Is it possible to have an HP-25C repaired? If so, could someone please recommend a trusted technician.


Specifically, what is the problem with your 25C?



I've rebuilt the battery pack using 1.2v,700mAH NiCads but they don't seem to charge - calculator does not get warm in charge mode. Also, after it has been on and running for a while, the digits start to flicker. It appears that, for some reason, the calculator is not receiving proper voltage. So, I'm basically looking for someone who is a knowledgeable, careful repair person to check the unit over. Thanks for your reply.


It sounds like your charger may be bad. Check it with a meter. It outputs around 8 volts AC without a load on it (if I remember right).


You are going to 'rub some of us up the wrong way' by asking for a 'trained technician'!. Many of us here are self-trained, but still manage to get things to work.
You may want to ignore the rest of this message, since I have no formal training in electronics or calculator repair at all. But in case you find it interesting...
The charger circuit in the Woodstocks consists of a diode and 8.2Ohm (I think) resistor in series, connected between one terminal of the AC adapter connector and the +ve side of the battery pack. The other side of the conenctor goes to the -ve side of the battery pack (and the system 0V rail). IIRC, the resistor is shorted out when the machine is turned on (and thus the total current (battery charging + calculator) needed is higher than when the just the battery is being charged.
These components are on the main PCB in the calculator on one of the thin 'strips' alongside the cutout for the battery pack. It's easy to test them.
However, are you sure the problem is in the calculator? Most of the time a no-charge fault is caused by the AC adapter -- often broken wires in the cable connecting the adapter to the calculator. Check you've got around 10V AC between the contacts in the calculator connector with the adapter plugged into the mains.


Yes, I've used another charger and still have the same problem. Actually, I really need a qualified HP "techie" specializing in vintage calculators for an evaluation. Do you happen to know of someone? If so, I could send them a lot of potential clients.


Just out of curiousity, what qualifications should this person have?


Tony, you seem to be the person I've been looking for. Do you happen to do outside work. Unfortunately, I do not pocess any of the electrical equipment need for testing or repair and you are highly recommended by the HP community. If possible, I'd like to send the calculator to you for your examination rather than try to explain all the quirky problems. One thing of note, the positive contact did have some leakage corrorsion that I was able to remove and I also placed a piece of aluminum foil between the batteries and spring inside the pack. Also, I've tried other batteries. At first, the new batteries seemed to charge since the calculator got "warm" to the touch. When this no longer occurred, I used a pair of freshly charged, standard NiCads. However, the 25C works fine for about 5-10mins then begins flickering digits. Also, another odd thing that sometimes occurs - in program mode, the displayed keycodes don't jive with the actual key. But, like I said, it's not consistant. This why I really need an expert like yourself to look it over.


I do _sometimes_ take on repairs for others. But _only_ if they can bring the machine to an HPCC meeting in London, and collect it from me at a meeting.
There are good reasons for this. Firstly, I've had too many things damaged in the post (even when well-packed). More importantly, I once had a very unpleasant experience (not with anyone here I hasten to add) where I was sent a machine in reasonable, but definitely used, condition, I repaired it, and was then accused ot damaging a 'minit condition' machine. So I only accept repairs when there are witnesses as to exactly want the machine was like when I received it and when I returned it.
Yes, I know most people are honest, and won't try tricks like this, but one bad experience was enough!


Paul, sir, the old HP that developed, sold and serviced calculators and such, has gone through many changes, layoffs and "re-invention" cycles over the years. Many of us here lament that, but that's part of the reason for belonging to a community of enthusiasts and collectors. Unlike HP, which offers NO support to products beyond its "support life", those of us here who CAN help each other, try VERY HARD to "steer" the owners of our favored calcs in the right direction.

It may well be that you will send off your 25c to someone to do the diagnosis and repair-- but NO ONE that I know of can claim the certification by Hewlett-Packard, to repair machines older than their End-Of-Life date. HP's attitude is similar to that of many companies these days: you merely need to throw out the old, and purchase the current offering.

If the product, considered now "disposable" by everyone in "certified" worlds, is only held onto by collectors and caring users, it will be difficult to find a "technician."

If the only parts available are "cannibalized" from other machines, as would be the case for a 25c, and the only people who care to DEAL with a 25c in the present are those who love them and make it their hobby, then I personally would say, your best-case scenario is that one or two here at the MoHPC forum would qualify "ex grege"(from the flock), not "ex-oficio" (having the title), by virtue of their having enough experience, knowledge, sense and decency to help you out.

TO reiterate: HP does not certify technicians for repair of these machines, and back in the long-ago day when these machines were supported, they were generally supported by a repair depot in Corvallis Oregon that no longer exists. If you must find someone whom you have more inherent trust in than those here, perhaps HP itself can point you to a facility it recommends. But I think a call to the service organization will confirm: your 25c is no longer considered a repairable item.

So, what do *we* do, here at MoHPC? We recommend, firstly, that you be willing to LET us help you by doing some troubleshooting from afar.

The 25c has no "diagnostic mode", but symptoms of various failures often point to the flaw. Knowing definitely about it is a matter of YOU making measurements, describing the situation as it exists, and being willing to follow recommended procedures.

Believe me, no person regularly here would EVER say anything that would lead you to risk the health of your calculator. They love these calcs too much to throw out their caution. They will ask questions and come to a conclusion based on the facts you can give. Sometimes, the conclusion they come to is that THEY have the ability to do the repair, in which case they'll make the offer to.

In MOST cases, people find that, if they are not too quick to pass on the advice offered, but actually engage in the process, that they themselves can get their calculator healthy again, repaired and restored. You will find many testaments here, if you stick around, of people who have been given advice, taken it, and had a happy result.

Here at the Forum, many of the respondents have EE's, doctorates, parchment of various types you'd be SO LUCKY to have paying you attention at any repair facility anywhere. There are self-taught technically-minded EXPERTS with no certifications, degrees or pedigrees OTHER THAN they have proven to be deeply committed and involved in the loving repair and restoration of these classic machines. Again, you should BE so lucky to find such expertise anywhere else.

The 25c is a good and generally robust machine, but it DOES have something of an Achille's Heel: the machine can eventually die a permanent, ugly death IF it receives too much power for too long by using a power supply meant to charge AND power the calc, WHEN the battery is not there, making bad connection, or otherwise disabled.

The advice most people will LOOK to give first therefore, will NOT focus on checkout of your CPU or display drivers; you have given them a reason to focus on the machine's most common failure points, so listening to them can SAVE YOU this machine, and continuing to use it WITHOUT following their train of enquiry may make us all sad.

Now *I* will offer some specific hardware advice. This is based on some things you have said. It is not offered as "expert". Peer review will be valuable, so other input is solicited.

1. You rebuilt your own battery pack. Fine.

2. You can operate the calc on AC power. You say the pack is not charging-- this implies to me that the calc, when on batteries, removing the charger, is not functioning.

3. You say you have tried another charger, and your theory is that one charger could have been bad, but two won't be. Although that logic is flawed, in my experience with chargers, I believe you are correct that they "work", because they turn on your calc. Verifying AC voltages would, nonetheless, be a GOOD THING TO DO.

4. Given what I have heard SO FAR, I would ask: have you checked your pack's connections? Have you checked that the connections BETWEEN the pack and the calculator (the "contacts") is secure? Have you sanded them lightly to remove corrosion? Have you put bits of aluminum foil between them and the contacts (small little intervening folded strip, just to see if contact is being made securely?)

I am suggesting that the calculator is not "seeing" the batteries, either because one or more of them are defective, or because the contact between pack and calc is not sufficiently made.

YOU can at least look at those possibilities, I am sure. The successful result would be batteries in the machine that DO charge, and subsequently run, the calc. Charge with the machine OFF, please.

Now let me tell you a little more about that Achille's Heel. IF you are not charging batteries because they are not properly connecting, they cannot act to "buffer" a little transformer which is not very "smart". WITH a sufficient load on it, the transformer puts out the right voltage, right current. If the battery does not exist in the circuit to put the proper load on it, the power supplied to the rest of the calc is a bit too much, but still passably regulated in most areas EXCEPT ONE: the "Continuous Memory" chips. If they are supplied extra power for long, they will die. You will not find replacement chips for them on the open market. A donor calc would have to be found, and those with functioning memory chips are damn scarce.

It seems possible to me that flickerinng in the display is a result only of poorly-regulated, "unbuffered" power due to a defective/disconnected battery pack. But BEWARE!-- it could ALSO mean that your memory is being adversely affected. I believe the damage you cause by operating the calc, if it has not got a healthy battery, is cumulative and may quickly become fatal. So, if you take away no OTHER advice, take this PLEASE: Turn off the calc and don't use it as a "desk" machine, not until you've investigated and resolved the issue of non-charging batteries.

By the way, Paul: we're folk who love what we do-- so when someone asks a question of us, we will point you in the direction we think will prove worthwhile to you-- but we are also people who sometimes take umbrage when someone comes in expecting one of us to be wearing an HP badge or waving a diploma from "HP School". Those of us who stay here, make it our home, know that listening to most of the regulars here is MORE valuable than what you'll find in your hometown (and a lot cheaper, too!)

Most of us here know Tony Duell to be one of the most capable, electronic-saavy and knowlegeable voices in our community (if I were LOOKING to get myself a diploma in vintage calc repair, I would go to Tony to teach me, and I'd show a diploma signed by HIM with pride)-- and know David Smith and Trent Moseley to be valuable and experienced members fully able to help; in fact all three have proven their help over and over, as many others here have. Some of the crowd here are experts at one model, others are widely-experienced. If you have friends/colleagues who need assistance with vintage calculators, do tell them about the community here, for we exist to further the future of these old machines and their use.

Thanks. Let us know what you find.



Greeting Glynn,
Thank you so much for your reply. I'm basically trying to find someone who could examine and hopefully repair my HP-25C. It doesn't matter if they they have an HP badge or not, just someone that maybe you could recommend. My first 25C was bought back by HP many years ago since they were not in a position to perform repairs; it was exchanged for a 42S with printer. The current HP25C was purchased via eBay and is in very excellent to near mint condition except for a corroded terminal that I was able to clean. In doing so, I also cleaned a polished the other. Regarding the battery pack, I removed the original batteries and replaced them with 1.2v Panasonic NiCads and, initially, they seemed to be charging since the calculator got warm during the process. That does not happen now. I can use the 25C for about 5-10mins until the digits begin to blink and flicker, when powered off for a bit, it begins to work for a short while again. Since I'm not an electriction or electrical engineer, my common sense tells me the for some reason the unit is not getting the required voltage but don't have a clue regarding the charging issue. Any insight or advice would be appreciated


The 5-10 minutes of okay behavior before the blinking begins is a pointer that this is almost certainly a THERMAL problem, meaning something is getting hot, and yes, that could be due to voltage regulation problems, but THAT in itself is a large subject, ranging from the power adaptor wall-brick spoken of in earlier posts in this thread, to the resistor and diode mentioned in another post, to, well, chips that have already seen their share of abuse and just can't take it anymore.

You mentioned some interesting points in your post to Tony. One, that there WAS some terminal corrosion. Two, and maybe a real factor in whether we're talking silk purse or sow's ear here, is the mismatch between displayed program codes and the keys they correspond with. Of course, we need to rule out obvious things first: Do all your digits display when in plain-ol' "Run" mode?

RAM chips are not forever. ROM chips have gone bad, too, and they also are not easy to find replacements for. I wouldn't rule out pc-board corrosion or solder-joint degradation at the moment. But I am probably going to agree that what this machine will need IS a deeper checkout than what you can yourself comfortably provide.

Having these additional observations of yours switch the line of inquiry slightly and make your request for hands-on expertise understandable, where it sort of seemed elitist to me before.

You may want to ask a more general question of yourself first: how much is this e-Bay investment worth to you as a working machine? What you've put into it by buying it, plus your battery-pack rebuild, plus what someone will charge you to discover the exact problem and replace whatever is necessary, may be MORE than setting this one aside or re-selling it as a "parts" machine, and acquiring a functional, fairly clean example of the model.

If economics are not really a consideration (i.e., this one is the one you want to keep), and you want to get it working as it should, because you want to program and use it, then that's all good; I just want you aware that costs depend on the trouble the unit is in, and the demand/supply of the parts it requires. Costs rarely seem "fair"at the moment: "Dang, it cost me $$$ and now it needs ### and that adds up to ***...".

Assuming people you hear of here WANT to take on the job, Mr. Siegmund, please understand that there is a special dynamic in user-communities that makes your relationship with those in them slightly different.

If I am in a business to make a profit and I say I can do something for you, I will set aside dinner and not do any sleeping and I'll apologize to the wife for coming home late-- because my PROFESSIONAL commitment is to YOU, the customer, and I depend on your business and referrals for my survival. And I will charge you whatever the going rate is, regardless if the repair means just touching up a dull connector with a nail-file.

Most EVERYBODY understands and expects this relationship in "the BUSINESS world", as YOU DO-- which is obviously what you were searching for: "Actually, I really need a qualified HP "techie" specializing in vintage calculators for an evaluation." and, "...If so, I could send them a lot of potential clients." ;-)

In user-communities, however, if there WERE businesses the community itself would rely on them and not be as likely to individually aim a screwdriver in the direction of a valuable piece of gear. It is the LACK of reliable paid-professional sources that makes users band together more and talk through problems and learn as much about the internals as can be gleaned from old documentation, periodicals, and rooting around them with voltmeters.

The upshot of this is that most of us have projects of our own, lives to live, jobs that keep food on the table, and we NEED nothing from you (or any referrals) to do what we want to do.

We do it because we want to, IF we want to, and it becomes a project for us only IF we think that, at the end of it all, you'll be happy, and we'll be happy we became involved.

We will take time out when we feel overwhelmed by other events, we will not like to be "bugged" every day with frantic e-mails wondering what happened to us, we will look around amongst our buddies for parts, and even buy things on eBay just for parts--- and you'll think you are forgotten before you finally hear that some "framistan" was replaced, and your calc is on the way.

Now this may sound absolutely topsy-turvy to the normal relationship of business to customer, and that's because the relationship IS different, even if money is involved in the repair.

The money will likely be a slight recompense to the time largely freely given, and parts will have to be valued at some arbitrary level based on what our reclamation baskets hold and how likely we are to ever see one again.

The MAIN thing is that the professionalism values held by an enthusiast are often seen in the absolute care with which he treats the calculator, the insistence on getting things right and not just putting a band-aid on it.

The old business values of price and delivery are tied only to the profit-motive, not the volunteer-motive. Chances are, the volunteer wants to do "right" by you, give you the most value he can-- but HE must be free to define what that entails exactly.

The enthusiast often enters the relationship giving you fair warning: "I don't know till I look at it. My plate is full. I'll get to it when I can. I will contact you, when I know. I will try to make you the best deal I can on the parts. I'll need you to be patient". This does not mean he or she is inept or unaware of the demands of business. But this is NOT a business, not meant to be lucrative. It is a Hobby, meant to be personally satisfying.

It is often the customer who does not understand why his offer of money has not changed the relationship over to the "business" model. But until and unless someone makes a specific commitment with prices, deadlines and all, don't make that assumption. Give the volunteer the opportunity to prove his worth to you. He may not be in business, but he does not like to feel he has taken advantage of you or let you down. Let him know that you are interested in learning more about calcs, in being more than just another project on his stack, and likely yours will be first on his stack because he wants to help you and enjoys unravelling a mystery.

Well, I have written this in the hope that you will understand now why some of the things you have been asking are more than just a matter of looking into the "MoHPC Yellow Pages". Tony may want to help, he may be too busy with other things right now, or he may actually feel that your calc is a basket-case. But it's HIS choice, he hasn't yet *(as of my writing this)* offered anything but advice.

Neither have I. In fact, shortly after reading your post this morning to Tony, indicating that we had nominated him to be asked to take your calc on as a project, I felt I was going to have to apologize directly to Tony, for taking away his choice to offer, and to the community at large, for appearing to ignore the many 25c enthusiasts who might have also otherwise cared to get involved. I had wanted to let you know that the guys helping you are DAMN GOOD at their craft, and reliable sources of information. I saw that you had taken it to be the equivalent of me handing out a guy's business-card. If that turns out to be the way he wants, fine. But if not, I want you to know that business amongst collectors and in a user-community setting IS a strange animal to outsiders, but that you should be able to get help here-- if you understand who you are dealing with, and WHY they are willing to do what they do.

By the way-- I've traded a few things in my collection for a few things I've wanted. I've bought stuff from individuals here. I've had calcs repaired, by folk in our group. I'VE NEVER been burned HERE (wish I could say that about eBay). I am comfortable with everybody on this board, even those I fight with!! So observe and know: this is a GREAT group of people.


A couple of quick points. Firstly, it's safer to charge the batteries with the calculator _ON_, not off. The extra load of the calculator will pull the AC adapter voltage down a bit even if the battery pack goes open-circuit, which may save the RAM chip. The worst possible situation for the RAM is to have the adapter plugged in, no (or open-circuit) battery pack, and the machine off. The RAM then gets about 12V, and promptly expires.
Of course _the_ safest way to charge the battery is in a reserver power pack (a little holder for a battry pack containing the charger circuit). But reserve power packs are rare for the Woodstock series.
Secondly, thanks for supporting my view (shared, I suspect by almost everyone here) that the current best repaired are self-trained enthusiasts. FWIW, I do have some qualifications -- I'm a particle physicist -- but not in electronics or computing. And I am certainly not HP trained or certified.
On the other hand, even if I say so myself, I've worked on many models of HP calculator from the 9100 onwards.
Fianlly, if you really want me to teach you how to repair calculators, come to an HPCC meeting. I'll help you in any way I can :-)


Okay, charge while calc is ON!

Thanks Tony-- I needed to know that myself...

---glynn (no 25c, not yet anyway)

Possibly Related Threads...
Thread Author Replies Views Last Post
  HP-65 Repair/Restoration (Pictures) :) Dan Lewis 8 641 01-12-2013, 12:41 AM
Last Post: Dan Lewis
  HP-01 Restoration aj04062 4 417 10-01-2012, 08:52 PM
Last Post: BobVA
  A Guide for the Restoration of Hewlett Packard Calculators Jim Johnson 8 546 04-13-2012, 03:16 AM
Last Post: Geoff Quickfall
  9100B restoration progress David Ramsey 6 443 11-27-2011, 10:33 AM
Last Post: Etienne Victoria
  HP75C Restoration (zebra connector ??) Alberto Fenini 5 346 11-01-2010, 06:47 AM
Last Post: Alberto Fenini
  HP-67 keyboard restoration Pierre Brial 4 351 04-15-2010, 11:08 PM
Last Post: Pierre Brial
  Restoration of an HP-15C Karl-Ludwig Butte 7 434 09-15-2009, 03:19 PM
Last Post: Karl-Ludwig Butte
  Voyager HP-10C restoration and bezel replacement. and the book. Geoff Quickfall 11 699 08-13-2009, 02:33 PM
Last Post: Giovanni Jimenez
  early HP-80 restoration and Tallnut HP-41C question. Geoff Quickfall 20 1,054 08-10-2009, 02:32 PM
Last Post: geoff quickfall
  The HP Restoration Guide, progress report Geoff Quickfall 3 266 07-05-2009, 03:44 AM
Last Post: Marcus von Cube, Germany

Forum Jump: