. In this case, if your HP25 cannot hold registers data, chances are it is the type of unit with two RAM chips.

Would you have the chance to open it and describe what exists inside?

Success.

Luiz (Brazil)

Luis

Thanks for reply,

I have opened the unit and carefully cleaned PCB and all accesible contacts .

Doing an (X STO n) does not bring any errors but doing a (RCL n) always returns a 0 for any of the 8 registers.
(Last X) also returns 0 always.

It will not store progams either. Looks like RAM chip but
which one is it ? I can take a pic of insides if the forum allows this ?

George

Victor

Thanks for your suggestions, as I replied to Luis, I cleaned all I can inside all looks ok. I was also wondering about clocking problems I will let unit warm up as you suggest. The link you suggested has some interesting insites. Will keep in touch.

many thanks,
George

Hi, G. Evans;

about the RAM chip(s), have a look at the bottom of the PCB, close to the batteries contacts. There a big chip, the ACT, and in its left side you'll see one of the ROM chips. The other small chip cloe to it, most likely to be closer to the batteries contacts, is the RAM chip. I saw a unit's picture showing two chips, and if you look at the PCB's layout, you'll see that there's room for a third chip. The HP25C has almost the same layout, with a diferent power supply scheme to allow RAM chips to be fed while the calculator is OFF.

Unfortunately there's no such know replacement for these RAM chips other than original "donated organs" from deceased units. I hope you're lucky finding the problem. I had some HP25 and HP22 (both use the same RAM chip) and they have no good RAM to donate.

Best regards.

Luiz (Brazil)

Luis

I will check this out this week end.

thanks,
George

My 25 had the same keyboard but with different keys; my 4 key pressed sometimes resulted in a 6. I fixed it (not entirely) by dismantling the unit and stuffing a strip of isopropanol soaked paper under the metallic bubbles that form the backplane of the keys. (I can't remember though, what kind of paper I used) I also cleaned the six up as well. This got rid of 95% of my errors.

If I recall, there is a technical article somewhere on this forum on cleaning under keys. I'll see if I can track it down.

Good luck!

Well, it took me about 3.5 seconds to find the article. It is called "How to restore classic calculators" - (DUH, what else could it be called?)

http://www.hpmuseum.org/howtohp/howtohp.htm

I didn't use sandpaper, but rather I used a toothpick with alcohol on the "mystery" paper (that I can't remember).

You RAM chip is toast. This is a VERY common problem. I have seen dozens of HP25 machines that failed in this way. A similar amount failed with a bad ACT chip (the big wide 22 pin chip near the bottom of the circuit board). Only source of parts is another machine. Some HP25 machines have two RAM chips, others only one. The RAM chips are located either to the right of the ACT chip or directly above it next to the battery contacts.

The keyboard problem is caused by (often invisible) contamination on the keyboard. Clean it well with 91% isopropyl alcohol. Keep the alcohol off the LED window and the front keyboard surface.

If the problem persists, the contamination may be on the top side of the keyboard... bad news becuase the keyboard is held in place by a zillion melted heat staked plastic posts. If you cut them loose you can free the keyboard, but getting in back in so that it feels and works properly is VERY difficult. I would recommend using an ultrasonic cleaner. Use three different baths of distilled water. Shake out all the water afterwards and dry it overnight in an oven set on its very lowest setting.

I have just reanimated some dead keys on a HP 67 with a strip of plain paper soaked in contact spray. I applied some more spray afterwards and operated each contact a dozen times or two (with the calc off!). Got everything back to perfect working condition.

However, this is not going to help here because the 25's keys cannot be accessed from above, as pointed out by David. If the problem with your keyboard is indeed bad contact, you could try to apply contact spray from the back of the keyboard. There's a convenient little hole behind every single contact where you can apply the spray. I have cured the keys of my own HP 25 this way, although I had to repeat the procedure 3 or 4 times (spray - operate -let dry -assemble - test - disassemble ..., you get the idea).

Did warming up help?

Cheers, Victor

Hello,
My HP 25 also behavs in very mysterious ways as well.
It will not programme unless it has been on the battery charger for several hours. It appears to have to be 11o% charged to operate or else it has to be "hot" (the right hand side of the calculator gets quite hot after it has been on the charger for several hours).
Maybe yours is the same. I suggest leaving it the charger for several hours and try it when it is fully charged and still on the charger.

David Smith reports he' seen dozens of fried RAMs and a similar number of bad ACT chips. Many users make the experience that their HP 25s will only run when warmed up for a certain amount of time. And Katie provides at least two possible explanations plus the respective cures for these disorders in her excellent article (http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/articles.cgi?read=267).

I wonder if maybe most of these defective RAM or ACT chip problems could have been resolved in a similar matter, i.e. these chpis were not really bad, but just degraded to some point where even warming up would not suffice to get the calcs running. In that case it would still be worth trying Katie's suggestions on these calcs.

That said, if warming up with the charger (which is usually *very* effective) does not help, get a 20k resistor and a soldering iron and give it a try.

Cheers, Victor

Victor

No not yet, I will try this weekend and will also try Katie's suggestions.

thanks for you interest will keek the thread posted.

George

One of the first things I did when I opened the calc for the fist time was to clean with PCB cleaner(alcohol based) the exposed side of the key pad. This, I must admit, did help A LOT ( several other keys did funny things) now only the 1 registers a 3 about once in very 6 to 10 presses.

I will repeat the cleaning as others have suggested .

thanks to all.

George.

thanks for the link -

I will try to clean it again it helped the first time.

thanks
George

Thanks for the great article by Steve Loboyko.

Just printed it.

George

Nope, the chips I have pulled are well and truly bad. The thermal problems of the 25C and 27 are a different problem. I do have a couple of thermally sensitive ACT chips, but they are not very common.

Any idea, then, what is causing these chips to fry ?
Maybe trying to use the calc with charger and no batteries ?

George

It's definitely a power supply type of problem. The HP67 and HP97 use the same ACT chip. They have a different type of charger system and their chips (almost) never go bad.